Selective Recovery of Zinc from Metallurgical Waste Materials from Processing Zinc and Lead Ores.

A method for processing of metallurgical waste materials (chemically defined as sulfur-bearing zinc-ferric materials) produced by plants processing zinc ores and their concentrates is proposed. The method proposed is a combination of pyro- and hydrometallurgical treatments of the waste material. The crucial steps in the developed method include: roasting the material at 450 °C to generate sulfur dioxide (SO2), absorption of SO2 in an aqueous system to form sulfuric acid (IV), carbothermic decomposition of zinc ferrite compounds, and leaching of zinc from the roasted material using sulfuric (IV) acid. The method allows one to extract up to 40% of zinc from the waste material and, consequently, to generate a fraction of material with substantially higher content of iron oxides. The proposed method takes advantage of the presence of sulfur in the processed material which upon roasting is converted to sulfuric acid (IV)-a leaching agent for selective extraction of zinc. The properly adjusted pH of the aqueous medium in which the leaching process is carried out is the key factor determining the quantitative and selective separation of zinc. If the amount of sulfur in the processed material is insufficient, it may be supplemented by adding sulfuric acid (VI) to adjust the pH. The method proposed was tested at a laboratory scale and quarter industrial scale using the real samples taken from stockpiles in the vicinity of the plant processing zinc and lead ores in Poland. It may also work for any zinc-ferric materials from various sources.

Highly efficient and selective leaching of silver from electronic scrap in the base-activated persulfate - ammonia system.

A system composed of persulfate salt and ammonia in highly alkaline aqueous solution is developed and examined for leaching metallic silver from elements of the electronic waste materials (e-scrap). Strong base activates persulfate ions providing in situ generation of highly reactive oxygen molecules. The oxidized metal forms then well soluble complex ions with ammonia ligands. The kinetic studies of the leaching process were performed for pure metallic silver. They revealed that the efficiency of the process is affected by the type of the persulfate salt. By employing potassium persulfate one obtains significantly (more than 50% for silver plates and more than 100% for silver powder) increased efficiency of silver dissolution compared to the solution composed of either sodium or ammonium persulfates. In the range of persulfate concentrations between 0.02 and 0.23mol/L the apparent reaction order with respect to the persulfate concentration was similar for all persulfate salts and was estimated to be around 0.5. The room temperature (22±2°C) seems to be an optimal temperature for the leaching process. An increase in the temperature resulted in the significant drop in the silver dissolution rate due to the decreased solubility of oxygen. Based on these results a possible mechanism of dissolving silver is discussed and the optimal composition of the leaching solution is formulated. The obtained formulation of the leaching solution was applied for the extraction of silver coatings of Cu-based e-waste scrap and the obtained results revealed an important effect of copper in the mechanism of the leaching process. The regression analysis of the leaching curve indicated that each gram of base-activated potassium persulfate under the specified conditions may leach almost 100mg of silver coatings in a form of well soluble diamminesilver (I) complex. The silver complex can be relatively easy reduced to metallic silver. The method developed is relatively cheap, low toxic and does not produce harmful by-products.