Wet zinc smelting - water release impurities in neutral leaching process (2)

Water lifting iron
Iron in the leachate to the divalent iron salt and a trivalent iron salt FeS0 ₄ Fe ₂ (S0 _{4) 3} is present in the form of, when the end of the leaching is controlled at pH 5.2 to 5.4, not hydrolyze ²⁺ divalent iron Fe It must be oxidized to ferric Fe ³⁺ . Industrial software usually natural manganese ore and anode slime (including manganese dioxide) as an oxidizing agent, in an acidic medium in the oxidation of ferrous sulfate, the reaction of

    It can be seen from the above reaction equation that in order to make the Fe ²⁺ oxidation reaction proceed to the right, H ⁺ must exist, that is, in an acidic solution, and the index of the activity of divalent iron ions and hydrogen ions can be known from the reaction equilibrium constant K _a . Higher, the impact on the reaction balance is extremely great, so the use of Mn0 ₂ as an oxidant is always carried out in a more acidic solution.
The high iron hydrolysis reaction proceeds as follows.

Fe ₂ ( S0 ₄ ) ₃ + 6H ₂ 0 ==== 2Fe ( OH ) ₃ + 3H ₂ S0 ₄

It can be seen from the above reaction that high-iron hydrolysis has an acid production, and as the acidity increases with hydrolysis, it is necessary to continuously neutralize the acid removal in order to maintain the pH of the high-iron hydrolysis. As the current industrial water and leachate iron is released in the neutral leaching stage end, high-iron acid hydrolysis of the pulp are consumed zinc oxide and other metal oxide is dissolved, generally not to meet extra neutralizer The need for endpoint pH control.
Water releases arsenic and antimony
As mentioned above, in the neutral leaching stage, arsenic in industrial leachate is mainly in the form of high-priced anion, and there are few simple cations; 锑 is mainly present in H ₃ Sb0 ₄ , H ₃ Sb0 ₃ colloid and Sb0 ₄ ^3- . When the pH of the leaching end point is controlled at 5.2~5.4, the instability in the leaching solution cannot be removed by itself. The help of ferric iron must be used to remove arsenic and antimony. The role of strontium has two points: that is, it has a chemical action, so that arsenic and antimony become double salts of insoluble iron arsenate and iron citrate; at the same time, the iron hydroxide colloid adsorbs and coagulates and precipitates. Niobium is mainly composed of colloidal acid, so it is mainly adsorbed by Fe(OH) ₃ colloid. Ferric hydroxide is a kind of colloid. Because its colloidal particles have the same electrical charge, they repel each other and are not easy to settle. Fe(OH) ₃ has different charges under different acidities, and the charge is different. When the pH value of the solution is <5.2, it adsorbs Fe ³⁺ ions and is positively charged. When the pH value is >5.2, it is negatively charged. The localized ion is OH ^- and its isoelectric point is near pH = 5.2. [next]
Since the Fe(OH) ₃ colloid is positively charged at pH < 5.2, As0 ₄ ^3- , Sb0 ₄ ^3- will become its counterion. Generally speaking, various negative ions in the solution can be attracted to the "counter ion" by the core, and some of them can enter the micelle and move together with the micelle. In industrial leachate, there are many substances that can become counterions, such as SO ₄ ^2- , OH ^- , As0 ₄ ^3- , Sb0 ₄ ^3- , GeO ₃ ^2-, etc., but the amount of them entering the micelle adsorption layer will depend on The concentration of these ions and the magnitude of the charge are more likely to enter the adsorption layer when the concentration is high and the charge is high. The concentration and charge are more effective than the charge, so the negative ions entering the adsorption layer of the ferric hydroxide colloid will be mainly As0 ₄ ^3- , Sb0 ₄ ^3- , SO ₄ ^2- , and there will be a small amount of SiO ₃ ^2- and OH. ^- Wait.
Arsenic and antimony can exist in the form of cations As ⁵⁺ and Sb ⁵⁺ only when the acidity of the solution is high. For neutral leaching, the solution with the endpoint pH value of 5 or higher, arsenic and antimony will mainly be ionized with As0. ₄ ^3- , Sb0 ₄ ^3- form exists, metal arsenic, antimony ions will be minimal. Although As0 ₄ ^3- , Sb0 ₄ ^3- is much lower in concentration than SO ₄ ^2- , it has a great advantage in terms of charge and can be adsorbed in the surface layer by the iron hydroxide core.
The arsenic and antimony entering the colloidal adsorption layer are directly proportional to the concentration of arsenic and antimony in the solution. The amount of arsenic and antimony is also proportional to the amount of ferric hydroxide. The more the amount of ferric hydroxide, the amount of adsorbed arsenic and antimony. The more the amount, the less the amount of arsenic and antimony remaining in the solution. In order to completely remove arsenic and antimony from the solution in production practice, it is generally required that the iron content in the solution is 10 to 15 times the amount of arsenic and 20 to 40 times the amount of antimony. For this reason, when the amount of iron in the solution is insufficient, it is necessary to add iron to the solution, but the amount of iron added should not be too much, otherwise a large amount of iron hydroxide precipitate will be produced, which is disadvantageous for clarification filtration and too large amount of leaching slag. of.
Water Relief Silicon It is well known that silicon is present in a colloid rather than an ionic state in the leaching slurry. Excessive silica gel will make the slurry clear and the sedimentation performance deteriorate. The colloidal particles in the colloidal solution are charged. The isoelectric point of the silica gel is near pH = 2. When the pH is >2, the silica gel particles are negatively charged. When the pH is <2, the silica gel particles are positively charged. When the leaching end point is controlled at pH=5.2. The iron hydroxide and the silicate colloid have opposite charges, and under the action of electrostatic attraction, the two will coalesce together and cause them to co-precipitate from the solution. When the zinc roasting ore is neutrally leached, the silica gel aggregates and precipitates at a pH of 4.8 to 5.0, rather than being precipitated at a constant point. Practice has shown that the two colloidal coagulations are complementary.
It is clear that the co-condensation is related to the technical conditions of the process and to the amount of the two colloids. If there is more colloid, less colloid, or a large difference in charge of the gel particles, partial condensation and sedimentation may occur. In order to make the leaching slurry easy to settle or filter, the precipitated micelles should be bulky, have a firm structure and have a large settling velocity. If the precipitated particles are very fine and the structure is loose, it will cause difficulty in clarification and concentration. For this reason, industrial production usually adds polypropylene decylamine (No. 3 coagulant) to the solution to improve and accelerate the sedimentation process.
Polyacrylamide is a kind of polymer compound, which is a hydrophilic colloid. It usually has no charge. The stability of colloid is not dependent on the mutual repulsive force of the same charge, but by solvation. Adding polyacrylamide, hydrophobic iron hydroxide and hydrophilicity in leaching pulp

The polypropylene guanamine colloids coexist, and after the collision of the two colloids, the hydrocarbon group of the polyamidamine will be adsorbed on the surface of the ferric hydroxide colloid. Since the polypropylene guanamine molecular backbone is very long, in the case of a small amount, each polymer may stick to a plurality of ferrous hydroxide micelles, causing them to combine to promote coagulation. It can be seen from the role of polyacrylamide in slurry sedimentation that polypropylene decylamine cannot be added too much, otherwise the opposite phenomenon will occur. That is, the surface of the colloidal particles of iron hydroxide will be completely surrounded by it to form a film, which hinders the mutual aggregation of the colloidal particles.