Method for co-producing white carbon black, nano calcium carbonate and calcium sulfate whisker based on phosphorus chemical waste residue cascade separation

Abstract

The invention relates to the technical field of resource utilization of phosphorus chemical waste residues, in particular to a method for co-producing white carbon black, nano calcium carbonate and calcium sulfate whiskers based on stepped separation of phosphorus chemical waste residues. The method solves the problems that in the prior art, the phosphorus slag resource utilization rate is low, closed-loop circulation of acid-base media cannot be achieved, and the waste water discharge amount is large. According to the technical scheme, the method comprises the following steps: performing acidolysis on phosphorus slag powder and nitric acid to separate calcium and silicon components; after being purified, the acidolysis solution reacts with sulfuric acid to prepare calcium sulfate whiskers, and regenerated nitric acid is recycled for acidolysis; performing alkali dissolution on the active silicon slag, carbonizing to prepare white carbon black, causticizing mother liquor to prepare nano calcium carbonate, and recycling regenerated alkali liquor for alkali dissolution; and neutralizing the white carbon black filter cake to obtain a byproduct anhydrous sodium sulphate. According to the method, gradient high-valued utilization of calcium and silicon components in the phosphorus slag and double-independent closed-loop circulation of acid and alkali media are realized, and the production cost and the environmental protection pressure are remarkably reduced.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of resource utilization of phosphorus chemical waste slag, and in particular to a method for producing white carbon black, nano calcium carbonate and calcium sulfate whisker by cascade separation and co-production based on phosphorus chemical waste slag, a device and method for continuously preparing and modifying calcium sulfate whisker by using a metal filter concentrator, and a high-temperature-resistant and corrosion-resistant precision filtration and mother liquor recycling system in the production process of white carbon black. BACKGROUND

[0002] Yellow phosphorus is a basic chemical raw material and is widely used in the fields of pesticides, medicines, food additives and high-tech materials. The electric furnace method is the mainstream process for industrial production of yellow phosphorus. The basic process includes: the raw materials such as phosphate rock, coke and silica are mixed in proportion and then sent into an electric furnace for reduction reaction at high temperature to generate furnace gas containing phosphorus vapor and carbon monoxide; then, the furnace gas is recovered by a condensing system to obtain liquid yellow phosphorus, a crude phosphorus product and by-product yellow phosphorus tail gas.

[0003] In the process of producing yellow phosphorus by the electric furnace method, a large amount of yellow phosphorus production electric furnace slag, referred to as phosphorus slag, is produced in addition to yellow phosphorus and tail gas. The main component of phosphorus slag is calcium silicate, and it also contains a small amount of phosphate, fluoride, iron, aluminum, magnesium and other impurities. According to statistics, about 8 to 10 tons of phosphorus slag are produced for every ton of yellow phosphorus. At present, phosphorus slag is mainly used to produce cement, blocks and other low-value-added building material products, with low resource utilization rate and poor economic benefits. A large amount of phosphorus slag stored will occupy land and pollute the environment.

[0004] At present, although there are reports on the technology of preparing high-value-added products from phosphorus chemical waste slag, they are mostly developed for a single product, such as the preparation of calcium sulfate whisker or white carbon black from phosphorus slag alone. The calcium component utilization and silicon component utilization are not organically combined to form a complete cascade separation and comprehensive utilization process, resulting in low resource utilization efficiency.

[0005] Since phosphorus slag is rich in calcium and silicon, the treatment idea of cascade separation and co-production of white carbon black, nano calcium carbonate and calcium sulfate whisker is considered. The calcium component can be extracted by acid dissolution to prepare calcium-based products, and the insoluble silicon component can be prepared into silicon-based products by alkali dissolution. In terms of calcium-based products, the calcium salt solution obtained by acid dissolution can be reacted with sulfuric acid to prepare calcium sulfate whisker, which is a functional filler with high aspect ratio and is widely used in the fields of plastics, rubber, coatings, etc. In terms of silicon-based products, the silicon slag after acid dissolution can be reacted with alkali solution to generate sodium silicate solution, which can then be carbonated by passing carbon dioxide to prepare white carbon black. The sodium carbonate mother liquor produced in the carbonation process can also be used for causticization reaction with lime milk to regenerate alkali solution for recycling and co-produce nano calcium carbonate. However, based on the above idea, further research has found that there are still the following problems.

[0006] First, the acid dissolution process consumes a large amount of acid, and the alkali dissolution process consumes a large amount of alkali. Existing processes mostly adopt an open-loop production mode, where acid and alkali media are discharged or simply treated after use, making it impossible to achieve closed-loop recycling. This results in high production costs and generates a large amount of acidic or alkali-containing wastewater, increasing the burden on environmental protection.

[0007] Secondly, the preparation of calcium sulfate whiskers typically employs atmospheric pressure acidification or hydrothermal methods, requiring reactions to be carried out at relatively high temperatures to obtain optimal whisker morphology. Hydrothermal methods are generally conducted in closed reactors, with reaction temperatures reaching 95 to 105 degrees Celsius and pressures ranging from 0.1 to 0.8 MPa gauge pressure. After the reaction, traditional plate and frame filter presses and other filtration equipment cannot withstand the high temperature and pressure conditions. The material inside the reactor must first be cooled and depressurized to room temperature and pressure before solid-liquid separation can proceed. This cooling and depressurization process is time-consuming, leading to extended production cycles, reduced efficiency, and significant heat loss, thus increasing energy consumption. Furthermore, traditional plate and frame filter presses have low filtration accuracy, resulting in high moisture content in the filter cake and high energy consumption for subsequent drying, which also affects product quality.

[0008] Third, in the preparation of precipitated silica, the alkali dissolution process requires reacting silica slag with sodium hydroxide solution at a relatively high temperature to generate sodium silicate solution. The reaction temperature is typically 70 to 90 degrees Celsius, and the solution is strongly alkaline. The resulting sodium silicate solution contains a small amount of undissolved impurity particles, which need to be removed by filtration. However, conventional PA / PE plastic filter cartridges cannot withstand this high-temperature, strongly alkaline environment, are prone to aging and deformation, have a short service life, and require frequent replacement, increasing operating costs. To avoid damage to the filter cartridges, existing processes often require cooling the sodium silicate solution to a lower temperature before filtration, which not only increases the energy consumption for cooling and reheating but also makes the process more complex.

[0009] Fourth, the above-mentioned treatment approach generates various mother liquors, including sodium carbonate mother liquor from the carbonation process, alkali liquor from the causticization process, and sodium sulfate mother liquor from the precipitated silica neutralization process. Currently, the recycling rate of these mother liquors is low. Sodium carbonate mother liquor and alkali liquor are not fully recovered and reused, and sodium sulfate mother liquor is often directly treated as wastewater rather than recycled as a byproduct. This leads to the loss of valuable resources, low byproduct recovery rates, and a heavy wastewater treatment load and significant environmental pressure. Summary of the Invention

[0010] To address the shortcomings of existing technologies, this disclosure provides a method for the cascade separation and co-production of silica, nano-calcium carbonate, and calcium sulfate whiskers based on phosphorus chemical waste residue. This method achieves the cascade separation and high-value utilization of calcium and silicon components in phosphorus chemical waste residue, while simultaneously realizing dual independent closed-loop circulation of acid and alkali media, thereby reducing production costs and wastewater discharge.

[0011] In a first aspect, this disclosure provides a method for the cascade separation and co-production of silica, nano-calcium carbonate, and calcium sulfate whiskers based on phosphorus chemical waste residue, comprising: Step 1, acid hydrolysis of phosphorus slag powder with nitric acid, followed by solid-liquid separation to obtain an acid hydrolysate containing calcium nitrate and an activated silica slag filter cake; Step 2, purification and impurity removal of the acid hydrolysate, followed by reaction with sulfuric acid in a metal filter cartridge thickener to generate calcium sulfate whiskers, and separation of regenerated nitric acid for recycling back to Step 1; Step 3, alkaline dissolution reaction of the activated silica slag filter cake with sodium hydroxide solution to generate sodium silicate solution. The solution is filtered through a high-temperature and corrosion-resistant precision filter and then reacted with carbon dioxide to produce silica and sodium carbonate mother liquor. In step four, the sodium carbonate mother liquor is concentrated and then reacted with lime milk to produce nano-calcium carbonate and regenerated sodium hydroxide solution. The regenerated sodium hydroxide solution is then returned to step three for recycling after fine filtration. In step five, the silica filter cake obtained from the carbonation separation in step three is neutralized by adding sulfuric acid to convert the residual sodium bicarbonate into sodium sulfate. After neutralization, solid-liquid separation is performed to obtain silica product and sodium sulfate mother liquor.

[0012] Furthermore, before step one, there is also a phosphorus slag powdering process: the molten slag from the electric furnace for yellow phosphorus production is fed into a vertical mill for grinding, and after being classified by a classifier, phosphorus slag powder with a fineness of 90% less than 200 mesh is obtained. The phosphorus slag powder is then pneumatically conveyed to the phosphorus slag intermediate silo for later use.

[0013] Furthermore, in step one, the acid hydrolysis reaction temperature is 70 to 90 degrees Celsius, and the final pH is controlled between 1 and 2; the activated silica slag filter cake obtained after solid-liquid separation after acid hydrolysis needs to be washed three times countercurrently with purified calcium solution and concentrated condensate until neutral before proceeding to step three.

[0014] Further, in step two, the purification and impurity removal process of the acid hydrolysate is as follows: lime milk is added to the acid hydrolysate in steps to first adjust the pH to above 3 to remove phosphate, iron, aluminum, fluorine and fluorosilicate impurities, and then adjust the pH to 7 to 9 to remove magnesium impurities; the purified liquid is concentrated by MVR evaporation to a calcium nitrate mass fraction of 20% to 30%; the neutralized residue filter cake generated after impurity removal is dried and transported for disposal.

[0015] Further, in step two, the metal filter element filtration concentrator is equipped with a filter element made of titanium-based alloy or 2205 duplex stainless steel and an ultrasonic transducer; the whisker preparation specifically involves: reacting the concentrated calcium nitrate solution with sulfuric acid at 95 to 105 degrees Celsius and a gauge pressure of 0.1 to 0.8 MPa, controlling the pH at 6.0, the stirring rate at 120 to 280 rpm, and the reaction time at 2 to 8 hours; after the reaction slurry passes through a conversion aging tank, a conversion temperature regulating tank, and a conversion thickening tank in sequence, it is directly separated into solid and liquid by passing through the metal filter element under high temperature and high pressure; the separated high-temperature regenerated nitric acid is returned directly to step one for acid dissolution of phosphorus slag without cooling.

[0016] Furthermore, step two also includes in-situ modification and cleaning processes: after solid-liquid separation is completed in the metal filter element filtration concentrator, the whiskers are washed three times in countercurrent using purified calcium solution concentrated condensate; after washing, stearic acid is directly added into the machine as a modifier for wet modification; during filtration or modification, the metal filter element is regenerated and cleaned online using the ultrasonic transducer to prevent the whiskers from scaling and clogging; the modified calcium sulfate whiskers are dried, pneumatically conveyed to the product silo, and then packaged for sale.

[0017] Furthermore, in step three, the alkaline dissolution reaction temperature is 70 to 90 degrees Celsius, and the modulus of the generated sodium silicate solution is 2.5 to 3.5; the high-temperature and corrosion-resistant precision filtration device is internally equipped with a silicon carbide ceramic membrane filter element or a metal filter element, which directly removes trace suspended impurities in the sodium silicate solution under a high-temperature and strong alkaline environment of 70 to 90 degrees Celsius; the alkali-insoluble filter cake generated after alkaline dissolution is dried and sold as neutralization residue.

[0018] Furthermore, in step three, the carbonization reaction is carried out using the carbon dioxide-containing waste flue gas generated from the combustion of yellow phosphorus tail gas; the white carbon black filter cake generated by the reaction is washed with the condensate water concentrated from the sodium carbonate mother liquor and then sent to step five; the separated sodium carbonate mother liquor is sent to step four.

[0019] Furthermore, in step four, the nano-calcium carbonate filter cake generated by the causticization reaction needs to undergo a re-slurry neutralization process: the filter cake is reacted with carbon dioxide in a re-slurry neutralization tank to convert the residual alkali solution into sodium carbonate, and the sodium carbonate filtrate after the reaction is returned to the causticization process as filter cake washing water; the neutralized calcium carbonate filter cake is dried by hollow paddle drying and rotary kiln drying to obtain nano-calcium carbonate product.

[0020] Furthermore, in step five, the sodium sulfate mother liquor obtained after solid-liquid separation after neutralization is concentrated and crystallized by MVR to obtain sodium sulfate by-product for sale; the condensate generated during concentration is recycled as process water; the neutralization washing filtrate is returned to the neutralization tank for recycling; the method realizes dual independent closed-loop circulation of acid and alkali media, wherein the acid circulation is that the nitric acid-containing filtrate separated in step two is mixed with the washing water and returned to step one for acid dissolution of phosphorus slag, and the alkali circulation is that the regenerated sodium hydroxide solution separated in step four is filtered and clarified by expanded polytetrafluoroethylene membrane and returned to step three for alkali dissolution of silicon slag.

[0021] Secondly, this disclosure provides an apparatus for the continuous preparation and modification of calcium sulfate whiskers using a metal filter cartridge concentrator, which can be used in step two of the above-mentioned method for the co-production of silica, nano-calcium carbonate and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue.

[0022] The device includes a reaction vessel and a metal filter cartridge filtration concentrator connected thereto; the metal filter cartridge filtration concentrator includes a pressure-resistant tank, a metal filter cartridge disposed inside the tank, a stirring device disposed inside the tank, and an ultrasonic transducer disposed outside the tank; the metal filter cartridge filtration concentrator is configured to directly receive the reaction slurry from the reaction vessel and perform continuous solid-liquid separation, washing, and modification operations.

[0023] Furthermore, the metal filter element is made of titanium-based alloy or 2205 duplex stainless steel; the metal filter element filtration concentrator is also equipped with a back pressure regulating valve for the clear liquid outlet, which is used to control the filtration pressure difference of the filter element by adjusting the back pressure of the clear liquid outlet; when the metal filter element is made of titanium-based alloy, the filtration pressure difference is controlled within 1 MPa; when the metal filter element is made of 2205 duplex stainless steel, the filtration pressure difference is controlled within 3 MPa.

[0024] Furthermore, the ultrasonic transducer is installed on the outer wall of the pressure tank and is configured to regenerate and clean the metal filter element online through ultrasonic vibration, so that the calcium sulfate whisker scale on the surface and inside of the filter element falls off, and at the same time, the equipment is deeply cleaned.

[0025] Furthermore, it also includes a conversion aging tank, a conversion temperature regulating tank, and a conversion thickening tank disposed between the reactor and the metal filter element filtration and thickening machine, for aging, cooling, and thickening treatment of the reaction slurry.

[0026] This disclosure also provides a method for continuously preparing and modifying calcium sulfate whiskers using a metal filter cartridge concentrator, utilizing the aforementioned apparatus for continuously preparing and modifying calcium sulfate whiskers using a metal filter cartridge concentrator. The method includes: Step A, reacting a solution of calcium nitrate with a mass fraction of 20% to 30% with sulfuric acid with a mass fraction of 98% in a reactor to generate a calcium sulfate whisker slurry; Step B, directly feeding the reaction slurry from Step A into the metal filter cartridge concentrator for solid-liquid separation and washing while maintaining the reaction temperature and pressure; Step C, after washing in Step B, adding a modifier to the metal filter cartridge concentrator for surface modification.

[0027] Further, in step A, the reaction temperature is 95 to 105 degrees Celsius, the reaction pressure is 0.1 to 0.8 MPa gauge pressure, the reaction pH is controlled at 6.0, the stirring rate is 120 to 280 revolutions per minute, and the reaction time is 2 to 8 hours; the reaction vessel is a closed reaction vessel.

[0028] Furthermore, in step B, the washing process is a continuous countercurrent washing: the washing liquid is added while stirring in the metal filter element filtration concentrator, and the clear liquid is continuously discharged through the metal filter element to achieve a three-stage countercurrent washing effect; the washing liquid is purified calcium solution concentrated condensate or pure water.

[0029] Further, in step C, the modifier is stearic acid; the modification process is carried out under closed conditions; for products that require ethanol replacement, ethanol is used for replacement washing after modification; the modified slurry is then packaged after solid-liquid separation, drying, and pneumatic conveying to the product silo to obtain calcium sulfate whisker products.

[0030] Furthermore, during the filtration or modification process, ultrasonic transducers are used to perform intermittent or continuous online cleaning of the metal filter element, which can complete filter element regeneration and deep cleaning of the equipment without stopping the equipment operation.

[0031] Furthermore, the clear liquid discharged in step B is a nitric acid-containing filtrate, which is kept at a high temperature and directly returned to the upstream process for acid dissolution of phosphorus residue.

[0032] Thirdly, this disclosure provides a high-temperature and corrosion-resistant precision filtration and mother liquor recycling system for the production process of silica, which can be used in steps three to five of the above-mentioned method for the co-production of silica, nano-calcium carbonate and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue.

[0033] It includes an alkali dissolution reaction unit, an alkali dissolution fine filtration unit, a carbonization reaction unit, a carbonization pressure filtration unit, a carbonization fine filtration unit, a silica neutralization unit, and a neutralization fine filtration unit connected in sequence; at least one of the alkali dissolution fine filtration unit and the carbonization fine filtration unit adopts a special filtration device; the special filtration device is internally equipped with a silicon carbide ceramic membrane filter element or a metal filter element.

[0034] Furthermore, the alkaline dissolution and fine filtration unit is located after the alkaline dissolution reaction unit and before the carbonization reaction unit, and is used to precisely filter the alkaline sodium silicate solution at a temperature of 70 to 90 degrees Celsius to remove trace suspended impurities; the slurry outlet of the alkaline dissolution and fine filtration unit is connected to an alkaline insoluble drying device.

[0035] Furthermore, the carbonization fine filtration unit is located after the carbonization pressure filtration unit and is used to precisely filter suspended impurities in the sodium carbonate mother liquor after carbonization pressure filtration; the fine filtration slurry from the carbonization fine filtration unit is returned to the carbonization pressure filtration unit.

[0036] Furthermore, the neutralization and fine filtration unit is located after the silica neutralization unit and is used to filter the neutralized sodium sulfate mother liquor; the neutralization and fine filtration unit adopts the special filtration device or a conventional filtration device.

[0037] Furthermore, it also includes a sodium carbonate MVR concentration unit connected to the carbonation and fine filtration unit; the sodium carbonate MVR concentration unit is used to combine the sodium carbonate mother liquor, sodium carbonate washing liquid and causticized filter cake washing liquid after carbonation and fine filtration and then evaporate and concentrate them.

[0038] Furthermore, it also includes a causticizing unit, a causticizing pressure filtration unit, and a causticizing fine filtration unit connected in sequence; the concentrate outlet of the sodium carbonate MVR concentration unit is connected to the causticizing unit; the causticizing unit is also connected to a lime digestion unit for providing lime slurry.

[0039] Furthermore, the causticizing and fine filtration unit adopts an expanded polytetrafluoroethylene membrane filtration device for precise filtration of the causticized pressure filtration solution; the fine filtration solution outlet of the causticizing and fine filtration unit is connected to the alkali dissolution reaction unit to form a closed-loop circulation of the alkali solution.

[0040] Furthermore, it also includes a calcium carbonate re-slurry neutralization unit and a calcium carbonate modified pressure filter unit connected to the causticizing pressure filter unit; the solid phase outlet of the causticizing pressure filter unit is connected to the calcium carbonate re-slurry neutralization unit; the calcium carbonate re-slurry neutralization unit is provided with a carbon dioxide inlet; the filtrate outlet of the calcium carbonate modified pressure filter unit is connected to the causticizing unit for filter cake washing water reuse.

[0041] Furthermore, it also includes a calcium carbonate drying unit connected to the calcium carbonate modified pressure filter unit; the calcium carbonate drying unit includes a hollow paddle dryer and a rotary kiln dryer connected in sequence.

[0042] Furthermore, it also includes a membrane concentration unit and a sodium sulfate MVR evaporation crystallization unit connected to the neutralization and fine filtration unit; the membrane concentration unit is used to pre-concentrate the sodium sulfate mother liquor after neutralization and fine filtration; the sodium sulfate MVR evaporation crystallization unit is used to evaporate and crystallize the pre-concentrated sodium sulfate solution to obtain sodium sulfate product; the condensate generated by the membrane concentration unit and the sodium sulfate MVR evaporation crystallization unit is recycled as process water.

[0043] The beneficial effects of this disclosure are: First, it has achieved the tiered separation and high-value utilization of phosphorus chemical waste residue. By acid hydrolysis, the calcium and silicon components in the phosphorus residue are separated to prepare three high-value-added products: calcium sulfate whiskers, silica, and nano-calcium carbonate, which significantly improves the resource utilization efficiency and economic value of the waste residue.

[0044] Secondly, it achieves dual independent closed-loop circulation of acid and alkali media. Nitric acid is regenerated through the calcium sulfate whisker preparation process and then returned to the phosphorus slag acid dissolution process for recycling. Sodium hydroxide is regenerated through the causticization reaction and then returned to the silicon slag alkali dissolution process for recycling, which significantly reduces acid and alkali consumption and production costs, while also reducing wastewater discharge.

[0045] Third, a metal cartridge filter thickener is used to achieve continuous solid-liquid separation, washing, and modification operations under high temperature and high pressure conditions. The reaction slurry is directly fed into the metal cartridge filter thickener for solid-liquid separation while maintaining the reaction temperature and pressure, eliminating the need for cooling and depressurization. This significantly shortens process time, reduces heat loss, and improves production efficiency and product quality. The high filtration precision of the metal cartridge allows for three countercurrent washing processes and in-situ wet modification within a single unit, simplifying the process flow.

[0046] Fourth, special filtration devices that use silicon carbide ceramic membrane filter elements or metal filter elements to replace traditional PA / PE plastic filter elements can directly perform precision filtration of sodium silicate solutions in a high-temperature and strong alkaline environment of 70 to 90 degrees Celsius, removing trace suspended impurities without the need for cooling treatment, thus reducing energy consumption and extending the service life of the filter element.

[0047] Fifth, the online regeneration and cleaning of the metal filter element is achieved through ultrasonic transducers, which removes the calcium sulfate whisker scale from the surface and inside of the filter element. Filter element regeneration and deep cleaning of the equipment can be completed without stopping the equipment, ensuring the continuous and stable operation of the equipment.

[0048] Sixth, the mother liquor is fully recycled. The sodium carbonate mother liquor, sodium carbonate washing liquid, and causticizing filter cake washing liquid are combined and concentrated by evaporation in the sodium carbonate MVR concentration unit before being used for the causticizing reaction. The fine alkali solution from the causticizing fine filtration unit is returned to the alkali dissolution reaction unit to form a closed-loop alkali solution cycle. The sodium sulfate mother liquor is pre-concentrated by the membrane concentration unit and evaporated and crystallized by the sodium sulfate MVR evaporation crystallization unit to obtain sodium sulfate product. The condensate from each process is recycled as process water, achieving near-zero wastewater discharge.

[0049] The present disclosure will now be further described in conjunction with the accompanying drawings and specific embodiments. Additional aspects and advantages provided by the present disclosure will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice. Attached Figure Description

[0050] The accompanying drawings, which form part of this specification, are used to aid in understanding this disclosure. The contents provided in the drawings and their related descriptions in this specification may be used to interpret this disclosure, but do not constitute an undue limitation of this disclosure.

[0051] Figure 1 This is a process flow diagram of a calcium sulfate whisker production line in an embodiment of a method for the co-production of precipitated silica, nano-calcium carbonate and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste. Figure 2 This is a process flow diagram of a silica production line in an embodiment of a method for the co-production of silica, nano-calcium carbonate and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste. Figure 3This is a process flow diagram of the causticization and nano-calcium carbonate production line in an embodiment of a method for the co-production of precipitated silica, nano-calcium carbonate and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste. Figure 4 This is a schematic diagram of the external structure of a metal filter cartridge filtration concentrator in an embodiment of a method for the cascade separation and co-production of silica, nano-calcium carbonate and calcium sulfate whiskers based on phosphorus chemical waste residue disclosed herein. Figure 5 This is an exploded view of the internal structure of a metal filter cartridge filtration concentrator, as described in an embodiment of a method for the cascade separation and co-production of silica, nano-calcium carbonate, and calcium sulfate whiskers based on phosphorus chemical waste residue.

[0052] The diagram is labeled as: ultrasonic transducer 1, protective casing 2. Detailed Implementation

[0053] The present disclosure will now be clearly and completely described in conjunction with the accompanying drawings. Those skilled in the art will be able to implement the present disclosure based on these descriptions. Before describing the present disclosure in conjunction with the accompanying drawings, it should be particularly noted that: The technical solutions and features provided in the various sections, including the following description, can be combined with each other without conflict. Furthermore, where possible, these technical solutions, features, and related combinations can be given specific technical subject matter and protected by relevant patents.

[0054] The embodiments described below are generally only some embodiments and not all embodiments. All other embodiments obtained by those skilled in the art based on these embodiments without creative effort should fall within the scope of patent protection.

[0055] The terms "comprising," "including," "having," and any variations thereof in this specification, the corresponding claims, and related sections are intended to cover non-exclusive inclusion. Other related terms and units can be reasonably interpreted based on the relevant content provided in this specification.

[0056] Example 1: A method for the co-production of silica, nano-calcium carbonate and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue.

[0057] This embodiment provides a method for the cascade separation and co-production of silica, nano-calcium carbonate, and calcium sulfate whiskers based on phosphorus chemical waste residue, combined with... Figure 1 , Figure 2 and Figure 3 The process flow shown is explained.

[0058] I. Phosphate Slag Powdering Process See Figure 1In the upper right section, molten slag from the electric furnace used in yellow phosphorus production is temporarily stored in a slag yard via conveyor belt. During production, the phosphorus slag is conveyed by conveyor belt into a vertical mill grinding system for grinding. The vertical mill uses hot air circulation drying. The ground phosphorus slag powder is classified by a classifier. Qualified fine powder (90% fineness less than 200 mesh) is sent to the phosphorus slag powder silo for storage and later use via a pneumatic conveying system, while coarse powder is returned to the vertical mill for further grinding. The dust-laden exhaust gas generated by the vertical mill system is treated by an exhaust gas dust removal device before being discharged in compliance with standards.

[0059] II. Acid dissolution process See Figure 1 In the upper left section, phosphorus slag powder is metered and conveyed from the phosphorus slag powder silo to the acid dissolution reaction tank. Simultaneously, circulating dilute nitric acid (heated by a plate heater) from the calcium sulfate whisker production process undergoes an acidolysis reaction with the phosphorus slag powder in the acid dissolution reaction tank. Steam is introduced for heating during the reaction, controlling the reaction temperature at 70 to 90 degrees Celsius, and the final pH is controlled at 1 to 2.

[0060] The chemical equation for the acidolysis reaction is:

[0061] After acidolysis, the reaction slurry undergoes solid-liquid separation through pressure filtration and washing. The filtrate, containing calcium nitrate, is sent to the neutralization and impurity removal process; the filter cake, consisting of activated silica slag, is subjected to three countercurrent washes with condensate from the calcium solution purification and concentration process until neutral, and then sent to the alkali dissolution process of the silica production line.

[0062] III. Purification and Impurity Removal Process See Figure 1 In the middle section, the acid hydrolysate enters the neutralization and impurity removal system. Coarse lime slurry from the lime digestion system is added to the acid hydrolysate in stages: The first step is to adjust the pH to above 3 to precipitate and remove impurities such as phosphate, iron, aluminum, fluorine, and fluorosilicate. The second step is to continue adding lime milk and adjust the pH to 7 to 9, so that magnesium ions are removed by precipitation in the form of magnesium hydroxide.

[0063] Purchased quicklime is fed into a lime digestion tank, where washing water is added for digestion. The resulting coarse lime slurry is used for neutralization and impurity removal, while the lime slag is transported off-site for treatment. The refined lime slurry is sent to the causticization process.

[0064] After impurity removal, the slurry undergoes solid-liquid separation via pressure filtration and washing. The filter cake, a neutralization residue, is dried and transported off-site for processing. The filtrate, a purified calcium solution, is concentrated to a calcium nitrate mass fraction of 20% to 30% via MVR evaporation. The condensate generated during MVR evaporation is returned to the system for reuse. The concentrated calcium nitrate solution is further heated by a plate heater before being sent to the calcium sulfate whisker conversion process.

[0065] IV. Preparation Process of Calcium Sulfate Whiskers SeeFigure 1 In the lower part, the concentrated and heated calcium nitrate solution reacts with 98% concentrated sulfuric acid in a calcium sulfate whisker conversion reactor. The reaction conditions are controlled as follows: temperature 95 to 105 degrees Celsius, pressure 0.1 to 0.8 MPa gauge pressure, pH controlled at 6.0, stirring speed 120 to 280 rpm, and reaction time 2 to 8 hours.

[0066] The chemical equation for the conversion reaction is:

[0067] The slurry generated from the reaction is successively passed through a conversion aging tank for crystal aging, a conversion temperature regulating tank for temperature adjustment, and a conversion thickening tank (marked as the upstream process of the conversion thickening tank in the figure) for concentration adjustment. Then, under the condition of maintaining high temperature and high pressure, it is directly sent to a metal filter element filter thickener for solid-liquid separation.

[0068] The separated high-temperature regenerated dilute nitric acid is returned directly to the acid dissolution process through a circulation pipeline without cooling, and is used for the acid hydrolysis of phosphorus slag, thus realizing a closed-loop circulation of the acid medium.

[0069] V. Calcium Sulfate Whisker Modification Process After solid-liquid separation is completed in the metal filter cartridge filtration concentrator, the whisker filter cake is subjected to three countercurrent washes using purified calcium solution and concentrated condensate to remove residual nitric acid and soluble impurities. After washing, stearic acid is added directly to the metal filter cartridge filtration concentrator as a modifier, and wet surface modification is carried out under closed high-temperature conditions.

[0070] During the filtration and modification process, ultrasonic transducers installed on the outer wall of the metal filter element filtration concentrator tank are used to perform intermittent or continuous online regeneration cleaning of the metal filter element, causing the calcium sulfate whisker scale on and inside the filter element to fall off, preventing filter element blockage, and ensuring continuous and stable operation of the equipment.

[0071] After solid-liquid separation and drying, the modified calcium sulfate whisker slurry is sent to the product silo through a pneumatic conveying system, and after packaging, the calcium sulfate whisker product is sold externally.

[0072] VI. Alkali dissolution process See Figure 2 In the upper part, the activated silica slag filter cake from the acid dissolution process and the circulating alkaline solution from the causticizing process undergo an alkaline dissolution reaction in an alkaline dissolution reaction tank. Steam is introduced for heating during the reaction, and the reaction temperature is controlled at 70 to 90 degrees Celsius.

[0073] The chemical equation for the alkali dissolution reaction is:

[0074] The reaction conditions were controlled so that the modulus (molar ratio of SiO2 to Na2O) of the generated sodium silicate solution was 2.5 to 3.5.

[0075] After alkali dissolution, the reaction slurry undergoes alkali dissolution pressure filtration to achieve solid-liquid separation. The alkali-insoluble residue filter cake is flash-dried and then packaged and sold together with the neutralization residue; the filtrate, a sodium silicate solution containing trace amounts of suspended impurities, is sent to the alkali dissolution fine filtration process.

[0076] The filtrate from alkali-soluble pressure filtration undergoes precision filtration via an alkali-soluble fine filtration unit. This unit is equipped with a silicon carbide ceramic membrane filter or a metal filter element, enabling direct precision filtration of sodium silicate solution at a high temperature and strong alkaline environment of 70 to 90 degrees Celsius, removing trace suspended impurities without the need for cooling. The clarified liquid after fine filtration is sent to the carbonization process, while the thickened slurry is returned to the alkali-soluble pressure filtration process.

[0077] VII. Carbonization Process See Figure 2 In the middle section, the finely filtered sodium silicate solution enters the carbonization reaction tank. Simultaneously, carbon dioxide-containing waste gas generated from the combustion of yellow phosphorus tail gas is used as a carbonizing agent and introduced into the carbonization reaction tank. The carbonization reaction proceeds in the presence of seed crystals, which are provided by a seed crystal preparation tank. The seed crystal preparation tank utilizes condensate and carbon dioxide to pre-prepare carbonized seed crystals.

[0078] The chemical equation for the carbonization reaction is:

[0079] As the reaction proceeds, sodium bicarbonate is further produced:

[0080] After carbonization, the reaction slurry undergoes solid-liquid separation via carbonization pressure filtration. The filter cake, a silica filter cake (containing residual sodium bicarbonate), is washed with condensate from the concentration of sodium carbonate mother liquor and then sent to the silica neutralization process; the filtrate is sodium carbonate mother liquor.

[0081] The filtrate from carbonation pressure filtration undergoes precision filtration in a carbonation filtrate fine filtration unit to remove residual suspended impurities. The clarified liquid after fine filtration is sent to the sodium carbonate MVR concentration process, while the finely filtered thick slurry is returned to the carbonation pressure filtration process. The carbonation washing liquid is also sent to the sodium carbonate MVR concentration process after carbonation washing liquid fine filtration.

[0082] VIII. Neutralization and Drying Process of Silica See Figure 2 In the lower right section, the silica filter cake from the carbonization process enters the silica neutralization tank. Sulfuric acid is added to the neutralization tank to convert the residual sodium bicarbonate in the silica filter cake into sodium sulfate.

[0083] The neutralized slurry undergoes solid-liquid separation via neutralization and pressure filtration with silica. The silica filter cake is then washed and dried using either spray drying or fluidized bed drying methods, depending on product specifications. The dried silica is then packaged and sold as finished silica products.

[0084] The filtrate from neutralization and pressure filtration is then filtered through a fine filtration process using silica to obtain sodium sulfate mother liquor. This mother liquor is then concentrated and crystallized using a sodium sulfate MVR evaporation process to obtain sodium sulfate as a byproduct, which is sold externally. The condensate from the MVR evaporation is recycled as process water.

[0085] IX. Causticizing Process See Figure 3 The sodium carbonate concentrate from the sodium carbonate MVR concentration process is first precisely filtered by a sodium carbonate fine filtration device to remove suspended impurities before entering the causticization reaction tank.

[0086] Meanwhile, quicklime is digested with water in a lime digester to produce refined lime milk, which is then sent to a causticizing reaction tank to undergo a causticizing reaction with a sodium carbonate solution.

[0087] The chemical equation for the causticizing reaction is:

[0088] After causticization, the reaction slurry undergoes solid-liquid separation via causticization pressure filtration. The filter cake is a calcium carbonate filter cake, which is sent to the calcium carbonate modification process; the filtrate is a regenerated alkali solution.

[0089] The filtrate produced by causticizing pressure filtration still contains a small amount of suspended impurities, which are then precisely filtered by a causticizing fine filtration unit. This fine filtration unit uses an expanded polytetrafluoroethylene (ePTFE) membrane filter, which effectively removes fine calcium carbonate particles from the alkaline solution. The clarified liquid (regenerated sodium hydroxide solution) after fine filtration is returned to the alkali dissolution process for recycling, achieving a closed-loop circulation of the alkaline medium. The thick slurry from fine filtration is returned to the causticizing pressure filtration process.

[0090] 10. Modification and Drying Process of Nano-Calcium Carbonate See Figure 3 In the lower section, the calcium carbonate filter cake from the causticizing and pressure filtration process enters the re-slurry neutralization tank. Carbon dioxide is introduced into the modification tank to neutralize the residual alkali in the calcium carbonate filter cake and convert it into sodium carbonate.

[0091] Meanwhile, calcium carbonate is re-slurryed using the filtrate (from the sodium carbonate filtration process) to improve its dispersibility and surface properties.

[0092] The neutralized slurry is then subjected to calcium carbonate-modified pressure filtration to achieve solid-liquid separation. The filtrate (containing sodium carbonate) is returned to the causticizing reaction tank and recycled as filter cake washing water; the filter cake is modified calcium carbonate.

[0093] The filtrate produced by the modified pressure filtration is then subjected to calcium carbonate modified fine filtration. The fine filtrate is returned to the re-slurry neutralization tank for recycling, while the finely filtered thick slurry is returned to the calcium carbonate modified pressure filtration process.

[0094] The neutralized calcium carbonate filter cake is then processed through a calcium carbonate drying process. This process includes preliminary drying and dehydration using a hollow paddle dryer and deep drying using a rotary kiln dryer. The dried nano-calcium carbonate is then packaged to obtain the nano-calcium carbonate product for sale.

[0095] The method in this embodiment, based on the cascade separation of phosphorus chemical waste to co-produce precipitated silica, nano-calcium carbonate, and calcium sulfate whiskers, achieves dual independent closed-loop circulation of acid and alkaline media. Acid cycle: The nitric acid-containing filtrate separated by the metal filter cartridge thickener in step four is mixed with washing water and then returned directly to step two for acid dissolution of phosphorus slag while maintaining a high temperature. Since nitric acid is released during the reaction of sulfuric acid and calcium nitrate to form calcium sulfate whiskers, the nitric acid is regenerated during the cycle, and only a small amount of loss needs to be replenished to maintain the acid dissolution reaction.

[0096] Alkali cycle: The regenerated sodium hydroxide solution separated by causticization filtration in step nine is clarified by filtration through an expanded polytetrafluoroethylene membrane and then returned to step six for alkali dissolution of silicon slag. Since sodium hydroxide is released during the causticization reaction as sodium carbonate reacts with calcium hydroxide to form calcium carbonate, sodium hydroxide is regenerated during the cycle, requiring only a small amount of replenishment to maintain the alkali dissolution reaction.

[0097] In addition, the condensate generated in each process is recycled as process water, and the washing filtrate is returned to the corresponding process for recycling, achieving near-zero wastewater discharge.

[0098] Example 2: Apparatus and method for continuous preparation and modification of calcium sulfate whiskers using a metal filter cartridge concentrator.

[0099] This embodiment provides an apparatus and method for the continuous preparation and modification of calcium sulfate whiskers using a metal filter cartridge concentrator, which can be used in the calcium sulfate whisker preparation and modification processes described in steps four and five of Embodiment 1. Combined with... Figure 4 and Figure 5 The structure of the equipment shown will be explained.

[0100] See Figure 4 and Figure 5 The apparatus for continuous preparation and modification of calcium sulfate whiskers using a metal filter cartridge concentrator provided in this embodiment includes a reaction vessel and a metal filter cartridge filtration concentrator connected thereto.

[0101] The metal filter cartridge filtration concentrator consists of the following main components: Pressure Tank: The tank body is made of corrosion-resistant and high-temperature-resistant material, with a design pressure of not less than 1.0 MPa, and can withstand high temperatures of 95 to 105 degrees Celsius and working pressures of 0.1 to 0.8 MPa gauge pressure. The top of the tank is equipped with a removable end cap, which is connected to the tank body via a flange, facilitating the replacement and maintenance of the internal filter element. The sides of the tank have material inlets, washing liquid inlets, and modifier inlets; the bottom has a concentrated slurry outlet; and the lower side has a clear liquid outlet.

[0102] Metal Filter Cartridges: Metal filter cartridges are installed inside the tank, arranged in a cylindrical shape within the annular space between the inner wall of the tank and the central stirring shaft. The material of the metal filter cartridges is selected from titanium-based alloys or 2205 duplex stainless steel. Titanium-based alloy filter cartridges have excellent corrosion resistance and are suitable for strongly acidic environments, with a filtration pressure differential controlled within 1 MPa; 2205 duplex stainless steel filter cartridges have higher mechanical strength, and the filtration pressure differential can be controlled within 3 MPa. The filtration accuracy of the metal filter cartridges can be selected according to product requirements, generally from 1 to 50 microns.

[0103] Agitation device: The agitation device includes an agitator shaft located at the center of the tank and agitator blades mounted on the shaft. The agitator shaft is driven by a drive motor at the top of the tank. The agitation device is used to maintain the slurry in uniform suspension during filtration, washing, and modification processes, prevent sedimentation and agglomeration, and enhance mass and heat transfer.

[0104] Ultrasonic transducer: The ultrasonic transducer is installed on the outer wall of the pressure tank, fitting tightly against the inner wall. The ultrasonic vibrations generated by the transducer are transmitted through the tank wall to the interior of the tank, acting on the surface of the metal filter element. During filtration or modification, activating the ultrasonic transducer causes calcium sulfate whiskers and scale to detach from the surface and internal pores of the filter element, enabling online regeneration and cleaning of the filter element without stopping equipment operation, thus ensuring continuous and stable operation of the equipment.

[0105] Back pressure regulating valve: A back pressure regulating valve is installed on the clear liquid outlet pipeline to control the filtration pressure difference of the filter element by adjusting the back pressure of the clear liquid. The filtration pressure difference refers to the difference between the internal pressure of the tank and the pressure at the clear liquid outlet, which is a key parameter affecting the filtration rate and the moisture content of the filter cake.

[0106] like Figure 5 The diagram shown is an exploded view of the external structure of the metal filter cartridge filtration concentrator in this embodiment, focusing on the arrangement of the ultrasonic auxiliary device. The cylindrical component in the diagram is a pressure-resistant tank with a removable sealing cap at the top. Several ultrasonic transducers 1 are densely arrayed on the outer wall of the pressure-resistant tank. These transducers are directly attached to the tank wall and are used to emit ultrasonic waves into the tank, thereby cleaning the filter cartridge online, preventing filter cartridge clogging, and preventing scale buildup on the tank wall. A protective outer shell 2 is provided around the tank to protect the ultrasonic transducers and shield noise.

[0107] This device also includes a conversion aging tank, a conversion temperature control tank, and a conversion thickening tank, which are connected sequentially between the reaction vessel and the metal filter cartridge thickener to form a slurry pretreatment system. The conversion aging tank is used to age the slurry after the reaction, allowing calcium sulfate whiskers to grow and develop fully; the conversion temperature control tank is used to adjust the slurry temperature to a suitable filtration temperature; and the conversion thickening tank is used to initially thicken the slurry, increasing the concentration of the slurry entering the thickener.

[0108] The method for continuously preparing and modifying calcium sulfate whiskers using the above-mentioned apparatus with a metal filter cartridge concentrator includes the following steps: Step A: Transformation reaction A calcium nitrate solution (calcium nitrate mass fraction of 20% to 30%) from the purification and impurity removal process is pumped into a closed reactor. Simultaneously, concentrated sulfuric acid (98% mass fraction) is added to the reactor according to a stoichiometric ratio.

[0109] The reaction conditions were controlled as follows: temperature 95 to 105 degrees Celsius, pressure 0.1 to 0.8 MPa (gauge pressure), pH 6.0, and stirring speed 120 to 280 rpm. Under these conditions, calcium nitrate reacted with sulfuric acid to produce calcium sulfate whiskers and nitric acid.

[0110] The reaction time is determined based on the required aspect ratio and crystallinity of the whiskers, and is generally 2 to 8 hours. A longer reaction time is beneficial for obtaining whisker products with a high aspect ratio, but it will reduce production efficiency.

[0111] After the reaction is complete, the reaction slurry is sequentially passed through a conversion aging tank for crystal aging (30 to 60 minutes), a conversion temperature-regulating tank for cooling, and a conversion thickening tank for concentration adjustment. Throughout the pretreatment process, the slurry is maintained at a high temperature and high pressure, without any cooling or depressurization treatment.

[0112] Step B: High-temperature and high-pressure filtration separation and washing The pretreated slurry is fed directly into a metal filter cartridge thickener while maintaining the reaction temperature and pressure. The slurry enters from the material inlet on the side of the tank and is kept in uniform suspension by the agitator.

[0113] Open the back pressure regulating valve at the clear liquid outlet to control the appropriate filtration pressure difference. Under the action of the pressure difference, the liquid phase (containing nitric acid) in the slurry passes through the metal filter element and is discharged as clear liquid, while the solid phase (calcium sulfate whiskers) is retained in the tank and gradually concentrated.

[0114] Once the slurry concentration in the tank reaches the set value, feeding is stopped and the washing operation begins. The washing process employs a continuous countercurrent washing method: washing liquid (concentrated condensate from purified calcium solution or pure water) is continuously added from the washing liquid inlet while stirring and filtering are maintained, ensuring that the washing liquid fully contacts the whiskers before being discharged through the filter element. By controlling the balance between the amount of washing liquid added and the amount of clear liquid discharged, the effect is equivalent to three countercurrent washings, effectively removing residual nitric acid and soluble impurities from the surface of the whiskers.

[0115] During filtration and washing, ultrasonic transducers are intermittently activated for online cleaning based on changes in the filter element's permeability. The ultrasonic vibrations cause the whisker-like scale adhering to the filter element's surface and pores to detach, restoring the filter element's filtration performance without requiring equipment shutdown.

[0116] Step C: In-situ wet modification After washing, the whiskers are not removed from the filter concentrator, but are directly subjected to in-situ wet modification.

[0117] Stearic acid is added into the tank as a modifier through the modifier inlet. The amount of stearic acid added is determined according to the amount of whiskers and the required degree of modification, and is generally 0.5% to 3% of the dry weight of the whiskers.

[0118] Under sealed, high-temperature conditions, the stirring device is activated to ensure thorough mixing and contact between stearic acid and the whiskers. Stearic acid molecules adsorb onto the whisker surface, forming a hydrophobic organic coating layer that improves the compatibility between the whiskers and the polymer matrix. The modification time is generally 30 to 60 minutes.

[0119] For special specifications of products that require ethanol replacement, the whiskers are replaced and washed with ethanol after modification to remove excess modifier and moisture.

[0120] Step D: Post-processing After modification, the modified whisker concentrate is discharged from the concentrated slurry outlet at the bottom of the tank. After solid-liquid separation (which can be done by centrifugation or secondary pressure filtration) to remove residual liquid, the concentrate enters the drying system for drying.

[0121] The dried calcium sulfate whiskers are fed into the product silo through a pneumatic conveying system, and after weighing and packaging, the finished calcium sulfate whiskers are sold.

[0122] The clear liquid discharged in step B is a high-temperature nitric acid filtrate. It is kept at a high temperature and directly returned to the acid dissolution process through the circulation pipeline for acid hydrolysis of phosphorus slag, realizing closed-loop circulation of acid medium and heat recovery and utilization.

[0123] The apparatus and method for continuous preparation and modification of calcium sulfate whiskers using a metal filter cartridge concentrator provided in this embodiment have the following technical advantages: First, it enables continuous solid-liquid separation under high temperature and high pressure conditions. The reaction slurry is directly filtered and separated while maintaining a temperature of 95 to 105 degrees Celsius and a gauge pressure of 0.1 to 0.8 MPa, eliminating the need for cooling and depressurization processes. This significantly shortens the process cycle, reduces heat loss, and improves production efficiency.

[0124] Secondly, metal filter elements offer high filtration accuracy, high temperature resistance, and corrosion resistance. Titanium-based alloy or 2205 duplex stainless steel filter elements can operate stably for extended periods in high-temperature and strong acid environments, boasting a long service life and low maintenance costs.

[0125] Third, it integrates filtration, washing, and modification into a single device. The continuous countercurrent washing effect is equivalent to three conventional washes, with high washing efficiency and low water consumption; in-situ wet modification eliminates the need for material transfer, simplifying the process and ensuring stable product quality.

[0126] Fourth, ultrasonic online cleaning technology ensures continuous equipment operation. The vibration generated by the ultrasonic transducer can effectively remove scale from the filter element without stopping the machine for cleaning, thus improving equipment utilization and production continuity.

[0127] Fifth, the high-temperature regenerated nitric acid is directly recycled. The separated nitric acid-containing filtrate is kept at a high temperature and returned to the acid dissolution process, which not only realizes the recycling of the acid medium, but also recovers heat energy, reducing production costs and energy consumption.

[0128] Example 3: High-temperature and corrosion-resistant precision filtration and mother liquor recycling system in the production process of silica.

[0129] This embodiment provides a high-temperature and corrosion-resistant precision filtration and mother liquor recycling system for the production of silica, which can be used in the silica production and nano-calcium carbonate co-production processes described in steps six to ten of Embodiment 1. Combined with... Figure 2 and Figure 3 The process flow shown is explained.

[0130] See Figure 2 and Figure 3 The high-temperature and corrosion-resistant precision filtration and mother liquor recycling system for silica production provided in this embodiment includes the following units: Alkali dissolution reaction unit: located in Figure 2 The upper part includes an alkali-dissolving reaction tank and supporting heating and stirring equipment. The alkali-dissolving reaction tank is equipped with a silicon slag inlet, a circulating alkali inlet, and a steam heating port. The alkali-dissolving reaction unit is used to react activated silicon slag with sodium hydroxide solution at a temperature of 70 to 90 degrees Celsius to produce sodium silicate solution.

[0131] Alkali-soluble pressure filter unit: Located after the alkali-soluble reaction unit, it is used for solid-liquid separation of the slurry after the alkali-soluble reaction. The filtrate produced by the alkali-soluble pressure filter unit is a sodium silicate solution containing trace amounts of suspended impurities, and the filter cake is an alkali-insoluble substance.

[0132] Alkali-soluble fine filtration unit: Located after the alkali-soluble pressure filtration unit and before the carbonization reaction unit. This unit employs a special filtration device, internally equipped with a silicon carbide ceramic membrane filter element or a metal filter element. The silicon carbide ceramic membrane filter element possesses excellent high-temperature and corrosion resistance, enabling direct and precise filtration of sodium silicate solutions in a high-temperature, strongly alkaline environment of 70 to 90 degrees Celsius, removing trace suspended impurities with a filtration accuracy of 0.1 to 1 micrometer. The fine filtrate outlet of the alkali-soluble fine filtration unit is connected to the carbonization reaction unit, while the slurry outlet returns to the alkali-soluble pressure filtration unit. The unit also includes an interface for an alkali-insoluble matter drying device to treat the alkali-insoluble filter cake produced by the alkali-soluble pressure filtration.

[0133] Carbonization reaction unit: located in Figure 2 The central section includes a carbonization reaction tank and associated stirring and gas distribution equipment. The carbonization reaction tank is equipped with an inlet for sodium silicate solution, a gas inlet for carbon dioxide, and a seed crystal inlet. The carbonization reaction unit is also connected to a seed crystal preparation tank, used to pre-prepare carbonization seeds using condensate and carbon dioxide. The carbonization reaction unit is used to react the finely filtered sodium silicate solution with carbon dioxide in the presence of the seed crystals to produce silica and sodium carbonate mother liquor.

[0134] Carbonization and filtration unit: Located after the carbonization reaction unit, it is used for solid-liquid separation of the slurry after the carbonization reaction. The filter cake produced by the carbonization and filtration unit is precipitated silica filter cake, and the filtrate is sodium carbonate mother liquor.

[0135] Carbonization Fine Filtration Unit: Located after the carbonization pressure filter unit. The carbonization fine filtration unit includes a carbonization filtrate fine filtration device and a carbonization washing liquid fine filtration device, used to precisely filter the filtrate produced by carbonization pressure filtration and the washing liquid of carbonization filter cake, respectively, to remove residual suspended silica particles. The fine filtrate from the carbonization fine filtration unit is returned to the carbonization pressure filter unit for recycling.

[0136] Silica neutralization unit: located in Figure 2 The lower right section includes a silica neutralization tank and associated stirring and acid-adding equipment. The silica neutralization unit is used to neutralize the silica filter cake produced by carbonization and pressure filtration with sulfuric acid, converting residual sodium bicarbonate into sodium sulfate.

[0137] The silica neutralization and pressure filtration unit is located after the silica neutralization unit and is used for solid-liquid separation of the neutralized slurry. The filter cake produced by the neutralization and pressure filtration unit is silica product, and the filtrate is sodium sulfate mother liquor.

[0138] Neutralization and fine filtration unit: Located after the silica neutralization and pressure filtration unit, this unit filters the sodium sulfate mother liquor produced during neutralization and pressure filtration to remove residual suspended silica particles. The neutralization and fine filtration unit can employ either specialized or conventional filtration devices.

[0139] Sodium carbonate MVR concentration unit: located in Figure 2 The lower left section connects to the carbonation and fine filtration unit. The sodium carbonate MVR concentration unit is used to combine the sodium carbonate mother liquor, sodium carbonate washing liquid, and causticized filter cake washing liquid after carbonation and fine filtration, and then evaporate and concentrate them. The MVR concentration uses mechanical vapor recompression technology, which is highly energy efficient and has low operating costs. The condensate generated during concentration is recycled as process water.

[0140] Causticizing unit: located in Figure 3 The upper part includes a causticizing reaction tank and supporting stirring equipment. The causticizing reaction tank is equipped with an inlet for sodium carbonate concentrate and an inlet for lime slurry. The concentrate outlet of the sodium carbonate MVR concentration unit is connected to the causticizing unit after sodium carbonate fine filtration. The causticizing unit is also connected to a lime slaking unit, used to slake quicklime to provide lime slurry. The causticizing unit is used to react the concentrated sodium carbonate solution with lime slurry to produce calcium carbonate and regenerated alkali solution.

[0141] Causticization and filtration unit: Located after the causticization unit, it is used for solid-liquid separation of the slurry after the causticization reaction. The filter cake produced by the causticization and filtration unit is calcium carbonate filter cake, and the filtrate is regenerated alkali solution.

[0142] Causticizing and Fine Filtration Unit: Located after the causticizing and pressure filtration unit. This unit uses an expanded polytetrafluoroethylene (ePTFE) membrane filtration device to precisely filter the alkali solution after causticizing and pressure filtration, removing fine suspended calcium carbonate particles. The ePTFE membrane has excellent alkali resistance and filtration accuracy, effectively ensuring the quality of the circulating alkali solution returned to the alkali dissolution process. The finely filtered alkali solution outlet of the causticizing and fine filtration unit is connected to the alkali dissolution reaction unit, forming a closed-loop alkali solution circulation. The finely filtered slurry is returned to the causticizing and pressure filtration unit.

[0143] Calcium carbonate re-slurry neutralization unit: located in Figure 3 The lower section connects to the causticizing filter press unit. The solid phase outlet of the causticizing filter press unit is connected to the calcium carbonate reslurry neutralization unit. The calcium carbonate reslurry neutralization unit includes a reslurry neutralization tank and supporting equipment, and is equipped with a fine filtrate inlet and a carbon dioxide inlet. This unit is used to reslurry the calcium carbonate filter cake and introduce carbon dioxide to neutralize the residual alkali solution and convert it into sodium carbonate.

[0144] Calcium carbonate modified filter press unit: Located after the calcium carbonate reslurry neutralization unit, it is used for solid-liquid separation of the modified calcium carbonate slurry. The filtrate (containing sodium carbonate) outlet of the calcium carbonate modified filter press unit is connected to the causticizing unit for filter cake washing water recycling.

[0145] Calcium carbonate modified fine filtration unit: Located after the calcium carbonate modified pressure filtration unit, it is used for precise filtration of the filtrate produced by modified pressure filtration. The fine filtrate is returned to the calcium carbonate re-slurry neutralization unit for recycling, while the fine filtrate slurry is returned to the calcium carbonate modified pressure filtration unit.

[0146] Calcium carbonate drying unit: Connected to the calcium carbonate modified filter press unit, it is used to dry the modified calcium carbonate filter cake. The calcium carbonate drying unit includes a hollow paddle dryer and a rotary kiln dryer connected in sequence. The hollow paddle dryer is used for preliminary drying and dehydration, while the rotary kiln dryer is used for deep drying. The dried nano-calcium carbonate is then packaged and sold as a product.

[0147] Sodium sulfate treatment unit: Connected to the neutralization and filtration unit, it includes a membrane concentration unit and a sodium sulfate MVR evaporation and crystallization unit. The membrane concentration unit is used to pre-concentrate the sodium sulfate mother liquor after neutralization and filtration, increasing the concentration of the feed solution entering the evaporation and crystallization process and reducing evaporation energy consumption. The sodium sulfate MVR evaporation and crystallization unit is used to evaporate and crystallize the pre-concentrated sodium sulfate solution to obtain sodium sulfate (anhydrous sodium sulfate) product. The condensate generated by both the membrane concentration unit and the sodium sulfate MVR evaporation and crystallization unit is recycled as process water.

[0148] The workflow of the high-temperature and corrosion-resistant precision filtration and mother liquor recycling system in the production of silica provided in this embodiment is as follows: The activated silica slag filter cake undergoes an alkaline dissolution reaction with the circulating alkaline solution in the alkaline dissolution reaction unit to generate a sodium silicate solution.

[0149] After the alkali dissolution reaction, the slurry undergoes solid-liquid separation in the alkali dissolution pressure filtration unit. The filtrate is then precisely filtered in the alkali dissolution fine filtration unit before being sent to the carbonization reaction unit. The alkali dissolution fine filtration uses silicon carbide ceramic membrane filter elements or metal filter elements, which can directly filter under high-temperature and strong alkaline conditions of 70 to 90 degrees Celsius without cooling.

[0150] The filtered sodium silicate solution undergoes a carbonization reaction with carbon dioxide in the carbonization reaction unit to produce silica and sodium carbonate mother liquor.

[0151] After carbonization, the slurry undergoes solid-liquid separation in the carbonization pressure filter unit. The silica filter cake is sent to the silica neutralization unit, and the sodium carbonate mother liquor is sent to the sodium carbonate MVR concentration unit after fine filtration in the carbonization fine filtration unit.

[0152] The silica filter cake undergoes a neutralization reaction with sulfuric acid in the silica neutralization unit, converting residual sodium bicarbonate into sodium sulfate. After neutralization, solid-liquid separation is performed in the silica neutralization pressure filtration unit. The silica filter cake is dried to obtain the product, while the sodium sulfate mother liquor is filtered through the neutralization fine filtration unit and then sent to the sodium sulfate treatment unit.

[0153] After being concentrated by evaporation in the sodium carbonate MVR concentration unit, the sodium carbonate mother liquor is sent to the causticizing unit after being filtered by sodium carbonate, where it undergoes a causticizing reaction with the refined lime milk provided by the lime digestion unit.

[0154] After the causticizing reaction, the slurry undergoes solid-liquid separation in the causticizing pressure filtration unit. The alkaline solution is then finely filtered in the causticizing fine filtration unit (using expanded polytetrafluoroethylene membrane filtration) and returned to the alkali dissolution reaction unit for recycling. The calcium carbonate filter cake is sent to the calcium carbonate re-slurry neutralization unit.

[0155] The calcium carbonate filter cake is neutralized by passing carbon dioxide through a calcium carbonate reslurry neutralization unit, converting the residual alkali solution into sodium carbonate. After neutralization, it is processed by a calcium carbonate modified pressure filter unit and a calcium carbonate modified fine filter unit. Finally, the calcium carbonate filter cake is dried by a calcium carbonate drying unit to obtain nano-calcium carbonate product.

[0156] Sodium sulfate mother liquor was pre-concentrated in a membrane concentration unit and then evaporated and crystallized in a sodium sulfate MVR evaporation and crystallization unit to obtain sodium sulfate by-product.

[0157] The condensate generated in each unit is recycled as process water, and the washing filtrate from each process is returned to the corresponding process for recycling.

[0158] The high-temperature and corrosion-resistant precision filtration and mother liquor recycling system provided in this embodiment for the production of silica has the following technical advantages: First, special filtration devices that use silicon carbide ceramic membrane filter elements or metal filter elements to replace traditional PA / PE plastic filter elements can directly perform precision filtration of sodium silicate solutions in a high-temperature and strong alkaline environment of 70 to 90 degrees Celsius without the need for cooling treatment, thus reducing energy consumption, extending the service life of the filter element, and reducing the frequency of replacement and maintenance costs.

[0159] Secondly, the causticizing and fine filtration unit uses expanded polytetrafluoroethylene membrane filtration, which can effectively remove fine calcium carbonate particles in the alkaline solution, ensure the quality of the circulating alkaline solution returned to the alkaline dissolution process, and avoid the accumulation of impurities that affect product quality.

[0160] Third, a complete closed-loop alkali solution cycle is formed. The regenerated sodium hydroxide solution after causticization and fine filtration is returned to the alkali dissolution reaction unit for recycling. Sodium hydroxide is regenerated in the causticization reaction, and only a small amount of loss needs to be replenished to maintain production operation, which greatly reduces alkali consumption and production costs.

[0161] Fourth, the mother liquor is fully recycled. The sodium carbonate mother liquor, sodium carbonate washing liquid, and causticizing filter cake washing liquid are combined and concentrated by MVR for use in the causticizing reaction. The sodium sulfate mother liquor is concentrated by membrane and then evaporated and crystallized by MVR to obtain sodium sulfate as a byproduct. The condensate from each process is recycled as process water, achieving near-zero wastewater discharge and maximizing the recovery of valuable resources.

[0162] Fifth, multi-stage fine filtration ensures product quality. Multiple precision filtration processes, including alkali dissolution filtration, carbonization filtration, neutralization filtration, and causticization filtration, effectively remove suspended impurities at each stage, ensuring the purity and quality stability of the silica and nano-calcium carbonate products.

[0163] The foregoing has described the relevant content of this disclosure. Those skilled in the art will be able to implement this disclosure based on these descriptions. All other embodiments obtained by those skilled in the art based on the foregoing content of this specification without inventive effort should fall within the scope of this disclosure.

Claims

1. A method for the cascade separation and co-production of silica, nano-calcium carbonate, and calcium sulfate whiskers based on phosphorus chemical waste residue, characterized in that: include: Step 1: The phosphorus slag powder is subjected to acid hydrolysis reaction with nitric acid. After the reaction, solid-liquid separation is performed to obtain an acid hydrolysis solution containing calcium nitrate and an activated silica slag filter cake. Step 2: After purifying and removing impurities from the acid hydrolysate, it is reacted with sulfuric acid in a metal filter cartridge filtration concentrator to generate calcium sulfate whiskers, and the regenerated nitric acid is separated and returned to Step 1 for recycling. Step 3: The activated silica slag filter cake is reacted with sodium hydroxide solution to generate sodium silicate solution. After being filtered by a high-temperature and corrosion-resistant precision filter, the solution is reacted with carbon dioxide to generate fumed silica and sodium carbonate mother liquor. Step four: The sodium carbonate mother liquor is concentrated and then subjected to a causticizing reaction with lime milk to generate nano-calcium carbonate and a regenerated sodium hydroxide solution; the regenerated sodium hydroxide solution is then returned to step three for recycling after fine filtration. Step 5: Neutralize the silica filter cake obtained from carbonization separation in Step 3, add sulfuric acid to convert the residual sodium bicarbonate into sodium sulfate, and then separate the solid and liquid to obtain silica product and sodium sulfate mother liquor.

2. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: Before step one, there is also a phosphorus slag powdering process: the molten slag from the electric furnace for yellow phosphorus production is fed into a vertical mill for grinding, and after being classified by a classifier, phosphorus slag powder with a fineness of 90% less than 200 mesh is obtained. The phosphorus slag powder is then pneumatically conveyed to the phosphorus slag intermediate silo for later use.

3. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step one, the acid hydrolysis reaction temperature is 70 to 90 degrees Celsius, and the final pH is controlled between 1 and 2. The activated silica slag filter cake obtained after solid-liquid separation after acid hydrolysis needs to be washed three times countercurrently with purified calcium solution and concentrated condensate until neutral before proceeding to step three.

4. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step two, the purification and impurity removal process of the acid hydrolysate is as follows: lime milk is added to the acid hydrolysate in steps to first adjust the pH to above 3 to remove impurities such as phosphate, iron, aluminum, fluorine and fluorosilicate, and then adjust the pH to 7 to 9 to remove magnesium impurities; the purified liquid is concentrated by MVR evaporation to a calcium nitrate mass fraction of 20% to 30%; the neutralized residue filter cake generated after impurity removal is dried and transported for disposal.

5. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step two, the metal filter element filtration concentrator is equipped with a filter element made of titanium-based alloy or 2205 duplex stainless steel and an ultrasonic transducer. The whisker preparation specifically involves reacting the concentrated calcium nitrate solution with sulfuric acid at 95 to 105 degrees Celsius and a gauge pressure of 0.1 to 0.8 MPa, controlling the pH at 6.0, the stirring rate at 120 to 280 rpm, and the reaction time at 2 to 8 hours. The reaction slurry is then passed sequentially through a conversion aging tank, a conversion temperature regulating tank, and a conversion thickening tank, and then directly through the metal filter element for solid-liquid separation under high temperature and high pressure. The separated high-temperature regenerated nitric acid is returned directly to step one for phosphate slag acid dissolution without cooling.

6. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 5, characterized in that: Step two also includes in-situ modification and cleaning processes: after solid-liquid separation is completed in the metal filter element filtration concentrator, the whiskers are washed three times in countercurrent using purified calcium solution and concentrated condensate; after washing, stearic acid is directly added into the machine as a modifier for wet modification; during filtration or modification, the metal filter element is regenerated and cleaned online using the ultrasonic transducer to prevent the whiskers from scaling and clogging; the modified calcium sulfate whiskers are dried, pneumatically conveyed to the product silo, and then packaged for sale.

7. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step three, the alkaline dissolution reaction temperature is 70 to 90 degrees Celsius, and the resulting sodium silicate solution has a modulus of 2.5 to 3.

5. The high-temperature and corrosion-resistant precision filtration device is equipped with a silicon carbide ceramic membrane filter element or a metal filter element, which directly removes trace suspended impurities from the sodium silicate solution in a high-temperature and strong alkaline environment of 70 to 90 degrees Celsius. The alkali-insoluble filter cake produced after alkaline dissolution is dried and sold as neutralization residue.

8. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step three, the carbonization reaction is carried out using the carbon dioxide-containing waste flue gas generated from the combustion of yellow phosphorus tail gas; the white carbon black filter cake generated by the reaction is washed with the condensate water concentrated from the sodium carbonate mother liquor and then sent to step five; the separated sodium carbonate mother liquor is sent to step four.

9. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step four, the nano-calcium carbonate filter cake generated by the causticization reaction needs to undergo a re-slurry neutralization process: the filter cake is reacted with carbon dioxide in a re-slurry neutralization tank to convert the residual alkali solution into sodium carbonate, and the sodium carbonate filtrate after the reaction is returned to the causticization process as filter cake washing water; the neutralized calcium carbonate filter cake is dried by hollow paddle drying and rotary kiln drying to obtain nano-calcium carbonate product.

10. The method for co-producing silica, nano-calcium carbonate, and calcium sulfate whiskers based on the cascade separation of phosphorus chemical waste residue according to claim 1, characterized in that: In step five, the sodium sulfate mother liquor obtained after solid-liquid separation after neutralization is concentrated and crystallized by MVR to obtain sodium sulfate by-product for sale; the condensate generated during concentration is recycled as process water; the neutralization washing filtrate is returned to the neutralization tank for recycling; the method realizes dual independent closed-loop circulation of acid and alkali media, wherein the acid circulation is that the nitric acid-containing filtrate separated in step two is mixed with the washing water and returned to step one for acid dissolution of phosphorus slag, and the alkali circulation is that the regenerated sodium hydroxide solution separated in step four is filtered and clarified by expanded polytetrafluoroethylene membrane and returned to step three for alkali dissolution of silicon slag.

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