Method for preparing cryolite from red mud and fluorine-containing solid waste

Abstract

The invention discloses a method for preparing cryolite from red mud and fluorine-containing solid waste, and belongs to the technical field of resource utilization, and the method comprises the following steps: step 1, ball milling: respectively carrying out ball milling on the red mud and the fluorine-containing solid waste, and respectively obtaining red mud powder and fluorine-containing solid waste powder after sieving; 2, mechanical activation: loading the red mud powder and the fluorine-containing solid waste powder into a ball mill, adding clear water, and starting the ball mill to obtain a composite admixture; step 3, leaching: mixing the composite admixture obtained in the step 2 with inorganic acid, stirring and leaching, washing after leaching, and filtering to obtain a leaching solution and leaching residues; and 4, cryolite is prepared, specifically, a neutralizer is added into the leaching solution, stirring is conducted to adjust the pH of the leaching solution, and after the reaction is finished, filtering and washing are conducted to obtain the cryolite. The method for preparing cryolite by using the red mud and the fluorine-containing solid waste is simple and convenient in process operation, is suitable for large-scale production, and can effectively improve the resource utilization additional value of the red mud and the fluorine-containing solid waste.

Description

Technical Field

[0001] The present invention relates to the technical field of resource utilization, and in particular to a method for preparing cryolite by using red mud and fluorine-containing solid waste. Background Art

[0002] Red mud is a solid waste containing iron oxide generated during the production of alumina. For every 1t of alumina produced, 0.8 - 1.5t of red mud will be generated. Currently, the annual output of red mud in China is about 100 million tons, and 90% of it is treated by stacking in dams. Red mud is a harmful substance with high alkalinity and corrosiveness, and a large amount of stacking brings a huge burden to the environment. In recent years, with the gradual increase in the domestic alumina production and the improvement of environmental protection requirements, the cost of red mud stacking has become higher and higher, restricting the development of alumina enterprises. The resource utilization of red mud has become an urgent problem to be solved in the alumina industry in China.

[0003] Fluorine-containing solid waste is a high-risk waste containing fluorides generated in the fields of metallurgy, chemical industry, etc., mainly including fluorite-rich tailings, rare earth tailings, spent potlining of electrolytic aluminum, and carbon slag. In rare earth mining, affected by the similar physical and chemical properties of symbiotic fluorite (CaF2) and rare earth ore, the flotation separation efficiency is low, resulting in the accumulation of tens of millions of tons of rare earth tailings with a fluorine content of 5% - 15% every year; while in the electrolytic aluminum industry, the annual emissions of fluorine-containing hazardous wastes such as carbon slag and spent potlining generated from the overhaul of smelting cells and anode reactions exceed one million tons, and the fluoride concentration is as high as 10% - 30%. At present, most of such solid wastes are mainly landfilled or stockpiled in the open air. However, fluoride ions are easily migrated through rain leaching, causing soil pollution and excessive fluoride in groundwater, seriously threatening the ecological environment. In recent years, with the shortage of fluorine resources and the upgrading of environmental protection policies, efficiently extracting fluorine from solid wastes and converting it into high-value-added products such as cryolite has become an important direction for the refined separation of rare earth tailings and the treatment of hazardous wastes in the electrolytic aluminum industry.

[0004] Combined utilization of the two wastes of red mud and fluorine-containing solid waste can, on the one hand, effectively solve the environmental pollution problems brought by the high alkalinity, corrosiveness of red mud and fluorine-containing solid waste, reduce the harm to soil, water bodies and the surrounding ecology, and reduce the potential safety hazards brought by tailings stacking. On the other hand, directly using the two solid wastes to prepare high-value-added products expands the resource utilization channels, reduces the environmental load of waste stacking, and reduces the environmental governance cost, which is of great significance for promoting resource recycling and green sustainable development.

[0005] Currently, the patent CN201910675919.3 involves the preparation of cryolite by mechanical activation. However, its preparation process requires a high-temperature environment and needs to be carried out under argon protection throughout the process. The equipment requirements are high, the preparation cost is high, the preparation process is long, and the production efficiency is low. It is difficult to achieve large-scale and rapid supply in the face of the increasing market demand for cryolite. Summary of the Invention

[0006] The object of the present invention is to provide a method for preparing cryolite by using red mud and fluorine-containing solid waste, which has simple process operation, is suitable for large-scale production, can effectively increase the added value of red mud and fluorine-containing solid waste, and provides an efficient and feasible comprehensive utilization technical route for red mud and fluorine-containing solid waste, opening up a new path for the efficient production of cryolite.

[0007] To achieve the above object, the present invention provides a method for preparing cryolite by using red mud and fluorine-containing solid waste, comprising the following steps:

[0008] Step 1, ball milling: ball mill red mud and fluorine-containing solid waste respectively, and after sieving, obtain red mud powder and fluorine-containing solid waste powder respectively;

[0009] Step 2, mechanical activation: load the red mud powder and the fluorine-containing solid waste powder into a ball mill, add clear water and start the ball mill to obtain a composite admixture;

[0010] Step 3, leaching: mix the composite admixture obtained in Step 2 with inorganic acid, stir and leach, and after leaching, wash and filter to obtain a leaching solution and a leaching residue.

[0011] Step 4, preparing cryolite: add a neutralizing agent to the leaching solution and stir to adjust the pH of the leaching solution, and after the reaction ends, filter and wash to obtain cryolite.

[0012] Preferably, in Step 1, both the red mud and the fluorine-containing solid waste are ball milled to a particle size less than 74 μm, the mesh number of the sieve is 150 - 300 meshes, and the un-sieved materials are returned to the ball mill for ball milling again.

[0013] Preferably, in Step 2, the red mud powder and the fluorine-containing solid waste powder are loaded into the ball mill according to the ratio of A:F, where A is the sum of the molar number a of aluminum element in each ton of red mud to be treated and the molar number b of aluminum element in the fluorine-containing solid waste, that is, A = a + b, F is the molar number of fluorine element in each ton of fluorine-containing solid waste to be treated, A:F = 1:3 - 8, when adding clear water, according to the ratio of adding 1 - 25 L of water per kilogram of material, the rotation speed of the ball mill is 100 - 1000 r / min, and the ball milling time is 30 - 240 min.

[0014] Preferably, in Step 3, the inorganic acid is one or more of hydrochloric acid, sulfuric acid and nitric acid, where the concentration of hydrochloric acid is 10 - 37%, the volume ratio of the added hydrochloric acid to the volume of clear water in Step 2 is 1:1 - 7, the concentration of sulfuric acid is 20 - 98%, the volume ratio of the added sulfuric acid to the volume of clear water in Step 2 is 1:1 - 5, the concentration of nitric acid is 20 - 68%, the volume ratio of the added nitric acid to the volume of clear water in Step 2 is 1:1 - 7, and after adding the inorganic acid, the ratio of the material to the liquid is maintained as: 1.3 - 50 L of liquid per kilogram of material.

[0015] Preferably, in step three, the leaching temperature is 20 - 120 °C, the leaching time is 30 - 180 min, and the stirring rate is 100 - 1000 r / min.

[0016] Preferably, in step four, the neutralizing agent is one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonium bicarbonate, ammonia water, calcium hydroxide, calcium oxide, magnesium oxide, dolomite powder, and calcium carbonate. The neutralizing agent is prepared into an aqueous solution or suspension, with a concentration of 0.5 - 1 mol / L, the pH is adjusted to 0.5 - 2, the stirring rate is 200 - 600 rpm, and the reaction time is 20 - 120 min.

[0017] Preferably, in step four, when the molar ratio of fluorine element to sodium element F:N > 0.5 in the composite admixture obtained in step two, first use one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, and sodium chloride as the sodium source and neutralizing agent to adjust the pH to 0.5, and the usage amount is calculated according to F:(N + X) = 1:0.5 - 0.8, where F is the molar number of fluorine element in the composite admixture, N is the molar number of sodium element in the composite admixture, and X is the molar number of sodium element in the neutralizing agent; subsequently, use a sodium-free neutralizing agent to continue adjusting the pH to 0.5 - 2;

[0018] When F:N ≤ 1:0.5, use a sodium-free neutralizing agent, select one or more of ammonium bicarbonate, ammonia water, calcium hydroxide, calcium oxide, magnesium oxide, dolomite powder, and calcium carbonate, and adjust the pH to 0.5 - 2.

[0019] Therefore, the method for preparing cryolite using red mud and fluorine-containing solid waste as described above in the present invention has the following beneficial effects:

[0020] (1) Using low-cost red mud and fluorine-containing solid waste as raw materials to prepare cryolite not only solves the problem of solid waste stacking pollution but also creates high-value products;

[0021] (2) Using mechanical activation to promote the reaction between sodium hydroxide and sodium oxide in red mud and fluorine-containing compounds such as calcium fluoride and aluminum fluoride in fluorine-containing solid waste to generate sodium fluoride that is easily soluble in water and calcium hydroxide or aluminum hydroxide that is insoluble in water;

[0022] (3) Fluoride ions in solid waste usually need to be separated from the solid waste in a strong acid environment, which causes problems of strong acid corrosion of equipment. In the present invention, since red mud is a strong alkaline solid waste, the whole process of mechanical activation of red mud and fluorine-containing solid waste is in an alkaline environment, avoiding the loss of equipment caused by strong acid.

[0023] The technical solution of the present invention will be further described in detail below through the accompanying drawings and examples. Description of the Drawings

[0024] Figure 1It is the front view preparation flowchart of an embodiment of the method for preparing cryolite using red mud and fluorine-containing solid waste in the present invention. Detailed implementation mode

[0025] The technical solution of the present invention will be further described below through the accompanying drawings and embodiments.

[0026] Unless otherwise defined, the technical terms or scientific terms used in the present invention should have the ordinary meaning understood by those of ordinary skill in the field to which the present invention belongs.

[0027] As Figure 1 shown, the present invention provides a method for preparing cryolite using red mud and fluorine-containing solid waste, including the following steps:

[0028] Step 1, ball milling: Ball mill red mud and fluorine-containing solid waste respectively, and after sieving, obtain red mud powder and fluorine-containing solid waste powder respectively.

[0029] Among them, both red mud and fluorine-containing solid waste are ball milled to a particle size less than 74 μm, the mesh number of the sieve is 150 - 300 meshes, and the un-sieved materials are returned to the ball mill for ball milling again.

[0030] Step 2, mechanical activation: Load the red mud powder and fluorine-containing solid waste powder into the ball mill, add clear water and start the ball mill to obtain a composite admixture.

[0031] Among them, the red mud powder and the fluorine-containing solid waste powder are loaded into the ball mill according to the ratio of A:F, where A is the sum of the molar number a of aluminum element in each ton of red mud to be treated and the molar number b of aluminum element in the fluorine-containing solid waste, that is, A = a + b, F is the molar number of fluorine element in each ton of fluorine-containing solid waste to be treated, A:F = 1:3 - 8, when adding clear water, it is in the ratio of adding 1 - 25 L of water per kilogram of material, the rotation speed of the ball mill is 100 - 1000 r / min, and the ball milling time is 30 - 240 min.

[0032] Step 3, leaching: Mix the composite admixture obtained in step 2 with inorganic acid, stir and leach, and after leaching, wash and filter to obtain a leachate and a leached residue.

[0033] Among them, the inorganic acid is one or more of hydrochloric acid, sulfuric acid and nitric acid. Among them, the concentration of hydrochloric acid is 10 - 37%, the volume ratio of the added hydrochloric acid to the volume of clear water in step 2 is 1:1 - 7, the concentration of sulfuric acid is 20 - 98%, the volume ratio of the added sulfuric acid to the volume of clear water in step 2 is 1:1 - 5, the concentration of nitric acid is 20 - 68%, the volume ratio of the added nitric acid to the volume of clear water in step 2 is 1:1 - 7, and after adding the inorganic acid, the ratio of the material to the liquid is maintained as: 1.3 - 50 L of liquid is prepared per kilogram of material.

[0034] The leaching temperature is 20 - 120 °C, the leaching time is 30 - 180 min, and the stirring rate is 100 - 1000 r / min.

[0035] Step 4. Preparation of cryolite: Add a neutralizing agent to the leaching solution and stir to adjust the pH of the leaching solution. After the reaction is completed, filter and wash to obtain cryolite.

[0036] Among them, the neutralizing agent is one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonium bicarbonate, ammonia water, calcium hydroxide, calcium oxide, magnesium oxide, dolomite powder, and calcium carbonate. The neutralizing agent is prepared into an aqueous solution or suspension, and its concentration is 0.5 - 1 mol / L. Adjust the pH to 0.5 - 2, the stirring rate is 200 - 600 rpm, and the reaction time is 20 - 120 min.

[0037] Generally, there is no fluorine in red mud. Therefore, the F value of the fluorine-containing solid waste and the composite admixture is the same. When the molar ratio of fluorine element to sodium element in the composite admixture obtained in Step 2, F:N > 0.5, first use one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, and sodium chloride as the sodium source and neutralizing agent to adjust the pH to 0.5, and the dosage is calculated according to F:(N + X) = 1:0.5 - 0.8, where F is the molar number of fluorine element in the composite admixture, N is the molar number of sodium element in the composite admixture, and X is the molar number of sodium element in the neutralizing agent; subsequently, use a sodium-free neutralizing agent to continue adjusting the pH to 0.5 - 2;

[0038] When F:N ≤ 1:0.5, use a sodium-free neutralizing agent, select one or more of ammonium bicarbonate, ammonia water, calcium hydroxide, calcium oxide, magnesium oxide, dolomite powder, and calcium carbonate, and adjust the pH to 0.5 - 2.

[0039] Traditional methods usually require separating fluoride ions from the solid phase in a strong acidic environment, which not only has a high cost but also has high requirements for the corrosion resistance of equipment and is difficult to achieve industrial utilization. Through mechanical activation, it promotes the reaction between sodium hydroxide and sodium oxide in red mud and fluoride-containing compounds such as calcium fluoride and aluminum fluoride in the fluorine-containing solid waste to generate sodium fluoride that is easily soluble in water and calcium hydroxide or aluminum hydroxide that is insoluble in water, enabling the efficient dissolution of fluoride ions under alkaline conditions while avoiding the corrosion problem of the equipment in an acidic environment. Subsequently, an inorganic acid solution is used to leach the aluminum element in the mixed admixture, so that the key elements (Na, Al, F) required for cryolite synthesis are fully dissolved into the leaching solution system; finally, the pH value of the leaching solution is regulated by a neutralizing agent to induce the selective precipitation of cryolite (Na3AlF6) crystals.

[0040] Example 1

[0041] The present invention provides a method for preparing cryolite using red mud and fluorine-containing solid waste, including the following steps:

[0042] (1) Grind 100 kg of red mud and 100 kg of fluorite-rich tailings separately until the particle size of the materials passes the 200-mesh sieve standard, obtaining red mud powder and fluorine-containing solid waste powder.

[0043] The specific components of the red mud and fluorine-containing solid waste used to prepare cryolite in this example are shown in Tables 1 and 2.

[0044] Table 1 Chemical composition of red mud used in Example 1, mass fraction %

[0045]

[0046] Table 2 Chemical composition of fluorite-rich tailings used in Example 1, mass fraction %

[0047] Chemical composition <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[Sc2O3]]> MgO <![CDATA[CaF2]]> CaO Mass fraction 0.29 0.41 8.72 1.61 88.62 1.03

[0048] (2) Add the ground red mud powder and fluorine-containing solid waste powder from the first step together with 300 L of clear water into a ball mill and mechanically activate for 150 min under the condition of a rotation speed of 600 rpm.

[0049] (3) Obtain a composite admixture through mechanical activation. Load the composite admixture into an acid leaching reactor, then add 200 L of 37% concentrated hydrochloric acid, and carry out leaching at 100 °C for 120 min. During the leaching process, continuously stir at a constant speed of 300 rpm. After leaching is completed, filter and wash in sequence to obtain a leaching solution and leaching residue.

[0050] (4) Add 4.6 kg of sodium chloride powder to the leaching solution. Then use calcium hydroxide to adjust the pH value of the solution to 1 and stir and react at a constant speed of 200 rpm for 1 h. After the reaction is completed, filter and wash in sequence to obtain 75.3 kg of cryolite.

[0051] Example 2

[0052] The present invention provides a method for preparing cryolite using red mud and fluorine-containing solid waste, comprising the following steps:

[0053] (1) Grind 100 kg of red mud and 200 kg of spent potlining from aluminum electrolysis separately until the particle size of the materials passes the 200-mesh sieve standard, obtaining red mud powder and fluorine-containing solid waste powder.

[0054] The specific components of the red mud and fluorine-containing solid waste used to prepare cryolite in this example are shown in Tables 3 and 4.

[0055] Table 3 Chemical composition of red mud used in Example 2, mass fraction %

[0056]

[0057] Table 4 Chemical composition of the aluminum electrolysis overhaul slag used in Example 2, mass fraction %

[0058] Chemical composition <![CDATA[Al2O3]]> <![CDATA[Na2O]]> <![CDATA[Fe2O3]]> MgO <![CDATA[CaF2]]> CaO Mass fraction 15.64 25.16 4.29 0.28 50.69 3.08

[0059] (2) Add the red mud powder and fluorine-containing solid waste powder after the first-step grinding into a ball mill together with 500 L of clear water, and mechanically activate for 130 min under the rotational speed condition of 650 rpm.

[0060] (3) Obtain the composite admixture through mechanical activation. Load the composite admixture into an acid leaching reactor, then add 200 L of 80% sulfuric acid, and carry out leaching at 110 °C for 120 min. During the leaching process, continuously stir at a uniform speed of 800 rpm. After the leaching is completed, filter and wash in sequence to obtain the leaching solution and the leaching residue.

[0061] (4) Use ammonia water to adjust the pH of the leaching solution to 1, and stir and react at a uniform speed of 200 rpm for 1 h. After the reaction is completed, filter and wash in sequence to obtain 206.3 kg of cryolite.

[0062] Example 3

[0063] The present invention provides a method for preparing cryolite by using red mud and fluorine-containing solid waste, comprising the following steps:

[0064] (1) Carry out ball milling treatment on 80 kg of red mud and 150 kg of fluorite-rich tailings respectively until the particle size of the materials passes through the 200-mesh sieve standard to obtain red mud powder and fluorine-containing solid waste powder.

[0065] The specific components of the red mud and fluorite-rich tailings used in this example for preparing cryolite are shown in Table 5 and Table 6 as follows.

[0066] Table 5 Chemical composition of the red mud used in Example 3, mass fraction %

[0067]

[0068] Table 6 Chemical composition of the fluorite-rich tailings used in Example 3, mass fraction %

[0069] <![CDATA[Al2O3]]> <![CDATA[SiO2]]> <![CDATA[Fe2O3]]> MgO <![CDATA[CaF2]]> CaO 0.16 5.24 1.12 4.22 76.62 11.63

[0070] (2) Add the red mud powder and fluorine-containing solid waste powder after the previous-step grinding into a ball mill together with 300 L of clear water, and mechanically activate for 60 min under the rotational speed condition of 650 rpm.

[0071] (3) The composite admixture is obtained by mechanical activation. The composite admixture is loaded into an acid leaching reactor, and then 200 L of 50% nitric acid is added. Leaching is carried out at 100 °C for 120 min, and during the leaching process, uniform stirring is continuously carried out at a speed of 300 rpm. After the leaching is completed, filtration and washing treatments are carried out in sequence to obtain a leaching solution and leaching residues.

[0072] (4) 70 kg of sodium chloride powder is added to the leaching solution. Subsequently, the pH of the leaching solution is adjusted to 1.5 using ammonia water, and uniform stirring reaction is carried out at a speed of 200 rpm for 1 h. After the reaction is completed, filtration and washing are carried out in sequence to obtain 87.3 kg of cryolite.

[0073] Therefore, the present invention adopts the above method for preparing cryolite using red mud and fluorine-containing solid waste. The process operation is simple and suitable for large-scale production, which can effectively improve the added value of red mud and fluorine-containing solid waste, and provide an efficient and feasible technical path for the comprehensive utilization of red mud and fluorine-containing solid waste, opening up a new path for the efficient production of cryolite.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing cryolite using red mud and fluorine-containing solid waste, characterized in that: The following steps are involved: Step 1, ball milling: ball milling the red mud and the fluorine-containing solid waste respectively, and sieving to obtain red mud powder and fluorine-containing solid waste powder respectively; Step 2: Mechanical activation: red mud powder and fluorine-containing solid waste powder are loaded into a ball mill, clean water is added and the ball mill is started to obtain a composite admixture; Step 3, leaching: mixing the composite admixture obtained in step 2 with an inorganic acid, stirring and leaching, and washing and filtering after leaching to obtain a leachate and a leaching residue; Step 4, preparing cryolite: adding a neutralizing agent into the leachate and stirring to adjust the pH of the leachate, filtering and washing after the reaction to obtain cryolite.

2. The method for preparing cryolite using red mud and fluorine-containing solid waste according to claim 1, characterized in that: In step 1, red mud and fluorine-containing solid waste are ball-milled to a particle size of less than 74 μm, and the mesh size of the sieve is 150-300 meshes, and the unscreened material is returned to the ball mill for ball milling.

3. The method for preparing cryolite using red mud and fluorine-containing solid waste according to claim 1, characterized in that: In step 2, red mud powder and fluorine-containing solid waste powder are loaded into a ball mill in a ratio of A:F, wherein A is the sum of the molar number a of aluminum element in each ton of red mud to be treated and the molar number b of aluminum element in the fluorine-containing solid waste, that is, A=a+b, F is the molar number of fluorine element in each ton of fluorine-containing solid waste to be treated, A:F=1:3-8, and when adding clean water, 1-25L of water is added per kilogram of material. The speed of the ball mill is 100-1000r / min, and the ball milling time is 30-240min.

4. The method for preparing cryolite using red mud and fluorine-containing solid waste according to claim 1, characterized in that: In step three, the inorganic acid is one or more of hydrochloric acid, sulfuric acid and nitric acid, wherein the concentration of hydrochloric acid is 10-37%, the volume ratio of hydrochloric acid added to the volume of clean water in step two is 1:1-7, the concentration of sulfuric acid is 20-98%, the volume ratio of sulfuric acid added to the volume of clean water in step two is 1:1-5, the concentration of nitric acid is 20-68%, the volume ratio of nitric acid added to the volume of clean water in step two is 1:1-7, and after adding the inorganic acid, the ratio of material to liquid is maintained at: 1.3-50L liquid per kilogram of material.

5. The method for preparing cryolite using red mud and fluorine-containing solid waste according to claim 1, characterized in that: In step 3, the leaching temperature is 20-120° C., the leaching time is 30-180 min, and the stirring rate is 100-1000 r / min.

6. The method for preparing cryolite using red mud and fluorine-containing solid waste according to claim 1, characterized in that: In step 4, the neutralizing agent is one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonium bicarbonate, ammonia water, calcium hydroxide, calcium oxide, magnesium oxide, dolomite powder and calcium carbonate. The neutralizing agent is prepared into an aqueous solution or suspension with a concentration of 0.5-1 mol / L, the pH is adjusted to 0.5-2, the stirring rate is 200-600 rpm, and the reaction time is 20-120 min.

7. The method for preparing cryolite using red mud and fluorine-containing solid waste according to claim 6, characterized in that: In step 4, when the molar ratio F:N of the fluorine element and the sodium element in the composite admixture obtained in step 2 is greater than 0.5, first use one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, and sodium chloride as a sodium source and a neutralizing agent to adjust the pH to 0.5, and the amount used is calculated according to F:(N+X)=1:0.5-0.8, wherein F is the number of moles of the fluorine element in the composite admixture, N is the number of moles of the sodium element in the composite admixture, and X is the number of moles of the sodium element in the neutralizing agent; then, use a neutralizing agent that does not contain sodium to continue adjusting the pH to 0.5-2; When F:N≤1:0.5, use a sodium-free neutralizer, select one or more of ammonium bicarbonate, ammonia water, calcium hydroxide, calcium oxide, magnesium oxide, dolomite powder and calcium carbonate, and adjust the pH to 0.5-2.

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