A method for treating waste acid obtained from a smelting acid production system

Abstract

The invention discloses a method for treating waste acid obtained from a smelting acid production system. The method comprises pretreatment, acid salt separation, intermediate leaching, iron, arsenic and lead removal, secondary purification, fluorine removal, evaporation saturation and freezing crystallization. The method adopts resource-optimized waste acid treatment technology, can directly treat heavy metal ions, as well as chemical elements such as fluorine and chlorine in the waste acid, avoids large amounts of reagent consumption, utilizes zinc fume produced as a by-product of the enterprise itself to prepare zinc sulfate solution and utilizes high-purity zinc slag to replace medium and heavy metal elements in the waste acid. After evaporation saturation, freezing crystallization is performed to produce zinc sulfate heptahydrate product. No waste acid treatment liquid is generated, and waste acid treatment is resource-utilized.

Description

Technical Field

[0001] The invention belongs to the technical field of waste acid wastewater recovery, and particularly relates to a method for treating waste acid obtained in a smelting acid production system. Background Art

[0002] Smelting acid wastewater contains high concentrations of arsenic, fluorine, and chlorine, primarily from zinc and nickel smelting flue gases. During the purification and scrubbing process, as the gas phase enters the liquid phase, these elements, such as arsenic, fluorine, and chlorine, remain in the acid in a free state. Smelting acid is highly corrosive, causing significant damage to soil, equipment, and converter catalysts.

[0003] Currently, common methods for treating acid wastewater in China include: 1) pretreatment + tertiary neutralization; 2) lime-iron salt method; 3) sulfidation neutralization; and 4) high-efficiency gas-liquid enhanced sulfidation technology. Traditional acid treatment methods typically produce a large amount of neutralization residue, for which there is currently no effective treatment method. This residue is difficult to treat and expensive. Furthermore, traditional methods add a large amount of chemicals during the treatment process, resulting in higher salinity in the wastewater than before treatment, making it difficult to reuse. Annual operating costs can account for 30% or more of the system's investment cost.

[0004] To reduce the high levels of arsenic in the waste acid raw liquid, conventional processes often utilize high-efficiency gas-liquid enhanced sulfidation. This method initially involves the preparation of hydrogen sulfide gas, with varying processes. One method uses hydrogen from methanol cracking and sulfur to synthesize hydrogen sulfide gas in a synthesis tower. This method is extremely unsafe and requires stringent equipment material requirements. Another method uses the reagent sodium sulfide or sodium hydrosulfide to react with dilute sulfuric acid to produce hydrogen sulfide gas. This reagent requires high volumes and market prices, and subsequent removal of chlorine and fluoride ions requires triple-effect evaporation, which uses high volumes of steam and is prone to evaporator scaling. While this method can improve the purification of heavy metal arsenic, the large amount of slag and the difficulty in controlling arsenic contamination remain. Summary of the Invention

[0005] In order to solve the problems in the background technology, the purpose of the present invention is to provide a method for treating waste acid obtained from a smelting acid production system. The method adopts resource-optimized waste acid treatment technology, which can directly treat heavy metal ions, as well as chemical elements such as fluorine and chlorine in the waste acid. While avoiding the consumption of a large amount of reagents, the method utilizes the company's own by-product zinc smoke to produce zinc sulfate solution and utilizes high-purity zinc slag to replace medium and heavy metal elements in the waste acid. After evaporation and saturation, it is frozen and crystallized to produce zinc sulfate heptahydrate product. No waste acid treatment liquid is generated, and the waste acid treatment is resource-utilized.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a method for treating waste acid obtained from a smelting acid production system, comprising the following steps:

[0008] (1) Pretreatment: pretreating the waste acid to obtain pretreated waste acid;

[0009] (2) acid salt separation: separating the acid salt from the pretreated waste acid obtained in step (1) to obtain a salt solution and a waste acid solution;

[0010] (3) mid-leaching process: adding zinc fume to the waste acid solution obtained in step (2), performing filter pressing after leaching, and obtaining filter press liquid and leaching residue;

[0011] (4) Iron, arsenic and lead removal process: zinc slag is added to the filter press liquid obtained in step (3) to carry out a displacement reaction to produce acid salt precipitation of elements such as iron, arsenic and lead, and then hydrogen peroxide is added to change the valence state of the iron and arsenic elements. After solid-liquid separation, iron, arsenic and lead slag and waste acid after iron, arsenic and lead removal are obtained;

[0012] (5) Secondary purification: adding zinc slag to the waste acid after the removal of iron, arsenic and lead obtained in step (4), passing steam, and performing secondary purification to remove elements such as copper and cadmium to obtain waste acid after the removal of copper and cadmium;

[0013] (6) Defluorination step: adding calcium sulfate solution to the waste acid after copper and cadmium removal obtained in step (5) to obtain the waste acid after defluorination;

[0014] (7) evaporation saturation: evaporating the defluorinated waste acid obtained in step (6) to obtain a saturated zinc sulfate solution;

[0015] (8) Freeze crystallization: The saturated zinc sulfate solution obtained in step (7) is subjected to freeze crystallization. This process removes most of the Cl - , and obtain zinc sulfate heptahydrate product.

[0016] In a preferred embodiment, during the treatment process of the waste acid, zinc sulfate heptahydrate is produced as a product, so that the waste acid is utilized as a resource, zinc fume, a by-product produced by the enterprise itself, is used to produce zinc sulfate solution, and high-purity zinc slag is used to replace medium and heavy metal elements in the waste acid, thereby reducing the operating cost of the waste acid treatment process.

[0017] In a preferred embodiment, in step (1), the pretreatment is specifically as follows:

[0018] Adopting three-stage inclined plate continuous sedimentation, adding flocculant in the last stage settler to achieve graded purification and precipitation of the dirty acid; making the precipitation more thorough and the supernatant of the dirty acid clearer;

[0019] The acid sludge in the sludge thickening tank is pumped into the sludge centrifuge through the lifting pump. The separated sludge after dehydration is sent to the raw material plant for batching treatment.

[0020] In a preferred embodiment, in step (2), resin is used to separate the acid salts, thereby separating the sulfuric acid from metal ion salts such as zinc, thereby obtaining a salt solution free of sulfuric acid and a desalted waste acid solution. This significantly reduces the metal ion content in the desalted waste acid solution, purifies the impurities in the sulfuric acid for reuse, and saves the cost of removing the metal ions.

[0021] In a preferred embodiment, in step (3), the zinc fume contains 54-59wt% Zn, 1.0-2.4wt% Pb, 0.1-1.2wt% Cu, 6-10wt% Fe, 0-0.1wt% As, and 0.05-0.3wt% Cd; after adding the zinc fume to the contaminated acid solution obtained in step (2), the pH value and concentration of the zinc sulfate solution are adjusted.

[0022] In a more preferred embodiment, in step (3), the acid salt separation solution is pumped into a leaching tank, and the prepared zinc fume slurry is added, and the concentration is controlled at 1 to 1.05 kg / m 3 , control the zinc sulfate leaching reaction time to 0.5 to 5 hours, the pH value end point of the zinc sulfate solution is 4.5, and produce a zinc sulfate solution of a predetermined concentration.

[0023] In a more preferred embodiment, in step (3), the leached residue after filter pressing is dried by microwave and used as a slag-making solvent for nickel side blowing, thereby greatly reducing the amount of silica used.

[0024] In a preferred embodiment, in step (4), the zinc slag contains Zn 68-72wt%, F 0.01-0.1wt%, Cl 0.1-1.0wt%, Fe 0.05-0.3wt%, Sb 0.01-0.1wt%, Pb 0.1-2.0wt%, SiO2

[0025] 0.01~0.6wt%.

[0026] In a preferred embodiment, in step (4), zinc slag is added to the liquid after filter pressing, and the reaction time is controlled to be 2 to 12 hours. Most of the iron, arsenic, and lead in the solution are replaced, and the reaction pH is controlled to be 5 to 5.5. During the reaction, hydrogen peroxide with a concentration of 15 to 35 wt% is added, and an appropriate amount of iron salt is added. After the bottom flow liquid is released, it is filtered through a filter press, and the generated iron, arsenic, and lead slag is treated as hazardous waste.

[0027] When removing iron, arsenic, and lead, adding hydrogen peroxide changes the elements from their original divalent state to a trivalent state, which is conducive to the precipitation of arsenate. Later, some iron salts can be added to meet the iron element required for precipitation.

[0028] In a preferred embodiment, in step (5), the secondary purification is specifically as follows:

[0029] The dirty acid and zinc slag after the removal of iron, arsenic and lead are added into the reaction tank, steam is introduced, and the reaction temperature is controlled at 50-60°C for secondary purification. The copper and cadmium bottom flow is released to further remove elements such as copper and cadmium.

[0030] During the secondary purification, high-purity zinc slag is added. The zinc ions displace the inactive metal ions in the zinc sulfate solution without introducing new metal elements, which is conducive to the production of zinc sulfate. At the same time, external steam is added to ensure the reaction temperature is between 50 and 60°C, further removing elements such as copper, cadmium, and lead.

[0031] In a preferred embodiment, in step (6), a calcium sulfate solution is added to the waste acid after copper and cadmium removal obtained in step (5), wherein the calcium content is 20-40% and the concentration is controlled at 1-1.05 kg / m 3 , control the reaction time to 2 to 6 hours, the pH value of the waste acid solution to 6 to 8, the reaction generates calcium fluoride precipitation principle, and removes most of the fluorine elements in the waste acid.

[0032] In order to strengthen the precipitation of calcium fluoride, it is necessary to add an appropriate amount of one or more of lime, inorganic salt coagulant, and polymer coagulant to accelerate coagulation and precipitation; the inorganic salt coagulant is ferric chloride; the polymer coagulant is polyacrylamide.

[0033] Furthermore, in step (6), calcium sulfate solution generated by the ring-collected flue gas is added to recover and reuse the calcium sulfate solution generated by the ring-collected desulfurization process.

[0034] In a preferred embodiment, in step (7), after the waste acid is defluorinated, the waste acid is evaporated to saturation to increase the solubility of zinc sulfate, and the resulting saturated zinc sulfate solution is prepared for subsequent freeze crystallization.

[0035] In a preferred embodiment, in step (8), after the waste acid is evaporated to saturation, the waste acid is subjected to freeze crystallization, during which the chlorine element in the waste acid is removed to produce zinc sulfate heptahydrate crystals of qualified quality.

[0036] The present invention provides a method for treating waste acid obtained from a smelting acid production system. The method comprises: first, pre-treating the waste acid by using continuous three-stage precipitation and filtration to remove most suspended matter, heavy metals, sludge, etc. in the waste acid. The pre-treated waste acid is then separated using a resin for acid salt separation. The separated dilute acid containing fluorine and chlorine is then leached with zinc dust to produce a zinc sulfate solution. Zinc scum is then added to displace metal elements such as iron, copper, and cadmium carried by the zinc dust. Calcium sulfate and limestone are then added for defluorination. The defluorinated solution is subjected to evaporation and saturation, followed by freezing and crystallization. The waste acid is then subjected to control of the pH and temperature of the solution at different stages to produce a qualified zinc sulfate heptahydrate product. This technology not only solves the problem of the prior art requiring the consumption of a large amount of reagents, but also eliminates the production of waste acid treatment liquid. It fully utilizes the waste acid treatment liquid to produce products, thus achieving resource utilization of waste acid treatment.

[0037] Compared with the prior art, the beneficial technical effects of the present invention are:

[0038] 1) The present invention provides a method for treating waste acid generated by a smelting acid production system. This method significantly removes heavy metals, fluorine, chlorine, arsenic, and other elements from the waste acid stock solution, reducing the impurity content of the sold zinc sulfate heptahydrate product, meeting customer needs. It also significantly reduces the use of water, electricity, steam, and chemicals, significantly lowering operating costs. The reduced size or scale of the treatment facility indirectly reduces the construction costs of the waste acid treatment system.

[0039] 2) The present invention provides a method for treating waste acid obtained from a smelting acid production system, which converts the waste acid into a zinc sulfate heptahydrate product, thereby realizing waste acid recycling and reducing secondary emissions and environmental pollution.

[0040] 3) The present invention provides a method for treating waste acid from a smelting acid production system. The arsenic and iron removal design allows for both series and parallel operation, adapting to varying operating conditions and increasing operational flexibility. When waste acid levels are normal and the incoming water quality is poor, the system can operate in series; when waste acid levels are high and the water quality is normal, the system can operate in parallel; and when waste acid levels are low, the system can operate as a single tower.

[0041] 4) The present invention provides a method for treating waste acid obtained from a smelting acid production system. This method utilizes the reaction mechanism of zinc slag replacing copper and cadmium, thereby avoiding the introduction of other elements into the reagents, thereby facilitating the purity of the zinc sulfate solution and alleviating the subsequent quality standard issues of the product after the zinc sulfate solution is concentrated and crystallized.

[0042] 5) The present invention provides a method for treating waste acid obtained from a smelting acid production system, which avoids the production of waste acid treatment liquid and eliminates the chemical water treatment process. The condensed water generated by the evaporation and saturation process is reused for acid production and purification. By freeze-crystallizing the zinc sulfate solution, not only is chloride ions in the waste acid removed, but zinc sulfate heptahydrate product can also be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a process flow chart of Example 1 of the present invention. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present invention will be described clearly and completely below. 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 making creative work are within the scope of protection of the present invention.

[0045] The waste acid treated in this embodiment is the waste acid produced by the zinc and nickel smelting acid production system.

[0046] Combined waste acid emissions:

[0047] Table 1.1 Water quality characteristics of mixed waste acid (g / L)

[0048] Dilute acid emission m3 / h Sulfuric acid in dilute acid As Pb Zn Fe F Cl Hg 16 30～60 1.25 0.01 0.25 0.035 0.12 0.59 0.004

[0049] Note: 1) Waste acid temperature: 0℃±10℃; waste acid solid content ≤0.1g / l.

[0050] 2) The amount of sulfuric acid in the waste acid fluctuates greatly. It is assumed that the waste acid after merger contains 60g / l of sulfuric acid.

[0051] Table 1.2 Chemical composition of zinc fume (%)

[0052]

[0053] Table 1.3 Chemical composition of zinc slag (%)

[0054]

[0055]

[0056] Unless otherwise specified, the reagents used in this embodiment are all common commercial products or are prepared by conventional means, and the equipment used are all conventional equipment in the art. The following are some examples of the inventor's experiments:

[0057] The following is further described by specific embodiments and drawings:

[0058] Example 1

[0059] A method for treating waste acid obtained from a smelting acid production system comprises the following steps:

[0060] (1) Pretreatment: First, the waste acid is pretreated by three-stage inclined plate sedimentation, and the supernatant of the waste acid overflows into the waste acid salt separation system.

[0061] (2) Acid separation: The dirty acid solution is passed into the acid separation resin equipment, and the acid in the solution is adsorbed by the acid adsorption material to obtain desalted solution; after the adsorption is completed, water is pumped into the industrial water to elute into the acid separation equipment, and the acid is washed away by water to obtain high-purity acid solution, which is used to prepare zinc sulfate heptahydrate product; and the salt solution without sulfuric acid is a low-acid metal ion wastewater, which can be neutralized with a small amount of lime to obtain metal precipitation without the generation of gypsum slag, and the neutralization slag is greatly reduced, thus realizing the open circuit of metal ions and being incorporated into the comprehensive wastewater treatment station.

[0062] Utilizing the special adsorption properties of the resin, the waste acid solution is passed into the acid salt separation adsorption equipment, where the acid in the solution is adsorbed by the acid adsorption material, resulting in deacidified salt water. After the adsorption is completed, water is pumped into the acid salt separation adsorption equipment for elution, where the acid is washed away by the water, resulting in a recovered dilute acid solution.

[0063] Salt separation technology separates sulfuric acid from metal ion salts such as zinc, producing a desulfurized salt solution and a desalted sulfuric acid solution. The desalted sulfuric acid solution achieves an acid recovery rate of 80-90% and a salt removal rate of approximately 80%. The desalted salt solution achieves a salt recovery rate of 80% and an acid removal rate of approximately 80-90%. This significantly reduces the amount of zinc and other metal ions in the desalted sulfuric acid solution, purifies impurities in the sulfuric acid for reuse, and saves on the cost of removing zinc and other metal ions.

[0064] Table 1.4 Water quality and quantity of effluent from acid salt separation section

[0065]

[0066] (3) Intermediate leaching process: After the acid salt is separated, the dilute acid is recovered and pumped into the intermediate leaching tank for intermediate leaching reaction with the zinc roasting dust. The reaction time is controlled to be about 1.5 hours. By adding the prepared zinc dust slurry (the concentration is controlled at 1-1.05), the pH value of the solution is adjusted, and the reaction condition end point is controlled at 4.5. The original waste acid concentration is 60g / l, and the concentration of the zinc sulfate solution reaches 126g / l. The leaching residue produced in the intermediate leaching process is dried by microwave and used as a slag-making solvent for nickel side blowing, which greatly reduces the use of silica.

[0067] Zinc oxide dust is used to neutralize the acid wastewater and adjust the pH value. Zinc oxide and the acid are used to generate a relatively stable zinc sulfate compound. The sulfate ions introduced by the acid solution react with the metal ions such as Fe, Pb, and Cd in the zinc oxide dust. At the same time, a portion of the leached residue is filtered and dried to serve as a slag-making solvent ingredient. The chemical formula involved is:

[0068] Zn0+H2S04=ZnS04+H20

[0069] Cd+H2S04=CdS04+H2

[0070] Pb+H2S04=PbS04+H2

[0071] Fe+H2S04=FeS04+H2

[0072] (4) Iron, arsenic and lead removal process: The zinc sulfate solution after intermediate leaching enters the solution tank through the overflow pipe to remove metal ions such as arsenic, iron and lead. The zinc slag is used to replace the ferrous sulfate and lead sulfate in the zinc sulfate. The zinc slag is made into a solution and then pumped into the reaction tank through the pump body. The reaction time is controlled to 6h. Most of the lead, arsenic and iron are replaced. At the same time, the reaction pH value is controlled at 5-5.5. During the reaction, 27.5% hydrogen peroxide is introduced and an appropriate amount of iron salt is added. The arsenic, lead and iron slag produced are filtered and then treated as hazardous waste.

[0073] Zinc slag is used to neutralize the acid wastewater and adjust the pH value. Arsenic is removed by utilizing the property of arsenic and iron to form a relatively stable ferric arsenate compound, and ferric hydroxide and ferric arsenate co-precipitate. Iron hydroxide has strong adsorption and flocculation properties, achieving the purpose of removing harmful heavy metals such as arsenic and cadmium from the acid wastewater. Increasing the pH value removes heavy metal ions in the acid wastewater in the form of hydroxides. The chemical formula involved is:

[0074] Fe 3+ +AsO3 3- =FeAsO3↓

[0075] Fe 3+ +AsO4 3- =FeAsO4↓

[0076] In addition to forming ferric arsenate when iron ions react with arsenic, ferric hydroxide can act as a carrier to precipitate together with arsenate ions and ferric arsenate.

[0077] FeAsO4 is relatively stable, but when pH>10, it will produce a back-dissolution reaction, so the pH value is generally controlled at 5-6. The back-dissolution reaction formula is as follows:

[0078] FeAsO4+3OH - →Fe(OH)3+AsO4 3-

[0079] Add zinc slag to adjust the pH value of the system and then add hydrogen peroxide. Using the strong oxidizing property of hydrogen peroxide, Fe 2+ Oxidized to Fe 3 + , As 3+ Oxidized to As 5+ , which is conducive to the precipitation and precipitation of arsenic and iron ions.

[0080] Table 1.5 Water quality and quantity of effluent from arsenic and iron removal process

[0081]

[0082] (5) Secondary purification: Zinc slag is added to the waste acid after iron, arsenic and lead removal, and steam is introduced for secondary purification to remove elements such as copper and cadmium to obtain waste acid after copper and cadmium removal;

[0083] During the purification process, steam at 50-60°C is introduced into the top of the first-level purification tank, which is conducive to the precipitation of copper sulfate and cadmium sulfate. At the same time, the zinc slag is made into a solution and then pumped into the reaction tank through the pump body. The sulfate precipitates of copper, cadmium, lead and iron are discharged through the bottom flow. After filtering by the filter press, the slag is used for ore bunker mixing.

[0084] During the secondary purification process, the temperature of the zinc sulfate solution is maintained at 50-60°C, and the zinc slag solution is continuously introduced for reaction to remove the remaining unreacted copper, cadmium, and lead ions. The copper, cadmium, and other precipitates are discharged through the bottom flow and filtered through the filter press. The slag is then used for ore bunker batching. The chemical formula involved is:

[0085] Zn+CuS04=ZnS04+Cu

[0086] Zn+PbS04=ZnS04+Pb

[0087] Zn+CdS04=ZnS04+Cd

[0088] Table 1.6 Water quality and quantity of effluent from purification process

[0089]

[0090] (6) Defluorination process: After the zinc sulfate solution undergoes secondary purification to remove copper, cadmium and other ions, it enters the defluorination process;

[0091] Control the pH value of the solution to ≤2, add the liquid of calcium sulfate solution (calcium content 30%) produced by the ring-collected flue gas desulfurization with a pump (the concentration is controlled at 1-1.05), control the reaction time to 3h, and under the premise that the pH value of the waste acid solution is 6-8, in order to strengthen the precipitation of calcium fluoride, it is necessary to add an appropriate amount of lime or inorganic salt coagulant (ferric chloride), or polymer coagulant (polyacrylamide) to accelerate the coagulation and precipitation.

[0092] The lime precipitation method uses the Ca ions generated by the dissolution of CaO to react with the F ions in the water to form insoluble CaF precipitates, thereby removing F- from the water. The chemical formula involved is:

[0093] CaO+H2O=Ca(OH)2

[0094] Ca + 2F = CaF2↓

[0095] Lime is an alkaline substance. In order to control the pH value within a certain range, the amount of lime added must be controlled. In order to ensure that the F reaches the standard, a combined treatment method of lime and soluble salt (such as CaSO4) is adopted. First, lime is added to make the pH value of fluoride-containing water reach 6-8, and then the CaSO4 solution from the flue gas desulfurization is added to obtain excess Ca 2+ You can also add excess lime, then neutralize it with acid and adjust the pH value back to 3-4.

[0096] Table 1.7 Water quality and quantity of effluent from defluorination process

[0097]

[0098] (7) Evaporation saturation: The dirty acid liquid is pumped in from the bottom up by the circulation pump and flows upward along the heating chamber. When the circulating liquid flows through the heat exchanger, it is heated and then partially evaporated when the pressure in the separator is reduced. The steam and the liquid are separated in the evaporation chamber. The steam is discharged from the top and the liquid falls down. It is sucked into the circulation pump through the conical bottom and then enters the heating chamber to continue the cycle.

[0099] The defluorinated waste acid is evaporated to saturation to obtain a saturated zinc sulfate solution; during the evaporation process, the evaporation temperature is properly controlled to volatilize the solvent of the unsaturated zinc sulfate solution and gradually convert it into a saturated solution, so that the solubility of the zinc sulfate reaches the maximum.

[0100] Evaporation saturation technology preheats the brine before it enters the evaporator and partially evaporates within it. The resulting secondary steam is compressed by a compressor to increase its pressure and then introduced into the heating side of the evaporator. The steam condenses and is then withdrawn as product water, thus recycling the heat energy. The circulation of the zinc sulfate solution within the equipment primarily relies on the forced flow generated by an external power source (a forced circulation pump). An unsaturated zinc sulfate solution is pumped upward through the heating chamber by a circulating pump. The circulating liquid is heated as it flows through the heat exchanger and then partially evaporates as the pressure in the separator decreases, increasing the acidity of the waste acid.

[0101] Table 1.8 Water quality and quantity of effluent from evaporation and saturation process

[0102]

[0103] (8) Freeze crystallization: After evaporation and saturation, the zinc sulfate solution is pumped into a vacuum crystallizer, and the boiling point of the added solution is lower than its temperature. In this way, the solvent in the solution is partially vaporized and the solution is supersaturated, producing qualified zinc sulfate heptahydrate.

[0104] The "cooling hot saturated solution crystallization method," also known as the "cooling hot saturated solution method," is a method for separating or purifying zinc sulfate heptahydrate based on the varying solubility of zinc sulfate solutions at different temperatures (generally, higher solubility at high temperatures and lower solubility at low temperatures). This method is primarily used for substances whose solubility decreases significantly with decreasing temperature. The process involves evaporation and concentration, cooling crystallization, filtration, washing, and drying.

[0105] Table 1.9 Quality standard of ZnSO4·7H2O after freezing crystallization

[0106]

[0107]

[0108] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for treating waste acid obtained from a smelting acid production system, characterized in that: The following steps are involved: (1) Pretreatment: pretreating the waste acid to obtain pretreated waste acid; (2) acid salt separation: performing acid salt separation on the pretreated waste acid obtained in step (1) to obtain a salt solution and a waste acid solution; (3) leaching process: adding zinc fume to the waste acid solution obtained in step (2), performing filter pressing after leaching, and obtaining filter press liquid and leaching residue; (4) Iron, arsenic and lead removal process: zinc slag is added to the filter press liquid obtained in step (3) to carry out a displacement reaction to produce acid salt precipitation of iron, arsenic and lead elements, and then hydrogen peroxide is added to change the valence state of iron and arsenic elements. After solid-liquid separation, iron, arsenic and lead slag and waste acid after iron, arsenic and lead removal are obtained; (5) Secondary purification: adding zinc slag to the waste acid after the removal of iron, arsenic and lead obtained in step (4), passing steam, and performing secondary purification to remove copper and cadmium elements to obtain waste acid after the removal of copper and cadmium; (6) Defluorination step: adding calcium sulfate solution to the waste acid after copper and cadmium removal obtained in step (5) to obtain the waste acid after defluorination; (7) Evaporation saturation: evaporating the defluorinated waste acid obtained in step (6) to obtain a saturated zinc sulfate solution; (8) Freeze crystallization: The saturated zinc sulfate solution obtained in step (7) is subjected to freeze crystallization. This process removes most of the Cl - , obtaining zinc sulfate heptahydrate product; In step (2), resin is used to separate acid salts, and sulfuric acid and metal ion salts are separated to obtain a salt solution free of sulfuric acid and a waste acid solution free of salt.

2. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (1), the pre-processing is specifically as follows: Adopting three-stage inclined plate continuous sedimentation, adding flocculant in the last stage settler to achieve graded purification and sedimentation of the dirty acid; making the sedimentation more thorough and the supernatant of the dirty acid clearer; The acid sludge in the sludge thickening tank is pumped into the sludge centrifuge through a lifting pump. The separated sludge after dehydration is sent to the raw material plant for batching treatment.

3. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (3), the zinc fume contains Zn 54-59wt%, Pb 1.0-2.4wt%, Cu 0.1-1.2wt%, Fe 6-10wt%, As 0-0.1wt%, and Cd 0.05-0.3wt%. After adding the zinc fume to the contaminated acid solution obtained in step (2), the pH value and concentration of the zinc sulfate solution are adjusted.

4. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (4), the zinc slag contains Zn 68-72wt%, F 0.01-0.1wt%, Cl 0.1-1.0wt%, Fe 0.05-0.3wt%, Sb 0.01-0.1wt%, Pb 0.1-2.0wt%, and SiO2 0.01-0.6wt%.

5. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (4), zinc scum is added to the liquid after filtration, and the reaction time is controlled to 2-12 hours. Most of the iron, arsenic and lead in the solution are replaced, and the reaction pH is controlled to be 5-5.

5. During the reaction, hydrogen peroxide with a concentration of 15-35wt% is added, and an appropriate amount of iron salt is added. After the bottom flow liquid is released, it is filtered through a filter press, and the generated iron, arsenic and lead slag is treated as hazardous waste.

6. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (5), the secondary purification is specifically as follows: The dirty acid and zinc slag after the removal of iron, arsenic and lead are added into the reaction tank, steam is introduced, and the reaction temperature is controlled at 50~60℃ for secondary purification. The copper and cadmium bottom flow is discharged to further remove the copper and cadmium elements.

7. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (6), calcium sulfate solution is added to the waste acid after copper and cadmium removal obtained in step (5), with a calcium content of 20-40% and a concentration controlled at 1-1.05 kg / m 3 , control the reaction time to 2~6h, the end point pH value of the waste acid solution is 6~8, the reaction generates calcium fluoride precipitation principle, and most of the fluorine elements in the waste acid are removed.

8. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (7), after the waste acid is defluorinated, the waste acid is evaporated and saturated to increase the solubility of zinc sulfate, and the resulting saturated zinc sulfate solution is prepared for subsequent freezing and crystallization.

9. The method for treating waste acid obtained from a smelting acid production system according to claim 1, characterized in that: In step (8), after the waste acid is evaporated to saturation, the waste acid is subjected to freeze crystallization, during which the chlorine element in the waste acid is removed to produce zinc sulfate heptahydrate crystals of qualified quality.

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