Preparation of zinc sulfate from spent acid

1. Acid leaching of zinc ore

The water is fed to the spent acid leaching pan, heated and stirred, was slowly added 1 t of zinc oxide ore, while the mixed spent acid was added about 5 tons, the acid in an amount of 1.2 times the amount of zinc, heating was continued heated to 70 ℃, Add waste acid or mineral powder to adjust the pH to 4.2, continue stirring for 30 min, let it fully react, and finally adjust the pH to 5.4, which is the end point of leaching. Zinc oxide powder with the spent acid leaching process, the ore ZnO or ZnCO ₃ and H ₂ SO ₄ reaction to produce _4, other metal oxide also reacts with zinc in acid, and the action of acid soluble gangue ZnSO A silicic acid colloid is formed, and as a result, a slurry composed of a zinc sulfate-based sulfate solution and silica gel and an unreacted residue is obtained.

The slurry is subjected to plate frame pressure filtration, the filtrate is sent to the next process to remove impurities, the filter residue is added with waste acid and water for the second leaching, and after leaching, the lime milk is adjusted to pH 5.4, and polyacrylamide is added for coagulation sedimentation. Let it stand overnight, that is, press filter, filter residue is discarded, and the filtrate is used for the next leaching of zinc ore powder.

2, zinc liquid purification

â‘  primary iron oxide, manganese zinc bath Fe ³⁺ at pH 5.4, the precipitate hydrolyzed. However, the hydrolysis pH values ​​of Fe ²⁺ and Mn ²⁺ in the zinc solution were 6.5 and 9, respectively. If the hydrolysis method is used to remove Fe ²⁺ and Mn ²⁺ , the pH must be adjusted to 9, and at such a high pH value, zinc sulfate is also completely hydrolyzed into a zinc hydroxide precipitate which is removed together with the residue. In this regard, an oxidizing agent potassium permanganate may be added to oxidize Fe ²⁺ and Mn ²⁺ to Fe ³⁺ and Mn ⁴⁺ , and then hydrolyzed to Fe(OH) ₃ and MnO by adjusting the pH to 5.4, respectively. OH) ₂ precipitated and removed.

The specific operation is as follows: the crude zinc sulfate solution in the leaching process is sent to the first oxidation pot, and then heated and stirred. When the temperature reaches 70 ° C, the potassium permanganate solution (or bleaching powder or air) is added for oxidation. At this time, the red-brown iron hydroxide suspension appears in the oxidation pot, and the pH of the zinc solution is lowered due to oxidation to produce a new acid. Therefore, lime milk is added for neutralization, when the pH is checked to be 5.4 and the zinc solution is not tested. When iron is used, let it be aged for a while, then it can be sent to the frame filter press to filter, the filter residue is discarded, and the filtrate is used in the next process.

Fe ³⁺ starts to hydrolyze at a pH of 1.7, but due to the influence of zinc ions and sulfate ions and colloids, it needs to be hydrolyzed to a reddish-brown iron hydroxide precipitate at a pH of 3.2 to 3.5. When the pH value is <5.4, the iron hydroxide has a positive charge, strong water absorption, swelling, poor precipitation, difficult to filter, and iron removal is not complete; when the pH value is 5.4, the iron hydroxide has a negative charge and absorbs water at the same time. , the formation of expanded iron hydroxide, precipitation is not good, but also affects the filtration, so that the iron removal is not complete; only when the pH value is 5.4, is the isoelectric point of Fe(OH) ₃ , that is, the neutralization point, the charge is sensitive The mutation, which plays an abnormal agglomeration, gives the maximum filtration rate.

In conditions at pH 5.4, the hydrolysis of Mn ²⁺ oxidation generates manganite precipitate; A1 ³⁺ hydrolyzed aluminum hydroxide precipitate; hydrolysis also occurs arsenic and antimony, arsenic trioxide and generate a corresponding sub precipitate antimonate; and V Chromium is also hydrolyzed to form a precipitate and a corresponding double salt with Fe(OH) ₃ to accelerate precipitation; silicon is a colloidal precipitate of silicic acid colloid.

â‘¡ purged with copper, cadmium, nickel traditional and zinc substitution method in addition to copper, cadmium, solution of nickel, cobalt, arsenic, antimony, lead, indium, thallium, tin and other potential higher than that of zinc impurities in varying degrees It is replaced by a precipitate and removed.

The specific operation is as follows: the zinc liquid in the previous process is sent to the replacement pot, heated and stirred. When heated to 70 ° C, the theoretical amount of 2.5 times of zinc powder and the appropriate amount of copper sulfate are added, and copper and cadmium are replaced soon. Nickel, cobalt, etc. When testing zinc solution without cadmium, it can be discharged to the filter tank for rapid filtration to prevent re-dissolution of cadmium. The filter residue is used for separating copper, cadmium, nickel and cobalt, and the filtrate is sent to the next process for use.

3 Secondary oxidation In addition to iron and manganese, in order to remove iron and manganese more cleanly in order to improve product quality, secondary oxidation must be carried out, and the operation process is the same as primary oxidation. In order to prevent the incorrect introduction of manganese using potassium permanganate, the oxidant is changed to ammonium persulfate. After secondary decontamination, the impurities in the zinc solution are only magnesium , potassium, and sodium, and these impurities can be removed from the mother liquor when the zinc sulfate crystallizes. Because of the quality specifications of the finished product, there is no requirement for the content of these elements.

3. Preparation of zinc sulfate

The solution is heated to evaporate excess water. When the saturated concentration (relative density 1.56~1.58) is reached, the temperature can be lowered to normal temperature, and the solution is supersaturated and crystallized to obtain zinc sulfate crystals, which are then centrifuged to crystallize the crystals. Drying, cooling and packaging are the finished products. The filtrate was returned to evaporation and concentrated for reuse.

The process equipment is simple, the process is short, the operation is convenient, the investment is small, and the benefit is good, indicating that it is feasible to use zinc sulfate to produce zinc sulfate. The removal process of the zinc liquid also removes most of the impurities in the spent acid, and it is not necessary to separate the impurities separately. Solve the problem of environmental pollution in waste acid and achieve better social benefits. The secondary waste, waste water and waste gas in production will not cause secondary pollution.

After obtaining the pure zinc liquid, the metal zinc can be obtained by electrolysis; or by reacting with ammonium bicarbonate and ammonia water, filtering and roasting to obtain active zinc oxide, and the filtrate is made into ammonium sulfate; or synthesizing zinc antimony with barium sulfide and filtering and roasting And crushed to get lithopone.

It can reduce the cost of the main product and improve the economic benefits of the enterprise. An annual production capacity of 6,000 tons of titanium white powder manufacturers, the annual recycling sulfuric acid from spent acid 7,500 tons annual cost savings of $ 3 million sulfate; Moreover, this can also reduce the 7,500 tons of sulfuric acid and the spent lime and labor wages Production of zinc sulphate can also yield a certain profit. [next]

The concentration of sulfuric acid in waste acid is relatively rare. It is recommended to set up a factory near the titanium dioxide factory and use it with plastic pipes to avoid the transportation cost of adding a large amount of waste acid. The process flow of zinc products such as zinc sulfate from waste acid is shown in the figure below.