Method for preparing high-purity white gypsum from salt-containing waste dilute sulphuric acid

Abstract

The invention discloses a method for preparing high-purity white gypsum from salt-containing waste dilute sulphuric acid, and relates to the technical field of salt-containing waste dilute sulphuric acid treatment processes. The method specifically comprises the following steps: 1) waste acid pretreatment: adding a reducing agent into salt-containing waste dilute sulphuric acid; 2) adding a seed crystal into the pretreated waste acid, slowly adding the calcium salt prepared into slurry into the reactor, and growing the crystal after the pH value is controlled to be 1-4; 3) after crystal growing is finished, carrying out primary solid-liquid separation, and washing, filtering and drying a filter cake obtained by separation to obtain white gypsum; (4) slowly adding a precipitator into the filtrate, controlling the pH value to be 4-8, and then reacting and curing to obtain slurry containing phosphate precipitate; and 5) carrying out secondary solid-liquid separation on the slurry obtained after the curing reaction, washing, filtering and drying the separated filter cake to obtain a mixed phosphate filter cake containing iron, aluminum and magnesium for preparing a fireproof coating, iron phosphate or a heavy metal adsorbent, and separating to obtain a filtrate without aluminum ions for a fertilizer system or a phosphorus ore flotation homogenizing section.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of salt-containing waste dilute sulfuric acid treatment process, and particularly relates to a method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid. BACKGROUND

[0002] The solvent extraction method for purifying wet-process phosphoric acid has been widely concerned due to its low energy consumption, mild operating conditions and high purification efficiency. When organic extractant is used to extract and separate the impurity metal ions such as iron, aluminum and magnesium in the wet-process phosphoric acid, a large amount of waste dilute sulfuric acid solution is generated in the stripping process of the extractant loaded with impurity ions. Under normal conditions, 1 t of waste dilute sulfuric acid is generated corresponding to 1 t of wet-process phosphoric acid treated, and the waste dilute sulfuric acid contains 5-25 wt.% of SO4 2- 5-25 wt.% of SO4, 0.5-2.0 wt.% of Fe2O3, 0.1-1.0 wt.% of Al2O3, 0.1-1.0 wt.% of MgO, 0.5-2.0 wt.% of P2O5, and the rest is water. If the waste dilute sulfuric acid is directly treated as hazardous waste, it is not only expensive but also a waste of sulfur resources. At present, the treatment methods for the sulfuric acid salt-containing waste dilute sulfuric acid in titanium white waste liquid mainly include neutralization method, concentration method, solvent crystallization method, etc., and the neutralization method is more researched and applied due to the advantages of wide raw material sources, low energy consumption and mild reaction conditions.

[0003] Patent CN103572058A discloses a method for enriching rare earth and rare elements and preparing white gypsum from sulfuric acid method titanium white waste liquid. A calcium-containing alkaline compound is used to react with the waste liquid, and a magnesium-containing alkaline compound is used as a pH regulator, the influence of pH value on the content of titanium, vanadium and scandium elements in the gypsum is utilized, and the white gypsum is prepared under the condition of low pH value (1.5-3.0), the filter cake enriched with rare earth and rare elements and the iron-rich residual liquid are prepared under the condition of high pH value (5.5-7.0), and the titanium, vanadium and scandium in the filter cake enriched are enriched and purified through further hydrolysis and extraction processes. The gypsum obtained in the preferred embodiment can be used for preparing cement or gypsum board, and the yield of titanium, vanadium and scandium in the filtrate is 99.43%, 99.34% and 97.38% respectively.

[0004] Patent CN104016398A discloses a method for producing sulfate by using dilute sulfuric acid in industrial wastewater. Fresh high-purity CaCO3 is used to react with the dilute sulfuric acid in the industrial wastewater, and high-purity calcium sulfate dihydrate whiskers or crystals are generated by controlling the reaction conditions, i.e. the hydrogen ion concentration is greater than 0.01 mol / L, the reaction temperature is 0-90℃, the dilute sulfuric acid concentration is 0.5-50%, and the reaction time is 0.5-5 hours.

[0005] Patent CN113003599A discloses a process for producing high-strength gypsum using sulfate ions in industrial wastewater. After pretreatment, the sulfate-containing wastewater is added with complexing agent and precipitating agent, and after sufficient reaction, separation, filtration and washing, gypsum with standard whiteness is obtained. Then the gypsum is mixed with water in a reaction kettle for reaction, and after filtration and drying, high-strength gypsum powder is prepared. In the preferred embodiment, the whiteness of the gypsum is 98%, and all indicators meet the requirements of standard JC / T 2038-2010.

[0006] Patent CN118811849A discloses a method for preparing large-particle gypsum using sulfuric acid method titanium dioxide waste acid. First, high-quality calcium sulfate seeds are prepared from raw materials such as waste acid, water and alkaline calcium salt. In the presence of the seeds, the waste acid and calcium salt slurry are added to the reaction tank for neutralization reaction, and white gypsum is obtained after filtration. In the preferred embodiment, the white gypsum has a water content of 15.33% and an average particle size of 31.7 μm.

[0007] Patent CN105858705A discloses a method for preparing low-free water content dihydrate gypsum from waste sulfuric acid solution. By controlling the process conditions such as the concentration of the crystal modifier, the particle size of calcium carbonate and the concentration of waste sulfuric acid, dihydrate gypsum with regular morphology, maximum particle size of 39.84 μm and minimum free water content of 13.40% is obtained.

[0008] Patent CN117342596A discloses a method for directly preparing building gypsum from industrial waste sulfuric acid and waste stone powder. Through a normal pressure reaction kettle, building gypsum slurry is directly formed from waste stone powder and industrial waste sulfuric acid in the presence of salt and crystal modifier, and building gypsum crystals are obtained after solid-liquid separation, water washing and drying. By adjusting the types and amounts of salt and crystal modifier, building gypsum with crystal diameter of 10-30 μm and aspect ratio of 10:1-1:1 is prepared.

[0009] Waste sulfuric acid is widely available, and industries such as titanium dioxide production, metal pickling and wet metallurgy can produce salt-containing waste sulfuric acid. Due to differences in sulfuric acid concentration and impurity composition and content, different sources of waste sulfuric acid use different technical routes and process parameters in the treatment process. The trivalent iron ion content in the salt-containing waste sulfuric acid produced by the wet-process phosphoric acid extraction and purification device is high, and the metal ions, anion composition and content are different in different waste acids, which will inevitably be affected by different ions in the actual process. Therefore, it is urgent to develop a method for treating salt-containing waste dilute sulfuric acid in wet-process phosphoric acid. SUMMARY

[0010] The purpose of the present application is to provide a method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid, which solves the problem of difficult treatment of salt-containing waste dilute sulfuric acid produced in the prior art wet-process phosphoric acid.

[0011] To solve the above technical problems, the application adopts the following technical solutions: A method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid, comprising the following steps: 1) waste acid pretreatment: adding a reducing agent to the salt-containing waste dilute sulfuric acid to reduce Fe 3+ to Fe 2+ ; the reaction temperature is 20-80 DEG C, and the reaction time is 0.1-2 h; in the primary reaction process, the slurry pH is 1-4; under this condition, Fe 3+ is more likely to form a precipitate than Fe 2+ ; if the Fe 3+ content in the raw material acid is too high, the trivalent iron content in the gypsum will increase accordingly, thereby affecting the whiteness of the product; reducing Fe 3+ to Fe 2+ before the primary reaction can effectively reduce the Fe2O3 content in the gypsum, thereby ensuring the whiteness of the gypsum.

[0012] 2) primary reaction: adding seed crystals to the pretreated waste acid, stirring uniformly, and then slowly adding the prepared calcium salt slurry to the reactor under stirring; the feeding mode is batch feeding or continuous feeding, the addition amount per minute is controlled to be less than 5% of the theoretical addition amount, the pH value change is detected during the feeding process, the reaction endpoint pH value is controlled to be 1-4, the calcium salt slurry is stopped adding, and the temperature is maintained for 1-5 h; the primary reaction controls the reaction endpoint pH value to be 1-4, which is beneficial to reducing the impurity content of the gypsum; as the pH increases, the Fe2O3 content will also increase to a certain extent, and magnesium and aluminum also have a similar trend; by limiting the pH range, the impurity components of the product gypsum can be controlled, which can meet the higher requirements of the downstream market; however, the pH of the primary reaction cannot be too low or too high, otherwise the utilization efficiency of the sulfate radical in the waste acid will be affected.

[0013] 3) primary solid-liquid separation: after the step 2) is completed, the primary solid-liquid separation is carried out, the filter cake obtained by the separation is washed, filtered and dried to obtain white gypsum; the filtrate obtained by the separation is subjected to secondary reaction; in this step, the filter cake is washed with process water, the mass ratio of the washing water to the filter cake is 1-10:1, the washing temperature is 40-80 DEG C, and the washing time is 1-4 h; in this step, the gypsum is dried at a temperature of 40-60 DEG C for 1-8 h to ensure that the attached water content of the finished gypsum is ≤10 wt.%; after the drying is completed, the high-purity white gypsum product is prepared.

[0014] 4) secondary reaction: slowly add the precipitator to the filtrate obtained in step 3), the feeding mode is batch feeding or continuous feeding, the addition amount per minute is controlled to be less than 5% of the theoretical addition amount, the pH change is detected, the reaction end point pH value is controlled to be 4-8, the addition of the precipitator slurry is stopped and the reaction is continued to mature for 1-4 h, the reaction maturation temperature is 20-80℃, and a slurry containing phosphate precipitate is obtained. Compared with the primary reaction, under the condition of relatively high pH=4-8, with the addition of the precipitator, aluminum, iron and magnesium impurity ions will precipitate in the form of phosphate, among which the aluminum removal rate is the largest (≥99%), the iron removal rate is the second (≥80%), and the magnesium removal rate is the lowest (10%-50%). The main purpose of this step in the present application is to remove the aluminum element harmful to crops, so that the filtrate can be applied in the fertilizer system.

[0015] 5) secondary solid-liquid separation: the slurry obtained after the maturation reaction in step 4) is subjected to secondary solid-liquid separation, the filter cake obtained by separation is washed, filtered and dried to obtain a mixed phosphate filter cake containing iron, aluminum and magnesium, which is used for preparing fireproof coating, iron phosphate or heavy metal adsorbent; the filtrate obtained by removing aluminum ions is used in the fertilizer system or the phosphorite flotation section of the chemical industry.

[0016] A further technical solution is that the reducing agent is reduced iron powder, and the addition amount is 1.1-1.5 times the theoretical consumption; the SO4 2- The content of SO4 is 5-25%, the content of Fe2O3 is 0.5-2.0%, the content of Al2O3 is 0.1-1.0%, the content of MgO is 0.1-1.0%, and the content of P2O5 is 0.5-2.0%. Iron powder is selected as the reducing agent because the waste acid itself contains iron elements, and the use of iron powder can avoid introducing new impurities. At the same time, in order to ensure the reduction of trivalent iron ions to divalent iron ions, the reducing agent should be excessive.

[0017] A further technical solution is that in step 1), after the waste acid is pretreated, it is filtered and magnetically selected, and the excessive reduced iron powder is separated from the pretreated waste acid and recycled. By filtering, the excessive solid content iron powder and other solid impurities in the waste acid solution can be removed, and then by magnetic selection, the excessive iron powder can be separated out for recycling; A further technical solution is that in step 2), the seed crystal is any one of a flaky seed crystal, a needle-like seed crystal and a rod-like seed crystal.

[0018] A further technical solution is that the seed crystal is prepared by the following method: Flaky seed preparation: take the salt-containing waste dilute sulfuric acid solution 200 ml, as reaction liquid 1 for standby; prepare 5% lime milk solution 300 ml, as reaction liquid 2 for standby; using peristaltic pump slowly add reaction liquid 1 into the reactor, the feeding speed is 1-5 ml / min; reaction liquid 1 feeding at the same time, using another peristaltic pump to add reaction liquid 2 into the reactor, the feeding speed is 1.5-7.5 ml / min; after the feeding is completed, continue to react for 30 min; after the reaction is completed, vacuum filtration is carried out, the obtained filter cake is washed, filtered, and dried to prepare flaky seed; Needle-shaped seed preparation: take the salt-containing waste dilute sulfuric acid solution 200 ml, place in the reactor, as reaction liquid 1 for standby; prepare 5% lime milk solution 300 ml, as reaction liquid 2 for standby; using peristaltic pump to add reaction liquid 2 into the reactor, the feeding speed is 1.5-7.5 ml / min; after the feeding is completed, continue to react for 30 min; after the reaction is completed, vacuum filtration is carried out, the obtained filter cake is washed, filtered, and dried to prepare needle-shaped seed; Rod-shaped seed preparation: using CaO and pure water to prepare clear lime water 3000 ml (saturated Ca(OH)2 solution at ambient temperature), as reaction liquid 1 for standby; take the salt-containing waste dilute sulfuric acid solution 100 ml, as reaction liquid 2 for standby; add reaction liquid 1 and reaction liquid 2 into the reactor in turn, carry out thorough mixing under normal temperature and pressure, the stirring speed is 200-600 rpm, the reaction time is 30 min; after the reaction is completed, vacuum filtration is carried out to remove solid content, obtain filtrate; place the obtained filtrate in a distillation kettle, carry out evaporation concentration under normal pressure, the heating temperature is 100-200 ℃, stop heating when the concentrated liquid volume is 200 ml, obtain slurry; after the obtained slurry is cooled to room temperature, carry out liquid-solid separation by vacuum filtration method, the obtained filter cake is washed, filtered, and dried to prepare rod-shaped seed.

[0019] The initial concentration of sulfate has an influence on the size of the crystal, the lower the concentration, the larger the crystal shape; the feeding method has an influence on the shape, adding acid at one time and adding calcium carbonate in batches is easy to generate slender needle-shaped crystals; adding acid and calcium carbonate alternately is easy to generate flaky crystals; the rod-shaped crystal prepared by distillation method mainly utilizes slow concentration to ensure slow crystallization under the bottom supersaturation degree, so that the crystal is large and regular, and the over-high supersaturation degree is prevented to generate fine and broken crystals.

[0020] Further technical solutions are that the calcium salt of step 2) is one or more of calcium carbonate, calcium hydroxide, calcium oxide, calcium chloride, and calcium nitrate, which is stirred with water to prepare a slurry, and the concentration of the slurry is 1%-20%. After the powdered calcium salt is stirred with water to prepare a slurry, the particles are uniformly dispersed, dust flying during feeding is prevented, and problems such as caking of the powder, too fast local reaction, and low conversion rate caused by direct addition of the powder are avoided; Further technical solutions are that the precipitant of step 4) is one or more of ammonia, sodium hydroxide, potassium hydroxide, and calcium hydroxide, which is configured into an aqueous solution by adding water and stirring, and the concentration is 5% to 30%. The precipitant is configured into an aqueous solution, which can ensure uniform dispersion of the precipitant and avoid local rapid reaction; the use of different precipitants makes the pH end point of the secondary reaction different, and the removal rate of impurities in the filtrate obtained by the secondary reaction at different pHs is different; when the weak alkalinity, low concentration, and low cost of the precipitant are selected and the reaction end point pH is controlled to be 4 to 6, the filtrate obtained by the secondary reaction can be used in the phosphorite flotation homogenization section; when the strong alkalinity, high concentration, and high cost of the precipitant are selected and the reaction end point pH is controlled to be 6 to 8, the impurity content in the filtrate obtained by the secondary reaction is further reduced, the removal rate of aluminum ion impurities is greater than or equal to 99%, and the solution basically does not contain aluminum elements harmful to crops and can be used in a fertilizer production system as a solvent, a diluent, or a replacement for part of production water.

[0021] Further technical solutions are that step 5) adds a filter aid for filtration when the secondary solid-liquid separation is performed, and the filter aid is selected from one or more of white gypsum, calcium carbonate, diatomite, phosphogypsum, and quartz sand; and the addition amount of the filter aid is 1% to 10% of the yield of the secondary solid-liquid separation filter cake.

[0022] The application has the following advantages: 1. The salt-containing waste dilute sulfuric acid generated in the wet-process phosphoric acid is first added with iron powder to reduce Fe 3+ to Fe 2+ , and then seed crystals are added to prepare white gypsum; after the filtrate is precipitated and aged, the mixed phosphate filter cake containing iron, aluminum, and magnesium obtained by separation is used to prepare fireproof paint, iron phosphate, or a heavy metal adsorbent; and the filtrate from which aluminum ions are removed is used in a fertilizer system or a phosphorite flotation homogenization section.

[0023] 2. The application converts SO4 2- in the form of H2SO4 into high-purity white gypsum, and the conversion rate of corresponding SO4 2- is greater than or equal to 95%.

[0024] The prepared gypsum has a whiteness of greater than or equal to 90%, and the main component and impurity content indicators meet the requirements of GB / T 23456-2018 "Phosphogypsum" for first-grade phosphogypsum.

[0025] 3. The filtrate generated in the preparation of gypsum in the first reaction is subjected to secondary reaction and impurity removal to form a dilute ammonium sulfate solution, and the removal rate of aluminum ion impurities is greater than or equal to 99%, the solution basically does not contain aluminum elements harmful to crops, and can be used in a fertilizer production system as a solvent, a diluent, or a replacement for part of production water.

[0026] 4、The filter residue generated after the secondary reaction of the filtrate generated in the first reaction process of preparing gypsum is a mixture of iron, aluminum and magnesium phosphate, which can be applied to the preparation of fireproof paint, iron phosphate, heavy metal adsorbent, etc. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The process flow chart of the present application.

[0028] Figure 2 The detection results of the ferric iron reduction experiment of Example 4.

[0029] Figure 3 The polarizing microscope graph of the self-made flaky crystal seed in Example 1.

[0030] Figure 4 The polarizing microscope graph of the self-made needle-shaped crystal seed in Example 2.

[0031] Figure 5 The polarizing microscope graph of the self-made rod-shaped crystal seed in Example 3. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0033] Example 1 A method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid, comprising the following steps: The composition index of the salt-containing waste sulfuric acid produced by the wet-process phosphoric acid extraction purification device is shown in Table 1.

[0034] Table 1 1) Waste acid pretreatment: take 200g of salt-containing waste dilute sulfuric acid, add 1.51g of reduced iron powder, and reduce Fe 3+ to Fe 2+ . The reaction temperature is 25℃, the reaction time is 0.5h, and the stirring speed is 250rpm. After the reaction is completed, filtration and magnetic separation are performed to separate the excess reduced iron powder from the waste acid for recycling.

[0035] 2) Primary reaction: 0.5 g of self-made flaky seed crystals were added to the pretreated waste acid, and after stirring, the calcium salt slurry was slowly added to the reactor in batches. Every 5 minutes, 10 ml of calcium salt slurry was added, and the pH change was detected. The reaction endpoint pH value was controlled at 1, and the calcium salt slurry was stopped and the crystals were aged for 1 h. The calcium salt was calcium carbonate, which was prepared by stirring with water to form a slurry, and the slurry concentration was 1%.

[0036] The self-made flaky seed crystals were prepared by the following steps: (a1) Take 200 ml of salt-containing waste dilute sulfuric acid solution as reaction liquid 1; prepare 300 ml of 5% lime milk solution as reaction liquid 2; (a2) use a peristaltic pump to slowly add reaction liquid 1 to the reactor at a feeding speed of 1 ml / min; at the same time, use a peristaltic pump 2 to add reaction liquid 2 to the reactor at a feeding speed of 1.5 ml / min; after the addition is completed, continue to react for 30 min; (a3) after the reaction in step (a2) is completed, vacuum filtration is carried out, and the obtained filter cake is washed, filtered, and dried to obtain flaky seed crystals.

[0037] 3) Primary solid-liquid separation: after the aging in step 2) is completed, primary solid-liquid separation is carried out, and the primary solid-liquid separation method is gravity sedimentation. The obtained filter cake is washed, filtered, and dried to obtain white gypsum; the obtained filter cake is washed with process water, the mass ratio of washing water to filter cake is 1:1, the washing temperature is 40°C, and the washing time is 1 h; in this step, the gypsum is dried at a temperature of 40°C for 1 h to ensure that the attached water content of the finished product gypsum is ≤10%, and high-purity white gypsum is obtained after drying. The analysis indexes of white gypsum and primary filtrate are shown in Table 2, wherein the whiteness of gypsum is 94, and the purity is 99.61%.

[0038] 4) Secondary reaction: a precipitating agent is slowly added in batches to the filtrate obtained in step 3), and the pH change is detected. The reaction endpoint pH value is controlled at 4, and the addition of the precipitating agent slurry is stopped and the reaction is continued for 1 h. The corresponding removal rates of aluminum, iron, and magnesium are 99.31%, 84.08%, and 16.65%, respectively. The precipitating agent is lime milk with a concentration of 5%.

[0039] 5) Secondary solid-liquid separation: the slurry obtained after the aging reaction in step 4) is subjected to secondary solid-liquid separation, and the obtained filter cake is washed, filtered, and dried to obtain a mixed phosphate filter cake containing iron, aluminum, and magnesium, which is used for preparing fireproof coating, iron phosphate, or heavy metal adsorbent; the secondary filtrate from which aluminum ions are removed is used for fertilizer system or phosphate flotation section. The corresponding analysis indexes of the secondary filtrate and filter cake are shown in Table 2. The secondary solid-liquid separation method is vacuum filtration, and a filter aid is added for filtration. The filter aid is selected from white gypsum generated in the primary reaction, and the addition amount of the filter aid is 1% of the yield of the secondary solid-liquid separation filter cake (dry basis).

[0040] Table 2. Parameters of each reactant in Example 1 Example 2 A method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid, comprising the following steps: The composition of the salt-containing waste sulfuric acid produced by the wet-process phosphoric acid extraction and purification device is shown in Table 1.

[0041] 1) Waste acid pretreatment: Take 200 g of salt-containing waste dilute sulfuric acid, add 2.06 g of reduced iron powder, and stir to dissolve the iron powder. The reaction temperature is 25°C, the reaction time is 0.5 h, and the stirring speed is 250 rpm. After the reaction is completed, filtration and magnetic separation are performed to separate the excess reduced iron powder from the waste acid for recycling. 3+ The reduced iron is reduced to Fe 2+ . The reaction temperature is 25°C, the reaction time is 0.5 h, and the stirring speed is 250 rpm. After the reaction is completed, filtration and magnetic separation are performed to separate the excess reduced iron powder from the waste acid for recycling.

[0042] 2) First-stage reaction: Add 0.5 g of self-made needle-shaped seed crystals to the pretreated waste acid, stir uniformly, and then slowly add the prepared calcium salt slurry into the reactor. Add 10 ml of calcium salt slurry every 5 min and detect the pH change. The reaction endpoint pH value is controlled at 3.5, and the calcium salt slurry addition is stopped. The crystal growth is maintained for 5 h. The calcium salt is calcium hydroxide, which is prepared by stirring with water to form a slurry with a concentration of 20%.

[0043] The self-made needle-shaped seed crystals are prepared by the following steps: (b1) Take 200 ml of salt-containing waste dilute sulfuric acid solution and place it in a reactor as reaction liquid 1; prepare 300 ml of 5% lime milk solution as reaction liquid 2; (b2) use a peristaltic pump to add reaction liquid 2 to the reactor at a feeding speed of 7.5 ml / min; after the feeding is completed, continue the reaction for 30 min; (b3) after the reaction in step (b2) is completed, perform vacuum filtration, and the obtained filter cake is washed, filtered, and dried to obtain needle-shaped seed crystals.

[0044] 3) First-stage solid-liquid separation: After the crystal growth in step 2) is completed, perform first-stage solid-liquid separation. The first-stage solid-liquid separation method is vacuum filtration, and the obtained filter cake is washed, filtered, and dried to obtain white gypsum. The obtained filter cake is washed with process water at a mass ratio of washing water to filter cake of 10:1, a washing temperature of 80°C, and a washing time of 4 h. The gypsum is dried at a temperature of 60°C for 8 h to ensure that the attached water content of the finished gypsum is ≤10%. After drying, high-purity white gypsum is obtained. The analysis indexes of the first-stage filtrate and white gypsum are shown in Table 3, wherein the whiteness of the gypsum is 91 and the purity is 99.42%.

[0045] 4) Secondary reaction: The precipitant is added into the filtrate obtained in step 3) in batches slowly, the pH change is detected, the reaction end point pH value is controlled at 8, the addition of the precipitant slurry is stopped and the reaction is continued to mature for 4h, a slurry containing phosphate precipitate is obtained; the removal rates of aluminum, iron and magnesium are 99.88%, 99.65% and 24.90% respectively; the precipitant is ammonia water (26.5wt.%).

[0046] 5) Secondary solid-liquid separation: the slurry obtained after the maturation reaction in step 4) is subjected to secondary solid-liquid separation, the filter cake obtained by separation is washed, filtered and dried to obtain a mixed phosphate filter cake containing iron, aluminum and magnesium, which is used for preparing fireproof coating, iron phosphate or heavy metal adsorbent; the filtrate obtained by separation is used for fertilizer system or phosphate flotation section of chemical industry; the analysis indexes of the secondary filtrate and filter cake are shown in Table 3.

[0047] The secondary solid-liquid separation method in step 5) is vacuum filtration, and a filter aid is added for filtration; the filter aid is selected from calcium carbonate; the addition amount of the filter aid is 10% of the yield of the secondary solid-liquid separation filter cake.

[0048] Table 3: Parameters and indexes of each reactant in Example 2 Example 3: A method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid, comprising the following steps: The composition indexes of the salt-containing waste sulfuric acid produced by the wet-process phosphoric acid extraction and purification device are shown in Table 1.

[0049] 1) Waste acid pretreatment: 200g of salt-containing waste dilute sulfuric acid is taken, 1.65g of reduced iron powder is added, and Fe 3+ is reduced to Fe 2+ . The reaction temperature is 25℃, the reaction time is 0.5h, and the stirring speed is 250rpm. After the reaction is completed, filtration and magnetic separation are performed to separate the excess reduced iron powder from the waste acid for recycling.

[0050] 2) Primary reaction: 0.5g of self-made rod-shaped seed crystals is added into the pretreated waste acid, the calcium salt is slowly added into the reactor in batches after being uniformly stirred, the pH change is detected, the reaction end point pH value is controlled at 1.74, and the addition of the calcium salt slurry is stopped and the crystal is aged for 3h; the calcium salt is calcium nitrate, which is prepared into a slurry by adding water and stirring, and the slurry concentration is 5%.

[0051] The self-made rod-shaped seed crystals are prepared by the following steps: (c1) using CaO and pure water to prepare 3000 ml of clear lime water as reaction liquid 1; taking 100 ml of salt-containing waste dilute sulfuric acid solution as reaction liquid 2; (c2) sequentially adding reaction liquid 1 and reaction liquid 2 into the reactor, and fully mixing at normal temperature and pressure, the stirring speed is 200 rpm, and the reaction time is 30 min; (c3) after the reaction in step (c2) is completed, vacuum filtration is performed to remove the solid content, and a filtrate is obtained; (c4) the filtrate obtained in step (c3) is placed in a distillation kettle, and evaporation concentration is performed under normal pressure, the heating temperature is 100-200 ℃, and the heating is stopped when the concentrated liquid volume is 200 ml, and a slurry is obtained; (c5) the slurry obtained in step (c4) is cooled to room temperature, and then liquid-solid separation is performed by vacuum filtration, and a filter cake is obtained; after washing, filtering and drying, rod-shaped crystal seeds are prepared; 3) primary solid-liquid separation: after the crystal growth in step 2) is completed, primary solid-liquid separation is performed, and the primary solid-liquid separation method is vacuum filtration; the filter cake obtained by separation is washed, filtered and dried to obtain white gypsum; the filter cake is washed with process water, the mass ratio of washing water to filter cake is 5:1, the washing temperature is 60 ℃, and the washing time is 3 h; in this step, the temperature for drying the gypsum is 50 ℃, the drying time is 6 h, the attached water content of the finished product gypsum is ensured to be ≤10%, and high-purity white gypsum product is prepared after drying.

[0052] 4) secondary reaction: the precipitant is slowly added in batches to the filtrate obtained in step 3), the pH change is detected, the reaction endpoint pH value is controlled at 6, the addition of the precipitant slurry is stopped, and the reaction is continued to mature for 3 h, and a slurry containing phosphate precipitate is obtained; the removal rates of aluminum, iron and magnesium are 99.76%, 93.30% and 19.75%, respectively; the precipitant is calcium hydroxide, which is configured into an aqueous solution by adding water and stirring, and the concentration is 5%.

[0053] 5) secondary solid-liquid separation: the slurry obtained after the maturation reaction in step 4) is subjected to secondary solid-liquid separation, the filter cake obtained by separation is washed, filtered and dried to obtain a mixed phosphate filter cake containing iron, aluminum and magnesium, which is used for preparing fireproof coating, iron phosphate or heavy metal adsorbent; the filtrate obtained by removing aluminum ions is used for fertilizer system or phosphate flotation section of chemical industry; the analysis indexes of the secondary filtrate and filter cake are shown in Table 4.

[0054] The secondary solid-liquid separation method is vacuum filtration with the addition of a filter aid; the filter aid is selected from white gypsum generated in the first reaction, and the addition amount of the filter aid is 5% of the yield of the secondary solid-liquid separation filter cake.

[0055] Table 4: Parameters of each reactant in Example 3 Example 4 Gypsum 1# and Gypsum 2# were prepared in the same way as Example 3, except that in the preparation process of Gypsum 1#, the pH of the end point of the first reaction was 3; in the preparation process of Gypsum 2#, no reducing agent was added to the salt-containing waste dilute sulfuric acid before the first reaction to reduce Fe 3+ to Fe 2+ , and the pH of the end point of the first reaction was 3. The analysis index results of Gypsum 1# and Gypsum 2# prepared in Example 4 are shown in Table 5. At the same time, qualitative detection of ferric iron was carried out before and after the reduction of ferric iron in the preparation steps of Gypsum 1#, and the results are shown in Table 6. Figure 2 and Table 6.

[0056] Table 5 Analysis index results of Gypsum 1# and Gypsum 2# prepared in Example 4 Table 6 Qualitative detection results of ferric iron before and after the reduction of ferric iron in the preparation steps of Gypsum 1# in Example 4 As can be seen from Table 5, before the first reaction, the use of iron powder to reduce ferric iron in the waste acid will affect the iron content in the white gypsum. The results show that when the ferric iron is first reduced to divalent and then the reaction starts, the content of Fe2O3 in the gypsum is reduced from 2.5260 to 0.5827 wt.%.

[0057] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements to part of the technical features, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid, characterized in that: The steps include: 1) Pretreatment of waste acid: Add reducing agent to waste dilute sulfuric acid containing salt to reduce Fe 3+ Reduction to Fe 2+ ; 2) Primary reaction: Add seed crystals to the pretreated waste acid, stir evenly, and then slowly add the calcium salt slurry into the reactor under stirring. Monitor the pH change during the addition process. The pH value at the reaction endpoint is controlled at 1-4. Stop adding the calcium salt slurry and grow the crystals for 1-5 hours at a temperature of 20-80°C. 3) Primary solid-liquid separation: After step 2) crystal growth, primary solid-liquid separation is performed. The filter cake obtained by separation is washed, filtered, and dried to obtain white gypsum. The filtrate obtained by separation is subjected to secondary reaction. 4) Secondary reaction: Slowly add the precipitant slurry to the filtrate obtained in step 3) and monitor the pH change. When the pH value at the reaction endpoint is controlled between 4 and 8, stop adding the precipitant slurry and continue the reaction for 1 to 4 hours at a reaction temperature of 20 to 80°C to obtain a slurry containing phosphate precipitate. 5) Secondary solid-liquid separation: The slurry obtained after the aging reaction in step 4) is subjected to secondary solid-liquid separation. The separated filter cake is washed, filtered, and dried to obtain a mixed phosphate filter cake containing iron, aluminum, and magnesium, which is used to prepare fire retardant coatings, iron phosphate, or heavy metal adsorbents. The separated filtrate, from which aluminum ions have been removed, is used in fertilizer systems or in the phosphate rock flotation homogenization process.

2. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: The reducing agent is reduced iron powder, and the addition amount is 1.1 to 1.5 times of the theoretical consumption; SO4 2- Content is 5~25%, Fe2O3 content is 0.5~2.0%, Al2O3 content is 0.1~1.0%, MgO content is 0.1~1.0%, and P2O5 content is 0.5~2.0%.

3. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: In step 1), the reaction temperature is 20-80° C., and the reaction time is 0.1-2 h.

4. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: In step 1), the waste acid is pretreated and then filtered and magnetically separated to separate excess reduced iron powder from the pretreated waste acid and then recycled.

5. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: The seed crystals used in step 2) are any of flake seed crystals, needle seed crystals, and rod seed crystals.

6. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 5, characterized in that: The seed crystals are prepared by the following method: Preparation of flaky seed crystals: 200 ml of salt-containing waste dilute sulfuric acid solution was prepared as reaction solution 1; 300 ml of 5% lime milk solution was prepared as reaction solution 2; reaction solution 1 was slowly added to the reactor using a peristaltic pump at a feeding rate of 1 to 5 ml / min; while adding reaction solution 1, another peristaltic pump was used to add reaction solution 2 to the reactor at a feeding rate of 1.5 to 7.5 ml / min; after the addition was completed, the reaction was continued for 30 minutes; after the reaction was completed, vacuum filtration was performed, and the resulting filter cake was washed, filtered, and dried to obtain flaky seed crystals; Preparation of needle-shaped seed crystals: 200 ml of salt-containing waste dilute sulfuric acid solution was placed in a reactor as reaction solution 1; 300 ml of 5% lime milk solution was prepared as reaction solution 2; reaction solution 2 was added to the reactor using a peristaltic pump at a feeding rate of 1.5 to 7.5 ml / min; after the addition was completed, the reaction was continued for 30 minutes; after the reaction was completed, vacuum filtration was performed, and the resulting filter cake was washed, filtered, and dried to obtain needle-shaped seed crystals; Preparation of rod-shaped seed crystals: 3000 ml of clarified lime water is prepared using CaO and pure water, which is used as reaction liquid 1; 100 ml of salt-containing waste dilute sulfuric acid solution is used as reaction liquid 2; reaction liquid 1 and reaction liquid 2 are added to a reactor in sequence, and are fully mixed at room temperature and pressure, with a stirring speed of 200 to 600 rpm and a reaction time of 30 minutes; after the reaction is completed, solids are removed by vacuum filtration to obtain a filtrate; the obtained filtrate is placed in a distillation kettle and evaporated and concentrated under normal pressure at a heating temperature of 100 to 200° C., and heating is stopped when the volume of the concentrated liquid reaches 200 ml to obtain a slurry; the obtained slurry is cooled to room temperature and then subjected to liquid-solid separation by vacuum filtration. The obtained filter cake is washed, filtered, and dried to obtain rod-shaped seed crystals.

7. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: The calcium salt in step 2) is one or more of calcium carbonate, calcium hydroxide, calcium oxide, calcium chloride, and calcium nitrate, which are mixed with water and stirred to form a slurry with a slurry concentration of 1% to 20%.

8. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: The precipitant in step 4) is one or more of ammonia water, sodium hydroxide, potassium hydroxide, and calcium hydroxide, which are prepared into an aqueous solution by adding water and stirring, with a concentration of 5% to 30%.

9. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: Step 5) Adding a filter aid to assist filtration during the secondary solid-liquid separation, wherein the filter aid is selected from one or more of white gypsum, calcium carbonate, diatomaceous earth, phosphogypsum, and quartz sand; the amount of the filter aid added is 1% to 10% of the secondary solid-liquid separation filter cake yield.

10. The method for preparing high-purity white gypsum from salt-containing waste dilute sulfuric acid according to claim 1, characterized in that: Step 3) washing the obtained filter cake with process water, with a mass ratio of process water to filter cake of 1 to 10:1, a washing temperature of 40 to 80° C., and a washing time of 1 to 4 hours; drying temperature of 40 to 60° C., and a drying time of 1 to 8 hours. After drying, the attached water content of the white gypsum is ≤10%.

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