Low-calcium under-burnt lime as well as preparation method and application thereof

Abstract

The invention relates to the technical field of reutilization of phosphorus chemical by-product semi-hydrated gypsum and the technical field of filling materials, and discloses low-calcium under-burnt lime and a preparation method and application thereof.The low-calcium under-burnt lime is prepared by calcining a limestone raw material in a lime kiln, the coal consumption of the lime kiln is controlled to be 0.05-0.13 t / t lime, the stone consumption of the lime kiln is controlled to be 0.80-1.70 t / t lime, and the coal consumption of the lime kiln is controlled to be 0.05-0.13 t / t lime. The calcination temperature is 800-1000 DEG C, the calcination time is 15-40 h, the content of calcium carbonate in the raw material limestone is larger than or equal to 90 wt%, and the effective calcium content of the unburned lime prepared through the preparation method is 20-40 wt%. The low-calcium under-burnt lime provided by the invention is used for modifying the semi-hydrated gypsum with low free water content, so that acidic ions of the semi-hydrated gypsum can be cured in a short time to meet the standard requirements, and the problem that the pH value is strongly alkaline for a long time due to low free water content in the conversion process of the modified semi-hydrated gypsum is solved.

Description

Technical Field

[0001] The present invention relates to the technical fields of the reuse of by - product hemihydrate gypsum in phosphoric acid chemical industry and filling materials, and particularly relates to a low - calcium under - burned lime, a preparation method thereof and an application thereof. Background Art

[0002] Phosphoric acid chemical enterprises' wet - process phosphoric acid systems produce a large amount of gypsum as by - products every year. After modification, the resource utilization of these by - product gypsums in filling fields such as mine ecological restoration project filling, roadbed filling, and industrial site filling projects has become one of the mainstream ways to deal with by - product gypsum in China at present. According to statistics, the output of phosphogypsum in 2023 was 81 million tons, the comprehensive utilization amount was 45 million tons, and the comprehensive utilization rate was 55.6%. Among them, the utilization rate in the fields of mine ecological restoration project filling, roadbed filling, and industrial site filling projects was about 20 - 30%.

[0003] For the modification of gypsum, currently commonly used modifiers include materials such as lime, slaked lime, carbide slag, and lime milk. The effective calcium content of traditional lime is usually greater than 70wt%, which is a high - calcium modifier, while slaked lime (effective calcium ≥ 50wt%), carbide slag (effective calcium ≥ 30wt%), and lime milk are slaked calcium - based alkaline modifiers. Since lime promotes the reaction due to heat release during the hydration process, compared with modifiers such as slaked lime, carbide slag, and lime milk, it has a better fluorine - fixing effect. However, due to the high effective calcium content in traditional lime, when lime is added in excess and / or the free moisture content of gypsum is relatively low, resulting in insufficient reaction of lime, the pH will remain in a strong alkaline state for a long time.

[0004] Phosphoric acid chemical by - product gypsum is divided into dihydrate gypsum and hemihydrate gypsum. Since hemihydrate gypsum is in a metastable state and is very easy to absorb water and convert into dihydrate gypsum, there are some problems in using hemihydrate gypsum as a gypsum cementitious material after modification.

[0005] When applying by - product gypsum from the phosphoric acid industry, it is necessary to ensure that the water - soluble P, F and other acidic ions and pH value meet the requirements in the standard GB / T32124 - 2024 "Specification for the Treatment and Disposal of Phosphogypsum" (P≤0.5mg / L, F≤10mg / L in the leachate, and pH is 6 - 9). When the gypsum - based cementitious material prepared from traditional lime - modified hemihydrate gypsum is used in filling projects, in order to ensure its compaction degree and strength, strict control requirements for the free - water content are imposed. The initial optimal free - water content for the layered rolling of modified hemihydrate gypsum is generally 20±2%. Since modified hemihydrate gypsum is in a metastable state, as the reaction proceeds, within a short period (1 - 3 days), the crystal water can be quickly converted to more than 16%, becoming aged dihydrate gypsum. Correspondingly, the free - water content rapidly decreases to less than 10%. When cured for 7 - 28 days, the free - water can be reduced to 1 - 5%. As a result, the free - water content in the subsequent reaction of modified gypsum is too low, and the reaction rate gradually slows down. This makes it impossible for the lime and water - soluble P, F and other acidic ions in hemihydrate gypsum to react fully. It is necessary to add an excessive amount of lime to promote the reaction. Although the water - soluble P, F and other acidic ions in hemihydrate gypsum can be well solidified after the excessive addition of lime, the pH is in a strong - alkaline state, and there is no sufficient water content for the material to continue the neutralization reaction. As a result, the material is in a strong - alkaline state. Even with long - term watering and curing for 180 - 360 days on the surface of the compacted and solidified filling body, the pH is very difficult to be reduced to the standard requirements of 6 - 9.

[0006] When lime is applied to modified hemihydrate gypsum, in addition to the excessive addition of lime to solidify P and F ions, by improving the accuracy of traditional lime feeding equipment to ±0.1%, the solidification of water - soluble P and F ions can also be quickly achieved. The content of acidic ions (P, F) in the leachate meets the requirements of GB / T32124 - 2024, and at the same time, the pH can be adjusted to 6 - 9. However, in the actual operation process, due to reasons such as the lime feeding bin being hygroscopic and caking, the vibration of the screw equipment, large fluctuations in the quality of hemihydrate gypsum (water - soluble P, water - soluble F), and large fluctuations in the conveying volume, the lime screw metering fluctuates greatly. It is very difficult to achieve continuous and stable production under high - precision metering, resulting in large fluctuations in the quality of modified hemihydrate gypsum, and it is impossible for P, F and pH in the leachate to meet the standards simultaneously.

[0007] In addition to traditional lime as a modifier, slaked lime, carbide slag, lime milk and other slaked calcium - based alkaline modifiers have poor curing rates for fluoride ions compared with CaO dry powder during the modification of hemihydrate gypsum because Ca(OH) 2 is a poorly soluble substance, and at the same time, due to the premature release of reaction heat, usually an excessive amount needs to be incorporated and the reaction needs to proceed for more than 28 days, otherwise the short - term curing of fluoride ions cannot be achieved.

[0008] The use of traditional hemihydrate gypsum modifiers usually mainly involves the compound mixing of various modifiers. However, due to the large density differences between different modifiers, after pre-mixing, under common pneumatic conveying methods (such as Raymond mills, tanker transportation, pneumatic conveying in lime silos, etc.), there is often a certain sorting segregation phenomenon (lime-calcium carbonate mixture segregation), resulting in large fluctuations and unevenness in the quality of the modifiers, and ultimately leading to fluctuations in the quality of the modified hemihydrate gypsum.

[0009] In summary, currently, domestic hemihydrate gypsum is usually modified by compounding one or more of traditional modifiers such as lime powder, lime milk, cement, and carbide slag. Under excessive incorporation, although it has a good solidification effect on P and F in the modified hemihydrate gypsum, due to the rapid conversion of the crystal water in the modified hemihydrate gypsum, the free moisture content in the modified hemihydrate gypsum gradually decreases, and the pH cannot be reduced for a long time, and the material is in a strongly alkaline state. A large amount of modified hemihydrate gypsum cementitious material with a low moisture content is directly transported out of the factory to the external environment, and the pH cannot be reduced for a long time, which will pose a potential risk of alkali pollution to the soil and water quality in the environment. And by storing it in the factory for a long time until the pH is qualified, the activity of the modified hemihydrate gypsum cementitious material will be lost, and it will ultimately turn into aged dihydrate gypsum. At the same time, stacking in the factory will cause a large amount of stacking management fees. Summary of the Invention

[0010] In view of this, the present invention provides a low-calcium underburned lime and its preparation method and application to solve the problem that the modified hemihydrate gypsum used as a filling cementitious material has a long-term strong alkaline pH due to low free water content during the conversion process.

[0011] In order to achieve the above object, the present invention provides the following technical solutions.

[0012] First of all, the present invention provides a preparation method of low-calcium underburned lime, which uses a lime kiln to calcine limestone raw materials to prepare low-calcium underburned lime, wherein the coal consumption of the lime kiln is controlled at 0.05 - 0.13 t / t 石灰 and the stone consumption is 0.80 - 1.70 t / t 石灰 The calcination temperature is 800 - 1000 °C, the calcination time is 15 - 40 h, and the calcium carbonate content of the raw material limestone is ≥ 90 wt%. According to the applicable scenarios, advantages and disadvantages of different types of lime kilns, the present invention can select a suitable lime kiln from types such as shaft kilns, rotary kilns, and fluidized bed kilns according to the actual situation to calcine limestone to prepare lime.

[0013] The main component of limestone is CaCO 3, after being calcined in a lime kiln, calcium carbonate decomposes into calcium oxide, thus obtaining lime. Underburned lime refers to the situation where the temperature is insufficient or the time is not enough during the calcination of limestone, and calcium carbonate fails to completely decompose into calcium oxide. It mainly contains incompletely decomposed calcium carbonate and a small amount of calcium oxide. The present invention prepares underburned low-calcium lime with an effective calcium content of 20-40 wt% and an activity of 100-300 ml / 4N-HCl by controlling the coal consumption, stone consumption, calcination temperature and time parameters of the lime kiln.

[0014] Secondly, the underburned low-calcium lime obtained by the above preparation method should also be within the protection scope of the present invention.

[0015] Thirdly, the present invention also provides the application of the above underburned low-calcium lime in the modification of hemihydrate phosphogypsum. More preferably, the underburned low-calcium lime provided by the present invention is more suitable for the modification of hemihydrate phosphogypsum with low moisture content, and the initial free water content of the hemihydrate phosphogypsum with low moisture content is 5-25 wt%, and the crystal water content ≤ 9 wt%.

[0016] The mineral phase composition of the underburned low-calcium lime provided by the present invention is a composite phase in which the CaCO 3 phase coexists with the CaO phase, and the effective calcium content is 20-40%. Compared with traditional lime (effective calcium ≥ 70 wt%), its distribution area in the modified hemihydrate gypsum heap is wider. Even in the case where the diffusion of OH - , Ca 2+ is relatively blocked, it can also contact with surrounding acidic impurities, free water, etc. more fully. Therefore, when the underburned low-calcium lime of the present invention is used to modify hemihydrate gypsum, due to the good dispersion of the underburned lime and the large reaction contact area, under the self-insulation effect of hemihydrate gypsum, it can react fully with acidic impurities such as water-soluble P and F therein. The underburned low-calcium lime with low calcium is consumed quickly and the reaction is relatively complete. After the traditional high-calcium lime is hydrated at high temperature, the reaction intensifies. Under the condition of low moisture content, it is easy to form a wrapping hydration Ca(OH) 2 barrier layer on the surface of the traditional high-calcium lime, or Ca 3 (PO 4 ) 2 , CaF 2 insoluble matter barrier layer, forming reverse wrapping, and the reaction efficiency of the traditional high-calcium lime becomes poor. Therefore, using the underburned low-calcium lime of the present invention as a hemihydrate gypsum modifier can significantly reduce the pH under the condition of ensuring that P and F in the leachate are qualified.

[0017] Furthermore, in the above application technical solution, the modification of hemihydrate phosphogypsum includes the following raw material composition in mass ratio: underburned low-calcium lime powder: aluminum sulfate: hemihydrate phosphogypsum = (2-15): (0.0-0.3): (84.7-98.0).

[0018] Since the activity of underburned lime is relatively low, usually between 100 and 300 ml / 4N-HCl (a unit representing the lime hydration reaction rate), in order to avoid the occurrence of secondary dissolution, the initial pH of the modified hemihydrate gypsum after mixing needs to be ≥11 during the initial control process. Because if the initial pH is acidic, CaO or CaCO in the underburned multiphase lime at high temperature 3 is easily dissolved and reacts with HPO 4 2- and H 2 PO 4 - to form unstable CaHPO 4 and Ca(H 2 PO 4 ) 2 complex anionic groups, which are likely to cause secondary dissolution in the stockpile. When the initial pH is alkaline, stable Ca 3 (PO 4 ) 2 can be formed, and secondary dissolution is not likely to occur. Adding aluminum sulfate to the modified hemihydrate gypsum has a good pH adjustment effect, can effectively reduce the pH of the early leachate, promote the solidification of F ions, and there is no obvious secondary dissolution of P and F during the reaction. Adding an aluminum sulfate regulator in the modification of hemihydrate phosphogypsum can better solve the problem of difficult pH reduction of the modified hemihydrate gypsum. At the same time, compared with traditional lime, using the low-calcium underburned lime of the present invention can effectively reduce the usage amount of the aluminum sulfate regulator, thus controlling the admixture cost.

[0019] Furthermore, since aluminum sulfate is hygroscopic and prone to caking, and aluminum sulfate octadecahydrate contains crystal water and is more stable in storage and suitable for long-term preservation, in the above application technical solution, the aluminum sulfate is preferably aluminum sulfate octadecahydrate.

[0020] Furthermore, in the above application technical solution, the low-calcium underburned lime powder is obtained by grinding the above low-calcium underburned lime with a pulverizer. The fineness of the low-calcium underburned lime powder is ≥85 wt% passing through a test sieve with a 75 μm aperture. The activity of the low-calcium underburned lime powder with this fineness is ≥100, the true density is 2.700 - 3.017 g / cm 3 , and the loose bulk density is 0.712 - 0.878 g / cm 3 , meeting the modification requirements and the feeding accuracy requirements of powder-type feeding equipment.

[0021] Furthermore, in the above application technical solution, the chemical composition of the hemihydrate phosphogypsum includes 42 - 54 wt% of SO 3 , 31 - 36 wt% of CaO, and 0 - 6 wt% of SiO 2, 0.05 to 1.5 wt% of Al 2 O 3 , 0 to 0.7 wt% of Fe 2 O 3 , 0 to 0.5 wt% of MgO, 0.5 to 1.3 wt% of total P 2 O 5 , 0.1 to 0.8 wt% of water-soluble P 2 O 5 , 0.1 to 0.9 wt% of total F, 0.01 to 0.5 wt% of water-soluble F, 0.001 to 0.3 wt% of K 2 O, 0.01 to 0.6 wt% of Na 2 O.

[0022] Furthermore, in the above application technical solution, the modification of the hemihydrate phosphogypsum further includes the following raw material composition by mass ratio: active inhibitor: hemihydrate phosphogypsum = (0.05 - 0.2):(99.8 - 99.95), and the active inhibitor is selected from at least one of glucose and sodium citrate. The addition of the active inhibitor can delay the activity loss caused by the rapid conversion of the modified hemihydrate gypsum, ensure transportation, and improve the strength, durability and other properties of the filling body.

[0023] After the hemihydrate phosphogypsum modified by the low-calcium underburned lime of the present invention, without adding an active inhibitor, after 1 - 7 days of heat preservation and aging reaction in the storage yard, harmless complex-phase gypsum (hemihydrate - dihydrate coexisting-phase gypsum) or harmless dihydrate gypsum can be obtained, and its acidic ions (P, F) and pH value meet the requirements in GB / T 32124-2024 "Specification for Treatment and Disposal of Phosphogypsum" (P ≤ 0.5 mg / L, F ≤ 10 mg / L in the leachate, pH is 6 - 9). After the harmless complex-phase gypsum is transported to the construction site, paved, supplemented with water (20 - 25% moisture content), and compacted and formed (90% compaction degree), the 7-day compressive strength ≥ 1.5 Mpa, and it can be used as a filling cementing material in filling fields such as ecological restoration project filling, roadbed filling, and industrial site filling projects, while the harmless dihydrate gypsum after being stored in the yard can be used as an inert filler.

[0024] In summary, compared with the prior art, the present invention has the following advantages:

[0025] (1) The present invention can prepare low-calcium underburned lime with a stable effective calcium content of 20-40% by precisely controlling the coal consumption, stone consumption, and sintering temperature and time of the traditional lime sintering process. The low-calcium underburned lime is used as a modifier for hemihydrate gypsum. By utilizing the characteristics of high output temperature and good heat preservation effect of hemihydrate gypsum, the low-calcium underburned lime can increase the susceptibility of the low-calcium underburned lime to free water content and temperature sensitivity during the modification process of hemihydrate gypsum. The low-calcium underburned lime not only has a good effect of solidifying water-soluble acidic ions such as P and F, but also can significantly reduce the pH value, thereby solving the problem that the pH value of the existing modified hemihydrate gypsum used as a filling cementitious material is in a strong alkaline state for a long time during the conversion process due to the low free water content.

[0026] (2) The present invention uses low-calcium unburned lime as a modifier for hemihydrate gypsum. The low-calcium unburned lime is ground into powder by a pulverizer (such as a Raymond mill) for use. The powder has good uniformity, stable effective calcium content, and excellent air flow conveying performance. It can solve the lime-heavy calcium mixed segregation phenomenon of the low-calcium composite modifier in the prior art and ensure the quality uniformity of the modified hemihydrate gypsum.

[0027] (3) In the modification of hemihydrate gypsum, low-calcium underburnt lime powder does not require a high precision gradient of the dosage fluctuation of the feeding equipment used. The present invention increases the requirement for the precision gradient of the dosage fluctuation of the powder feeding equipment from ±0.1% to more than ±1.0%, greatly reducing the pressure of technical transformation on production equipment with high precision requirements.

[0028] (4) In the modification of hemihydrate gypsum, the present invention adds aluminum sulfate as a regulator, which can effectively adjust the pH of the gypsum cementitious material while ensuring that the acidic ions such as water-soluble P and water-soluble F in the hemihydrate gypsum solidified by the modifier are not affected, and can promote the solidification of fluoride ions. In addition, the water-soluble P and water-soluble F will not be secondary dissolved due to the addition of the regulator, further increasing the adjustability of the pH of the modified hemihydrate gypsum.

[0029] In summary, in order to solve the problem that the modified semi-hydrated gypsum cementitious material of the semi-hydrated gypsum with low free moisture content (5-25%) is in a strong alkalinity for a long time, the present invention is based on the good fluorine-fixing effect of CaO dry powder, and prepares a low-calcium underburnt lime for modifying the semi-hydrated gypsum with low free moisture content (5-25%), and supplemented with aluminum sulfate as a regulator, the acidic ions in the modified semi-hydrated gypsum cementitious material can be solidified within 1 to 7 days to meet the requirements of GB / T32124-2024 "Specifications for Treatment and Disposal of Phosphogypsum" that the leachate P is ≤0.5 mg / L, F is ≤10 mg / L, and the pH is 6 to 9, which greatly reduces the storage management cost of the enterprise in the factory, improves the comprehensive utilization efficiency of the semi-hydrated phosphogypsum of the phosphorus chemical enterprise, and reduces the environmental risk of alkali pollution of the cementitious material in the external environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the process flow chart of underburned lime in Embodiment 1 of the present invention. Specific Embodiments

[0031] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below in conjunction with specific embodiments and the accompanying drawings. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0032] Embodiment 1: Preparation of underburned lime powder

[0033] Using Figure 1 the traditional lime sintering process flow shown, the raw material limestone and coal are metered and fed according to the batching ratio. After the limestone undergoes processes such as top preheating, middle insufficient calcination, and bottom cooling in the lime kiln, it discharges ash from the bottom of the kiln through the four-sided ash discharge method, and then enters the Raymond mill (controlling the powder fineness to be ≥85wt% passing through a test sieve with a pore size of 75μm), and finally obtains the finished product of underburned lime powder.

[0034] According to the preparation process parameters shown in Table 1, the underburned lime powder was prepared respectively with reference to the above process flow. The effective calcium content in the underburned lime powder was tested according to T0811 - 1994 "Determination Method of Effective Calcium Oxide in Lime", and the activity (abbreviation: activity) in the underburned lime powder was tested according to YBT 105 - 2014 "Physical Inspection Method for Metallurgical Lime". The results are shown in Table 1.

[0035] Table 1. Preparation process parameters and effective calcium content of underburned lime powder

[0036]

[0037]

[0038] It can be seen from the data in Table 1 that when the calcination temperature is 800 - 1000°C, it is the under - decomposition temperature range of calcium carbonate. Under the condition of the same consumption of limestone, as the consumption of coal decreases, the high - point temperature during calcination is easier to control. That is, when the coal consumption is 0.10t / t 石灰 , the calcination temperature is 1000°C, and the calcination time is 30h, low - calcium lime with an effective calcium content of 39.87% can be obtained; when the coal consumption is reduced by 0.08t / t 石灰 and the calcination temperature is 900°C, and the calcination time is 25h, low - calcium lime with an effective calcium content of 26.27% can be obtained. However, when the coal consumption rises to 0.15t / t 石灰, the temperature will rise to 1100 °C and enter the full decomposition temperature range of calcium carbonate. Even if the sintering time is appropriately reduced, the effective calcium content is easily increased to 55.62%, exceeding the effective calcium control range of 20-40%. At the same time, the coal consumption is excessive, resulting in waste of fuel.

[0039] Example 2: Comparison of the pneumatic conveying uniformity between underburned lime and traditional compound mixture

[0040] The traditional compound mixture is prepared by mixing high-calcium lime blocks sintered according to traditional process parameters (the effective calcium content of the lumpy lime used in the experiment in Table 2 is 75 wt%) and heavy calcium stone in a certain mass ratio (the mass ratio used in the experiment in Table 2 is lumpy lime: heavy calcium stone = 30:70), and then directly entering a Raymond mill for grinding. The mixture of lumpy lime and heavy calcium stone is ground by the grinding roller and grinding ring in the Raymond mill. The powder that meets the particle size requirements is blown up by the fan and enters the cyclone separation system through the air classifier, while the powder that does not meet the particle size requirements continues to stay in the grinding chamber for further grinding. During this process, due to the high true density and low bulk density of the already sintered traditional lumpy lime, and the low true density and high bulk density of the unsintered heavy calcium stone, segregation occurs in the cyclone separation system of the Raymond mill, and the mass of the produced mixture fluctuates greatly. The density fluctuations are shown in Table 2 below.

[0041] Table 2. Density segregation of traditional compound mixture at the outlet of the grinding mill and the feeding port of pneumatic conveying

[0042]

[0043]

[0044] Table 3 shows the density segregation of the underburned lime prepared in Preparation Example 2 at the outlet of the grinding mill and the feeding port of pneumatic conveying.

[0045] Table 3. Density segregation of underburned lime at the outlet of the grinding mill and the feeding port of pneumatic conveying

[0046]

[0047] It can be seen from the data in Table 2 and Table 3 that in the Raymond mill and the pneumatic conveying process of the lime silo, which are the most commonly used in the lime preparation process, compared with the mixed grinding of lime and heavy calcium stone, the underburned lime powder has better uniformity, stable effective calcium content, and no sorting segregation phenomenon, and its pneumatic conveying performance is more excellent.

[0048] Example 3: Comparison of the modification effect of hemihydrate gypsum

[0049] The hemihydrate gypsum was modified with different types of modifiers, and the effects of solidifying P and F ions and the pH value were detected. The hemihydrate gypsum used in the following test examples was a by-product of the wet-process phosphoric acid system of the applicant of the present invention, and its chemical composition included 42-54 wt% of SO 3 、31-36 wt% of CaO, 0-6 wt% of SiO 2 、0.05-1.5 wt% of Al 2 O 3 、0-0.7 wt% of Fe 2 O 3 、0-0.5 wt% of MgO, 0.5-1.3 wt% of total P 2 O 5 、0.1-0.8 wt% of water-soluble P 2 O 5 、0.1-0.9 wt% of total F, 0.01-0.5 wt% of water-soluble F, 0.001-0.3 wt% of K 2 O, 0.01-0.6 wt% of Na 2 O.

[0050] Test Example 1. Traditional lime powder (effective calcium content of 75 wt%) was used as the modifier for hemihydrate gypsum, and the modification effect is shown in Table 4 below.

[0051] Table 4. Modification effect of traditional high-calcium lime powder

[0052]

[0053] Note: The free moisture content of hemihydrate gypsum is 20.00%, and the crystal water is 7.00%; "not detected" means not detected; the units of total P and fluoride are mg / L.

[0054] It can be seen from the data in Table 4 that when traditional lime modifies hemihydrate gypsum, the water-soluble P in it is relatively easy to solidify, while the fluoride ions are difficult to solidify. When the content of traditional lime is 1.8%, the fluoride in its 7-day leaching toxicity is 10.15 mg / L, and the pH is 8.59, and the fluoride exceeds the standard; when the content of traditional lime is 2.0%, the fluoride in its 7-day leaching toxicity is 7.91 mg / L, and the pH is 8.91, both meeting the limit requirements in B / T 32124-2024; when the content of traditional lime is 2.3%, the fluoride in its 7-day leaching toxicity is 7.18 mg / L, meeting the limit requirements in B / T 32124-2024, but the pH is 10.53, exceeding the standard requirements.

[0055] In summary, traditional lime-modified hemihydrate gypsum has excellent results in the early leaching toxicity test, but the dosage ratio needs to be controlled within 1.8 - 2.0%, that is: during industrial application, the accuracy of its feeding equipment needs to be controlled within ±0.1% to meet the requirements of P ≤ 0.5 mg / l, F ≤ 10 mg / l, and pH of 6 - 9 in the leachate after harmless treatment of gypsum in GB / T 32124-2024 "Phosphogypsum Treatment and Disposal Specification" for 3 - 28 days of leaching toxicity.

[0056] Test Example 2. Using digestion-type calcium-based alkaline modifiers (calcium carbide slag, lime milk) as hemihydrate gypsum modifiers, the modification effects are shown in Table 5 below.

[0057] Table 5. Modification effects of traditional digestion-type calcium-based alkaline modifiers

[0058]

[0059]

[0060] Remarks: The free moisture content of hemihydrate gypsum is 20.00%, and the crystal water is 7.00%; "not detected" means not detected; the units of total P and fluoride are mg / L.

[0061] It can be seen from the data in Table 5 that when using calcium carbide slag (effective calcium is 38.91%) to modify hemihydrate gypsum, when the dosage of traditional calcium carbide slag is 4.0%, the fluoride in the 7-day leaching toxicity is 18.99 mg / L, and the pH is 9.87, both of which exceed the limit requirements in GB / T 32124-2024. When the dosage of traditional calcium carbide slag is 5.0%, the fluoride in the 7-day leaching toxicity is 17.91 mg / L, and the pH is 11.68. It can be seen that with the increase of the dosage, fluoride ions are not significantly solidified and reduced in the early stage, while the pH rises rapidly. From the data of 28 days and 60 days, the solidification of fluoride ions mainly occurs in the later curing process.

[0062] When using lime milk (prepared with lime with an effective calcium of 75%) to modify hemihydrate gypsum, when the dosage of traditional lime milk is 2.0%, the fluoride in the 7-day leaching toxicity is 15.77 mg / L, and the pH is 9.76, both of which exceed the limit requirements in GB / T 32124-2024. When the dosage of traditional lime milk is 3.0%, the fluoride in the 7-day leaching toxicity is 14.16 mg / L, and the pH is 10.89. It can be seen that with the increase of the dosage, fluoride ions are not significantly solidified in the early stage, while the pH rises rapidly. Similarly, from the data of 28 days and 60 days, the solidification of fluoride ions mainly occurs in the later curing process.

[0063] It can be seen from the data in Table 4 and Table 5 that the following conclusions can be drawn by comparing digestion-type calcium-based alkaline agents such as calcium carbide slag and lime milk with traditional lime modification:

[0064] 1) When traditional lime and lime milk are at the same mass ratio, although lime milk can quickly increase the initial pH of modified hemihydrate gypsum and has certain advantages in solidifying soluble P, the Ca(OH) in lime milk 2 has a far worse short-term solidification effect on fluoride ions than lime CaO.

[0065] 2) When comparing traditional lime with carbide slag modified hemihydrate gypsum, due to the relatively low effective calcium content (38.91%) of carbide slag, its dosage is higher than that of traditional lime. At the same time, carbide slag is similar to lime milk, and in the early stage, the solidification effect of lime milk on fluoride ions is far worse than that of lime, and long-term curing is required.

[0066] 3) Whether it is carbide slag or lime milk, their short-term solidification effect on fluoride ions is poor. The modified hemihydrate gypsum needs to be cured for a long period (≥28d) for the fluoride ions to be solidified to meet the standard requirements. At the same time, without precise measurement or long-term stacking, it is very difficult to reduce the pH to neutral in the short term while solidifying fluoride ions to be qualified.

[0067] Test Example 3. Using underburned lime as a modifier for hemihydrate gypsum, the modification effect is as described below.

[0068] (1) The modification effect of the underburned lime (effective calcium content is 26.34wt%) prepared in Preparation Example 2 at room temperature of 20°C.

[0069] Table 6. Modification effect of underburned lime at room temperature of 20°C

[0070]

[0071] Remarks: The free moisture content of hemihydrate gypsum is 20.00%, and the crystal water is 7.00%; "not detected" means not detected; the units of total P and fluoride are mg / L.

[0072] It can be seen from the data in Table 6 that the effective calcium content in underburned lime is 26.34%. Compared with traditional high-calcium lime (effective calcium 75%), its dosage is relatively increased. When the dosage is 4 - 6%, the modified curing for 3 - 7d can meet the limit requirements in GB / T32124-2024; when its dosage is 4 - 7%, the modified curing for 3 - 28d can meet the limit requirements in GB / T32124-2024.

[0073] Its advantage lies in industrial application. The dosage accuracy of its feeding equipment can be increased from ±0.1% controlled by traditional lime to ±1.0% of underburned lime. At the same time, the modified hemihydrate gypsum only needs to be cured for 3 - 7d in the early stage to meet the requirements of P≤0.5mg / l, F≤10mg / l, and pH of 6 - 9 in the leachate after harmless treatment of gypsum in B / T32124-2024 "Phosphogypsum Treatment and Disposal Specification".

[0074] (2) Modification effect of underburned lime prepared in Preparation Example 2 (effective calcium content is 26.34 wt%) at a high temperature of 60°C.

[0075] Table 7. Modification effect of underburned lime (effective calcium content is 26.34 wt%) at a high temperature of 60°C

[0076]

[0077] Remarks: Free moisture content of hemihydrate gypsum is 20.00%, crystal water is 7.00%; "not detected" means not detected; units of total P and fluoride are mg / L.

[0078] It can be seen from the data in Table 7 that when traditional lime is used for curing under heat preservation conditions, its pH only decreases slightly. As the curing time prolongs, the trend and amplitude of its further pH decrease are not obvious. When underburned lime is used for curing under heat preservation conditions, due to the good dispersion and large reaction contact area of underburned lime, under the self-heat preservation effect of hemihydrate gypsum, it can fully react with acidic impurities such as water-soluble P and F in it. The low-calcium underburned lime is consumed quickly and the reaction is relatively complete. Under the condition of ensuring that P and F in the leachate meet the standards, the pH can be significantly reduced within 7 days.

[0079] (3) Modification effect of underburned lime with different effective calcium contents at normal temperature of 20°C.

[0080] Table 8. Modification effect of underburned lime with different effective calcium contents at normal temperature of 20°C

[0081]

[0082]

[0083] Remarks: Free moisture content of hemihydrate gypsum is 20.00%, crystal water is 7.00%; "not detected" means not detected; units of total P and fluoride are mg / L; the underburned lime used for 1# is prepared in Preparation Example 3, the underburned lime used for 2# is prepared in Preparation Example 1, and the underburned lime used for 3#-5# is prepared in Preparation Example 4.

[0084] It can be seen from the data in Table 8 that for low-calcium underburned lime (1-2#) with an effective calcium content of 20-40%, the limit requirements in GB / T 32124-2024 can be achieved after 3-7 days of modified curing; while for the underburned lime with an effective calcium content of 55.62% (3#) and a dosage of 6%, the fluoride content in the 28-day leaching toxicity is 5.13 mg / L and the pH is 9.48, with the pH exceeding the standard; when the dosage is 4%, the fluoride content in the 28-day leaching toxicity is 9.00 mg / L and the pH is 8.79, and neither can meet the limit requirements in GB / T 32124-2024 in the short term (3-7 days).

[0085] Test Example 4. Promoting effect of aluminum sulfate as a regulator.

[0086] Table 9. Promoting effect of aluminum sulfate regulator

[0087]

[0088] Remarks: The free moisture content of hemihydrate gypsum is 20.00%, and the crystal water is 7.00%; "not detected" means not detected; the units of total P and fluoride are mg / L; the underburned lime is prepared in Preparation Example 2.

[0089] It can be seen from the data in Table 9 that:

[0090] (1) Aluminum sulfate has a very good effect on reducing the pH in the early leachate, and at the same time can promote the further reduction of fluoride ions. During the use of regulators such as ferrous sulfate, dilute sulfuric acid, and hydrochloric acid, especially in the early stage of leaching toxicity, there will be a phenomenon that the concentrations of P and F ions are reversely dissolved and exceed the standard by 1-2 times. There is no obvious secondary reverse dissolution phenomenon of P and F during the reaction of aluminum sulfate.

[0091] (2) When traditional lime is used for modification, the dosage of aluminum sulfate regulator reaches 2% of the dry basis mass ratio of gypsum.

[0092] (3) When underburned lime is used for modification, the dosage of aluminum sulfate regulator reaches 0.1% of the dry basis mass ratio of gypsum.

[0093] (4) Whether it is underburned lime or traditional lime, if it is necessary to ensure that the fluoride ion solidification is qualified within 1-3 days, an excessive amount needs to be incorporated, but this will cause the pH to exceed the standard. The addition of aluminum sulfate can adjust the pH to meet the requirements of P≤0.5mg / l, F≤10mg / l, and pH of 6-9 in the leachate after harmless treatment of gypsum in B / T32124-2024 "Specification for Treatment and Disposal of Phosphogypsum" within 1-3 days of curing.

[0094] In summary, it can be seen that when the effective calcium content of lime in modified hemihydrate gypsum is higher and the pH value is higher, the higher the dosage of aluminum sulfate regulator is required. However, underburned lime can effectively reduce the dosage of aluminum sulfate regulator, thereby controlling the dosage cost and meeting the standard requirements in B / T32124-2024 "Specification for Treatment and Disposal of Phosphogypsum" within 1-3 days.

[0095] Test Example 5. Modification of underburned lime-aluminum sulfate at low moisture content.

[0096] Table 10. Modification effect of hemihydrate gypsum with different moisture contents

[0097]

[0098] Note: "Not detected" means not detected; the units of total P and fluoride are mg / L; the underburned lime is prepared in Preparation Example 2.

[0099] It can be seen from the data in Table 10 that:

[0100] The modification of underburned lime-aluminum sulfate at low moisture content can achieve the modification of 5-20% of the initial low-free moisture content hemihydrate gypsum within 3-7 days, which can meet the requirements of P≤0.5mg / l, F≤10mg / l, and pH of 6-9 in the leachate after harmless treatment of gypsum in "Code for Treatment and Disposal of Phosphogypsum" (B / T 32124-2024).

[0101] The above is only the preferred implementation mode of the present invention. It should be noted that the above preferred implementation mode should not be regarded as a limitation of the present invention, and the protection scope of the present invention should be subject to the scope defined by the claims. For those of ordinary skill in the art, several improvements and refinements can be made without departing from the spirit and scope of the present invention, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing low-calcium underburnt lime, characterized in that: The lime kiln is used to calcine limestone raw materials to prepare low-calcium underburned lime, wherein the coal consumption of the lime kiln is controlled at 0.05-0.13t / t 石灰 , stone consumption is 0.80~1.70t / t 石灰 The calcination temperature is 800-1000°C, the calcination time is 15-40h, and the calcium carbonate content in the raw material limestone is ≥90wt%.

2. The low-calcium underburnt lime obtained by the preparation method according to claim 1, wherein the effective calcium content of the low-calcium underburnt lime is 20-40wt% and the activity is 100-300ml / 4N-HCl.

3. Use of the low-calcium underburnt lime according to claim 2 in the modification of hemihydrate phosphogypsum.

4. The use according to claim 3, characterized in that: The initial free water content of the hemihydrate phosphogypsum is 5-25wt%, and the crystal water content is ≤9wt%.

5. The use according to claim 3, characterized in that: The modification of hemihydrate phosphogypsum includes the following raw material composition in mass ratio: low-calcium underburnt lime powder: aluminum sulfate: hemihydrate phosphogypsum = (2-15): (0.0-0.3): (84.7-98.0).

6. The use according to claim 5, characterized in that: The aluminum sulfate is selected from aluminum sulfate 18-hydrate.

7. The use according to claim 5, characterized in that: The low-calcium unburned lime powder is obtained by grinding the low-calcium unburned lime in claim 2 through a grinding mill, and the fineness of the low-calcium unburned lime powder is ≥85wt% when passing through a 75μm aperture test sieve.

8. The use according to claim 5, characterized in that: The chemical composition of the hemihydrate phosphogypsum includes 42-54wt% SO3, 31-36wt% CaO, 0-6wt% SiO2, 0.05-1.5wt% Al2O3, 0-0.7wt% Fe2O3, 0-0.5wt% MgO, 0.5-1.3wt% total P2O5, 0.1-0.8wt% water-soluble P2O5, 0.1-0.9wt% total F, 0.01-0.5wt% water-soluble F, 0.001-0.3wt% K2O, and 0.01-0.6wt% Na2O.

9. The use according to claim 5, characterized in that: The modification of the hemihydrate phosphogypsum also includes the following raw material compositions in mass ratio: Activity inhibitor: hemihydrate phosphogypsum = (0.05-0.2): (99.8-99.95), and the activity inhibitor is selected from at least one of glucose and sodium citrate.

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