What is zinc?

Zinc metal, which is produced and sold in the form of zinc ingots, is one of the important and strategic metals, which has many uses in the industry due to its valuable features such as color, high electron yield, malleability, and density.

The main production of zinc ingots in Iran is carried out in Zanjan province due to its proximity to Angoran mine and access to the main sources of Zn minerals such as zinc sulfide (ZnS) or sphalerite, zinc carbonate (ZnCO3) or smith zonite and zinc oxide (ZnS). ) or calamine, has caused the growth of this industry in the city and province of Zanjan, which produces more than 70% of zinc ingots and sheets in the country, and most of it is exported abroad.

Zinc ingot production method

Zinc metal is produced by mining zinc ore. Considering that most Zn mines are accompanied by lead, first, lead, and zinc should be separated by the flotation method. By performing the flotation operation, the lead element is physically separated from zinc by adding chemicals, and after collecting it, it becomes zinc concentrate, and it is transferred to the smelting furnace for regeneration and melting and turning into lead ingots, and with the help of Coke or anthracite is regenerated.

After flotation, the deleaded zinc soil resulting from flotation is ready to be transferred to the zinc factory for zinc extraction. At first, the leaching operation should be done so that the elements present in the zinc-oxidized or carbonated soil are dissolved in the reaction with the diluted solution of sulfuric acid and become water-soluble ions. The leaching operation starts at 65 degrees Celsius and pH = 1.5, and as a result of the reactions, the pH of the solution increases slowly and reaches about 3.5. After the end of leaching, to deposit iron, by adding hydrated lime or calcium carbonate, the pH of the solution increases to 4.5. At this stage, it should be noted that the pH of the solution does not increase more than 4.5, because this causes the re-precipitation of this chemical.

In the leaching operation, except for zinc, other elements such as copper, iron, fluoride, manganese, magnesium, cobalt, nickel, and cadmium in the soil are also dissolved in the solution. To obtain pure Zn ingots, all these elements must be Except for zinc ions, they are separated and precipitated to obtain pure ingots.

The presence of fluorine in the solution will cause the corrosion of the aluminum cathodes of electrolysis and the adhesion of the Zn sheet to the cathodes, and for its deposition, aluminum sulfate is used to form aluminum trifluoride (AlF3), which is deposited and at the end of the leaching stage, and After raising the pH, it is precipitated and separated from the solution.

The final slurry of leaching is purified by a filter press and silica, aluminum, and all its insoluble solids are separated and a clear solution plus leach cake is obtained. The obtained solution contains Zn, cobalt, manganese, nickel, and cadmium and will be ready for further purification.

Warm treatment

To separate cobalt and manganese from the solution, these ions must become precipitates, without zinc ions being precipitated. For this purpose, strong oxidants such as potassium permanganate or ozone gas can be used. By adding an oxidant to the solution, manganese and cobalt ions that have reached a higher oxidation number are precipitated at lower pH, and it is possible to separate them from the solution without this chemical precipitation. The oxidation of manganese and cobalt ions in the reaction with the oxidant will cause a drop in pH, for which calcium carbonate or hydroxide should be used to compensate. This operation is performed at high temperatures and it is called hot refining. The resulting black deposits will contain manganese dioxide and cobalt oxide, and it is called cobalt cake or warm cake.

Cold treatment

The solution resulting from the output of the filters of the hot purification stage contains zinc, nickel, and cadmium. To separate nickel and cadmium from the solution, zinc powder is added to the solution so that nickel and cadmium can be precipitated through cementation. By giving electrons to nickel and cadmium, zinc powder dissolves in the solution and reduces them to metal. This reaction is carried out without applying heat and at pH=3.5, the increase in temperature and acidity leads to the re-dissolution of nickel and cadmium. The resulting slurry is filtered and the pure and clear solution of zinc sulfate, as well as nickel and cadmium metal deposits with Zn metal cores, is obtained as a filter cake, which is called cold cake or nickel cake.

Electrolyze

To convert pure zinc sulfate solution into metal, electrolysis is performed. By transferring the solution to the electrolysis ponds with lead anode and aluminum cathode and applying a potential difference of 3.2 volts, zinc ions receive alkenron in the negative pole (cathode), reduce to Zn metal, and form sheets on the cathodes. After about 24 hours, a thick Zn sheet is placed on the cathode. Sheeting is done and after removing the sheets from the ponds, the zinc sheet is separated from the cathode.

In electrolysis, due to applying the potential difference to the lead anodes, slowly oxidized lead enters the solution, although lead sulfate has a very low solubility in water, some of it goes to the cathode and turns into a metal along with zinc. And it will cause the product to be unspecific. To protect the anodes, manganese carbonate, and manganese sulfate are used, which form a protective layer on the anode by adding to the electrolysis solution and oxidizing the manganese ion, which slows down the dissolution of lead in the solution. In addition to manganese compounds, strontium carbonate is also used to precipitate lead and gum Arabic to facilitate the separation of the zinc sheet from the cathode.