Quality-divided crystallization method of salt solution containing calcium and magnesium ions

Abstract

The invention discloses a quality-divided crystallization method of a salt solution containing calcium and magnesium ions, which adopts a quality-divided crystallization process technology (to realize separation of calcium and magnesium ions), generates byproducts with high added value, realizes recycling of valuable resources, not only avoids the problems of environmental pollution and secondary solid waste, but also can generate good economic value.

Description

Technical Field

[0001] The present application relates to the environmental protection field of resource recycling, and in particular to a method for fractional crystallization of a salt solution containing calcium and magnesium ions. Background Art

[0002] Wastewater salt solutions containing calcium and magnesium ions (calcium chloride and magnesium chloride) are commonly found in the following areas:

[0003] (1) Seawater carbonization process

[0004] A large amount of concentrated brine will be produced during the seawater desalination process. The brine contains high concentrations of calcium, magnesium ions, etc.

[0005] (2) High-salt wastewater directly discharged during industrial production

[0006] The inorganic salts in this type of wastewater mainly come from industrial wastewater and domestic sewage, including potassium ions, calcium ions, calcium ions, sodium ions, chloride ions, sulfate ions, etc.

[0007] (3) Brine produced by recycling industrial wastewater

[0008] For example, industrial industries with large discharge volumes, such as steel companies, coal chemical industry, and petroleum, need to recycle most of the water for reuse during the production process. This process will produce brine of a certain concentration, containing calcium, magnesium, potassium, sodium, chloride ions, carbonate ions, etc.

[0009] (4) Chemical production and reclaimed water treatment process

[0010] In these processes, the removal of calcium and magnesium ions is an important process unit, involving a variety of methods for removing calcium and magnesium ions, such as heating softening, chemical softening, ion exchange and membrane separation to produce wastewater containing calcium chloride and magnesium chloride.

[0011] (5) Desulfurization wastewater from coal-fired power plants

[0012] The desulfurization concentrated wastewater from coal-fired power plants has high concentrations of magnesium, calcium, sulfate and chloride ions, and special treatment methods are required to remove these ions.

[0013] (6) Industrial wastewater treatment

[0014] Wastewater generated in various industrial processes often contains harmful substances such as organic matter and heavy metals. The application of magnesium chloride can realize customized treatment solutions for different wastewater components.

[0015] If the wastewater generated in these fields is discharged directly without treatment, it will cause environmental pollution. If a simple treatment process is used, it will produce secondary solid waste problems. Summary of the invention

[0016] The purpose of the present invention is to provide a method for fractional crystallization of a salt solution containing calcium and magnesium ions, adopt a fractional crystallization process technology (to achieve separation of calcium and magnesium ions) and generate high value-added by-products, realize the recycling of valuable resources, not only avoid environmental pollution and secondary solid waste problems, but also can generate good economic value.

[0017] To achieve the above object, the present invention provides the following technical solutions.

[0018] The present application discloses a method for fractional crystallization of a salt solution containing calcium and magnesium ions, which comprises the following steps in sequence:

[0019] S01 uses a filter to remove trace insoluble impurities in the calcium and magnesium ion salt solution. The purified liquid is crystallized by liquid-solid reaction with sodium carbonate. The crystallization temperature is 20-40°C. The molar ratio of the added amount of sodium carbonate to the calcium and magnesium ions in the purified liquid is 1.00-1.05:1 to ensure complete precipitation of calcium and magnesium ions.

[0020] S02: the double salt precipitates of calcium carbonate and magnesium carbonate generated by the reaction crystallization in step S01 are subjected to solid-liquid separation and washing, and the wet materials enter the carbonization section; dilute hydrochloric acid is used to adjust the pH of the separated mother liquor to 6.0-7.0, and then the MVR evaporation crystallization process is used to evaporate and crystallize sodium chloride to obtain industrial grade sodium chloride with a purity greater than 97.5%;

[0021] S03: uniformly mix the wet material of the reaction crystallization in step S02 with the steam condensed water of the sodium chloride MVR evaporation crystallization, with a solid content of 50-150 g / L, control the pH of the slurry liquid to be 9.0-11.0, continuously introduce carbon dioxide gas for carbonization reaction, and convert the double salt precipitates of calcium carbonate and magnesium carbonate into soluble magnesium bicarbonate and insoluble calcium carbonate, the carbonization reaction temperature is 20-40° C., and the pH value adjustment solvent is one of sodium hydroxide and calcium hydroxide or a combination thereof;

[0022] S04: solid-liquid separation and washing of the slurry liquid after the carbonization reaction in step S03 to obtain high-purity light calcium carbonate with a purity greater than 97.0%;

[0023] In step S05, the separated mother liquor in step S04 is pyrolyzed and crystallized to prepare high-purity basic magnesium carbonate through solid-liquid separation and washing. The pyrolysis and crystallization temperature is 90.0-100.0°C. The carbon dioxide gas generated by the pyrolysis is sent to step S03 for recycling through a fan.

[0024] Preferably, in the above-mentioned fractional crystallization method of the calcium and magnesium ion-containing salt solution, it includes:

[0025] Reaction crystallization, filtration separation, acidification and MVR evaporation crystallization system, used for the reaction crystallization of calcium and magnesium ion salt solution and sodium carbonate in the fractionation crystallization process technology system of calcium and magnesium ion salt solution, including solid-liquid separation and washing of reaction crystallization product, pH adjustment of filtrate and MVR evaporation crystallization process of sodium chloride solution;

[0026] Carbonization and light calcium carbonate solid-liquid separation system, used for the slurry configuration and carbonization process of the reaction crystallization product of the fractional crystallization process technology system containing calcium and magnesium ion salt solution and the solid-liquid separation process of light calcium carbonate;

[0027] The pyrolysis and basic magnesium carbonate solid-liquid separation system is used in the process of preparing basic magnesium carbonate by pyrolysis and crystallization of the carbonization separation mother liquor of the fractionation crystallization process technology system containing calcium and magnesium ion salt solution.

[0028] Preferably, in the above-mentioned fractional crystallization method of the calcium and magnesium ion salt solution, the calcium and magnesium ion salt solution is filtered to remove trace insoluble impurities, sodium carbonate is added for reaction and crystallization to generate a double salt precipitate of calcium carbonate and magnesium carbonate, the double salt precipitate is solid-liquid separated and washed, the mother liquor is acidified, and the sodium chloride is evaporated and crystallized by MVR, and the chemical reaction equation involved is as follows:

[0029] CaCl2+MgCl2+2Na2CO3=MgCO3·CaCO3↓(double salt)+4NaCl.

[0031] Preferably, in the above-mentioned fractional crystallization method of the calcium and magnesium ion salt solution, the process of the carbonization and light calcium carbonate solid-liquid separation system includes introducing carbon dioxide under controlled pH conditions to convert the double salt precipitate of calcium carbonate and magnesium carbonate into soluble magnesium bicarbonate and insoluble calcium carbonate, and realizing calcium and magnesium separation after solid-liquid separation. The chemical reaction equation involved is as follows:

[0032] MgCO3·CaCO3+CO2+H2O=Mg(HCO3)2+CaCO3↓.

[0034] Preferably, in the above-mentioned fractional crystallization method of the calcium and magnesium ion-containing salt solution, the process of preparing basic magnesium carbonate by pyrolysis and crystallization of the carbonization separation mother liquor involves the following chemical reaction equation:

[0035] 5Mg(HCO3)2=4MgCO3·Mg(OH)2·4H2O↓+6CO2↑.

[0037] Compared with the existing technology, the advantage of this technical solution is that it realizes the recycling of valuable resources, which not only avoids environmental pollution and secondary solid waste problems, but also can generate good economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0039] Figure 1 It is a schematic diagram of the fractional crystallization method of the salt solution containing calcium and magnesium ions in an embodiment of the present invention;

[0040] Figure 2 Shown is the XRD pattern of the sodium carbonate reaction crystallization product of Example 1 of the present invention;

[0041] Figure 3 Shown is the XRD pattern of the pyrolysis crystallization product of Example 1 of the present invention. DETAILED DESCRIPTION

[0042] The following will describe the technical solutions in the embodiments of the present invention in detail in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] Combination Figure 1 As shown, the method for fractional crystallization of a salt solution containing calcium and magnesium ions, the system includes: purification of calcium and magnesium raw material liquid, sodium carbonate reaction crystallization, filtration separation, acidification and MVR evaporation crystallization; carbonization and light calcium carbonate solid-liquid separation; pyrolysis and basic magnesium carbonate solid-liquid separation system:

[0044] (1) Reaction crystallization, filtration separation, acidification and MVR evaporation crystallization system: It is applicable to the reaction crystallization system of calcium and magnesium ion salt solution and sodium carbonate in the fractional crystallization process technology system containing calcium and magnesium ion salt solution (calcium chloride and magnesium chloride system), including solid-liquid separation and washing of the reaction crystallization product, pH adjustment of the filtrate and MVR evaporation crystallization process of sodium chloride solution.

[0045] (2) Carbonization and light calcium carbonate solid-liquid separation system: Applicable to the core preparation process technology of the reaction crystallization product slurry configuration and carbonization process of the fractional crystallization process technology system containing calcium and magnesium ion salt solution (calcium chloride and magnesium chloride system) and the solid-liquid separation of light calcium carbonate.

[0046] (3) Pyrolysis and basic magnesium carbonate solid-liquid separation system: Applicable to the process of preparing basic magnesium carbonate by pyrolysis and crystallization of the carbonization separation mother liquor of the fractionation and crystallization process technology system containing calcium and magnesium ion salt solution (calcium chloride and magnesium chloride system).

[0047] The reaction crystallization system includes a precision filter containing a calcium and magnesium ion salt solution (calcium chloride and magnesium chloride system) to remove trace insoluble impurities, adding sodium carbonate for reaction crystallization to generate a complex salt precipitate of calcium carbonate and magnesium carbonate (the composition of the complex salt is related to the composition ratio of calcium and magnesium ions in the original aqueous solution), solid-liquid separation and washing of the complex salt precipitate, acidification of the separation mother liquor (pH adjustment) and MVR evaporation crystallization of sodium chloride. The chemical reaction equation involved is as follows:

[0048] CaCl2+MgCl2+2Na2CO3====MgCO3·CaCO3↓(double salt)+4NaCl

[0049] The carbonization and light calcium carbonate solid-liquid separation process system includes introducing carbon dioxide under certain pH conditions to convert the complex salt precipitates of calcium carbonate and magnesium carbonate into soluble magnesium bicarbonate and insoluble calcium carbonate, and realizes calcium and magnesium separation after solid-liquid separation. The chemical reaction equation involved is as follows:

[0050] MgCO3·CaCO3+CO2+H2O====Mg(HCO3)2+CaCO3↓

[0051] The process of preparing basic magnesium carbonate by pyrolysis and crystallization of carbonization separation mother liquor involves the following chemical reaction equation:

[0052] 5Mg(HCO3)2====4MgCO3·Mg(OH)2·4H2O↓+6CO2↑

[0053] Example 1

[0054] Take the concentrated brine after desulfurization pretreatment in a coal-fired power plant as an example, which contains 40.57 g / L (~3.8%) of magnesium chloride, 3.81 g / L (~0.36%) of sodium chloride, and 20.71 g / L (~1.94%) of calcium chloride.

[0055] (1) Reaction crystallization, filtration separation, acidification and MVR evaporation crystallization: Use precision ceramic filters (filtration accuracy of 0.1-0.15 μm) to remove trace insoluble impurities in the calcium chloride / magnesium chloride / sodium chloride salt solution. The purified liquid is reacted with sodium carbonate for liquid-solid reaction crystallization. The crystallization temperature is 30°C. The molar ratio of the amount of sodium carbonate added to the calcium and magnesium ions in the purified liquid is 1.03:1.00 (nCa 2+ +nMg 2+) to ensure that calcium and magnesium ions are completely precipitated; the complex salt precipitates of calcium carbonate and magnesium carbonate generated by the reaction crystallization are washed after solid-liquid separation, and the wet materials enter the carbonization section; dilute hydrochloric acid is used to adjust the pH of the separated mother liquor to 6.5 (to neutralize excess carbonate ions), and then the MVR evaporation crystallization process is used to evaporate and crystallize sodium chloride to obtain industrial grade sodium chloride with a purity of 98.0%. Figure 2 It can be known that the calcium carbonate and magnesium carbonate complex salts generated by the reaction crystallization are precipitated crystals of magnesian calcite (main component CaMg(CO3)2). The XRD phase structure shows that the precipitation crystallization product is mainly magnesian calcite, and also contains a small amount of other substances, which may be high-magnesium dolomite (main component Mg3Ca(CO3)4). The main reason for the appearance of this phase is that the molar ratio of magnesium ions and calcium ions in the raw materials is not 1:1.

[0056] (2) Carbonization and solid-liquid separation of light calcium carbonate: The wet material of sodium carbonate reaction crystallization is evenly mixed with the steam condensate of sodium chloride MVR evaporation crystallization, with a solid content of 100 g / L. The pH of the slurry liquid is controlled to be 10.5, and carbon dioxide gas is continuously introduced for carbonization reaction. The carbonization reaction temperature is 30°C, and the pH value adjustment solvent is calcium hydroxide. The slurry liquid after the carbonization reaction is completed is subjected to solid-liquid separation and washing to obtain high-purity light calcium carbonate with a purity of 98.0%.

[0057] (3) Pyrolysis and solid-liquid separation system of basic magnesium carbonate: The carbonization separation mother liquor (magnesium bicarbonate solution) is pyrolyzed and crystallized, and then solid-liquid separation and washing are performed to prepare high-purity basic magnesium carbonate with a purity of 99.2%. Figure 3 The carbon dioxide gas produced by pyrolysis at a temperature of 95.0°C is sent to the carbonization section for recycling through a fan.

[0058] For Example 1, the economic benefit analysis of a single ton of basic magnesium carbonate in the process system of the present invention is as follows:

[0059] (1) Raw material consumption per ton of basic magnesium carbonate:

[0060] Wastewater: 24.16m 3

[0061] Sodium carbonate: 1600.4kg

[0062] Carbon dioxide: ~485kg

[0063] (2) By-products produced per ton of basic magnesium carbonate:

[0064] Calcium carbonate: 450.8kg

[0065] Sodium chloride: 1731.9kg

[0066] Carbon dioxide: ~543.6 kg

[0067] (3) Cost per ton (excluding equipment depreciation, labor, marketing, management, etc.):

[0068] Raw material cost: 1.6*2000+100=3300 yuan

[0069] Revenue from light calcium carbonate and sodium chloride: 0.45*800+1.731*400=1052.4 yuan

[0070] Energy consumption cost: 1350 yuan

[0071] Basic magnesium carbonate sales price: 6500 yuan

[0072] (4) Gross profit: 1052.4 + 6500 - 3300 - 1350 = 2902.4 yuan

[0073] The above analysis shows that the process technology of the present invention not only avoids environmental pollution and secondary solid waste problems, but also prepares high value-added basic magnesium carbonate, light calcium carbonate and industrial-grade sodium chloride, which can generate good economic value.

[0074] Example 2

[0075] Take the washing water of membrane separation process in chemical production as an example, the magnesium chloride is 28.5g / L and the calcium chloride is 33.2g / L.

[0076] (1) Reaction crystallization, filtration separation, acidification and MVR evaporation crystallization: Use precision ceramic filters (filtration accuracy of 0.1-0.15 μm) to remove trace insoluble impurities in the calcium chloride / magnesium chloride / sodium chloride salt solution. The purified liquid is reacted with sodium carbonate for liquid-solid reaction crystallization. The crystallization temperature is 30°C. The molar ratio of the amount of sodium carbonate added to the calcium and magnesium ions in the purified liquid is 1.05:1.00 (nCa 2+ +nMg 2+ ) to ensure that calcium and magnesium ions are completely precipitated; the complex salt precipitates of calcium carbonate and magnesium carbonate generated by the reaction crystallization are washed after solid-liquid separation, and the wet materials enter the carbonization section; dilute hydrochloric acid is used to adjust the pH of the separated mother liquor to 6.2 (to neutralize excess carbonate ions), and then the MVR evaporation crystallization process is used to evaporate and crystallize sodium chloride to obtain industrial grade sodium chloride with a purity of 98.5%.

[0077] (2) Carbonization and solid-liquid separation of light calcium carbonate: The wet material of sodium carbonate reaction crystallization is evenly mixed with the steam condensate of sodium chloride MVR evaporation crystallization, with a solid content of 80 g / L. The pH of the slurry liquid is controlled to be 10.2, and carbon dioxide gas is continuously introduced for carbonization reaction. The carbonization reaction temperature is 40°C, and the pH value adjustment solvent is calcium hydroxide. The slurry liquid after the carbonization reaction is completed is subjected to solid-liquid separation and washing to obtain high-purity light calcium carbonate with a purity of 98.5%.

[0078] (3) Pyrolysis and basic magnesium carbonate solid-liquid separation system: The carbonization separation mother liquor (magnesium bicarbonate solution) is pyrolyzed and crystallized, and then washed by solid-liquid separation to prepare high-purity basic magnesium carbonate with a purity of 99.5%. The pyrolysis crystallization temperature is 100.0°C, and the carbon dioxide gas generated by pyrolysis is sent to the carbonization section for recycling through a fan.

[0079] The concentrations of calcium and magnesium ions in the raw water of Example 2 and Example 1 are slightly different, but both still have good economic effects. The higher the content of magnesium ions in the raw water, the better the economic value.

[0080] The above is only a specific implementation of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A method for fractional crystallization of a salt solution containing calcium and magnesium ions, characterized in that: The steps are as follows: S01 uses a filter to remove trace insoluble impurities in the calcium and magnesium ion salt solution. The purified liquid is crystallized by liquid-solid reaction with sodium carbonate. The crystallization temperature is 20-40°C. The molar ratio of the added amount of sodium carbonate to the calcium and magnesium ions in the purified liquid is 1.00-1.05:1 to ensure complete precipitation of calcium and magnesium ions. S02: the double salt precipitates of calcium carbonate and magnesium carbonate generated by the reaction crystallization in step S01 are subjected to solid-liquid separation and washing, and the wet materials enter the carbonization section; dilute hydrochloric acid is used to adjust the pH of the separated mother liquor to 6.0-7.0, and then the MVR evaporation crystallization process is used to evaporate and crystallize sodium chloride to obtain industrial grade sodium chloride with a purity greater than 97.5%; S03: uniformly mix the wet material of the reaction crystallization in step S02 with the steam condensed water of the sodium chloride MVR evaporation crystallization, with a solid content of 50-150 g / L, control the pH of the slurry liquid to be 9.0-11.0, continuously introduce carbon dioxide gas for carbonization reaction, and convert the double salt precipitates of calcium carbonate and magnesium carbonate into soluble magnesium bicarbonate and insoluble calcium carbonate, the carbonization reaction temperature is 20-40° C., and the pH value adjustment solvent is one of sodium hydroxide and calcium hydroxide or a combination thereof; S04: solid-liquid separation and washing of the slurry liquid after the carbonization reaction in step S03 to obtain high-purity light calcium carbonate with a purity greater than 97.0%; In step S05, the separated mother liquor in step S04 is pyrolyzed and crystallized to prepare high-purity basic magnesium carbonate through solid-liquid separation and washing. The pyrolysis and crystallization temperature is 90.0-100.0°C. The carbon dioxide gas generated by the pyrolysis is sent to step S03 for recycling through a fan.

2. The method for fractional crystallization of a calcium and magnesium ion salt solution according to claim 1, characterized in that: include: Reaction crystallization, filtration separation, acidification and MVR evaporation crystallization system, used for the reaction crystallization of calcium and magnesium ion salt solution and sodium carbonate in the fractionation crystallization process technology system of calcium and magnesium ion salt solution, including solid-liquid separation and washing of reaction crystallization product, pH adjustment of filtrate and MVR evaporation crystallization process of sodium chloride solution; Carbonization and light calcium carbonate solid-liquid separation system, used for the slurry configuration and carbonization process of the reaction crystallization product of the fractional crystallization process technology system containing calcium and magnesium ion salt solution and the solid-liquid separation process of light calcium carbonate; The pyrolysis and basic magnesium carbonate solid-liquid separation system is used in the process of preparing basic magnesium carbonate by pyrolysis and crystallization of the carbonization separation mother liquor of the fractionation crystallization process technology system containing calcium and magnesium ion salt solution.

3. The method for fractional crystallization of a calcium and magnesium ion-containing salt solution according to claim 2, characterized in that: The chemical reaction equations involved in the process of filtering the calcium and magnesium ion salt solution to remove trace insoluble impurities, adding sodium carbonate to react and crystallize to generate double salt precipitates of calcium carbonate and magnesium carbonate, washing the double salt precipitates by solid-liquid separation, acidification of the separated mother liquor, and MVR evaporation and crystallization of sodium chloride are as follows: CaCl2+MgCl2+2Na2CO3=MgCO3·CaCO3↓(double salt)+4NaCl.

4. The method for fractional crystallization of a calcium and magnesium ion-containing salt solution according to claim 2, characterized in that: The process of the carbonization and light calcium carbonate solid-liquid separation system includes introducing carbon dioxide under controlled pH conditions to convert the double salt precipitate of calcium carbonate and magnesium carbonate into soluble magnesium bicarbonate and insoluble calcium carbonate, and realize calcium and magnesium separation after solid-liquid separation. The chemical reaction equation involved is as follows: MgCO3·CaCO3+CO2+H2O=Mg(HCO3)2+CaCO3↓.

5. The method for fractional crystallization of a calcium and magnesium ion-containing salt solution according to claim 2, characterized in that: The process of preparing basic magnesium carbonate by pyrolysis and crystallization of carbonization separation mother liquor involves the following chemical reaction equation: 5Mg(HCO3)2=4MgCO3·Mg(OH)2·4H2O↓+6CO2↑.

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