The present work reports the direct leaching of zinc from a sphalerite concentrate in acid media. Lab-scale and pilot-scale experiments were conducted in atmospheric-pressure and low-pressure reactors, respectively. Leaching of zinc and precipitation of iron was achieved in the same stage using different reagents like Fe³⁺, O₂, O₃, and Fe²⁺ (which is continuously oxidized in the leaching solution by H₂O₂ and O₂). The highest percentage of zinc extraction (96%) was obtained in pilot-scale experiments using H₂SO₄, Fe²⁺, and O₂. Experimental results were compared with those of other researchers to provide a better understanding of the factors influencing the dissolution of zinc. In the first instance, it was determined from analysis of variance that leaching time and the use of an oxidant agent (O₂ or O₃) were the most influential factors during the direct leaching of zinc from the sphalerite concentrate. Kinetic models were also evaluated to determine the rate-limiting step of the sphalerite leaching; it was concluded that the type of the sulfur layer formed in the residue (porous or non-porous) depends on the type of the oxidant used in the leaching media, which determines the dissolution kinetics of zinc.

Jarosites are residues generated during the purification of zinc and are composed mainly of iron sulfates ((Na, K)Fe₃(SO₄)₂(OH)₆). Due to the large volume of jarosite generated during the process, these residues tend to be deposited in large land areas and are not used. In the present work, jarosite was used without heat treatment (JST) as an adsorbent of hexavalent chromium contained in a sample of wastewater from a chrome plating industry under the following conditions: C₀ = 200 mg/L of Cr, T = 25 °C, and pH = 3. It was only possible to remove 34% of Cr (VI). Subsequently, a thermal treatment of a jarosite sample (JTT) was carried out at 600 °C. The heat-treated sample was later used as an adsorbent in the same conditions as those for JST. The maximum chromium removal was 53%, and the adsorption capacity was 10.99 mg/g. The experimental data were fitted to the Langmuir model and to the pseudo-second-order kinetic model. It was determined that the adsorption process involved electrostatic attractions between the surface of the positively charged adsorbent and the chromium anions contained in industrial wastewater.

The sludge from a wet-off gas cleaning system of the iron blast furnace (BF) contains significant amounts of iron; however, they cannot be recycled due to their high content of zinc and alkalis. These compounds are detrimental to the optimal performance of iron and steelmaking furnaces. In this work, a comparative laboratory study to reduce zinc and alkali contained in the blast furnace sludge (BFS) is presented. The effect of leaching parameters such as oxidant (i.e., ferric ion, oxygen or ozone), aqueous solution media (i.e., 0.2 M NH₄Cl, 0.2 M HCl and 0.1 M H₂SO₄) and temperature (i.e., 27 and 80 °C) on Zn and alkalis (Na₂O and K₂O) removal were studied by applying an experimental design. The results obtained show that Zn and K₂O removal of 85% and 75% were achieved under the following conditions: Ozone as an oxidant agent and 0.1 M H₂SO₄ as an aqueous medium, temperature had no significant effect. The results are supported by thermodynamic diagrams and the possible chemical reactions are mentioned. Although the results also indicate that leaching under the above conditions dissolves up to 9% of iron, this loss is much less than leaching without the oxidizing conditions generated by the ozone. The BFS obtained from this treatment could be recirculated to the iron or steelmaking processes to recover iron values.

Rare earth elements comprise a group of 17 chemically similar elements, which increases the difficulty of separating them by traditional methods. For this reason, hydrometallurgy has been the most used method. However, it is important to evaluate the efficiency of the leaching processes used because, in addition to depending on the operating parameters of the leaching, they also depend on the mineralogical composition of the sample. In the present work, the extraction of Ce and La contained in the ferrocarbonatite mineral from the north of Mexico was studied. For the leaching tests, several leaching agents were used (HCl, H₂SO₄, HNO₃, and H₃PO₄ in different concentrations (0.5 [M], 1 [M], 1.5 [M]) and the temperature was modified to 20, 40, and 60 °C. A maximum recovery of 70% for Ce and La was obtained using HCl 1M in 4 h. The results of the kinetic study of the experiments showed that the best fitting model according to these kinetic models was the SCM controlled by a chemical reaction.

The purpose of this work is to obtain the optimum conditions for leaching the zinc contained in an industrial lead smelting slag. In this type of slag, zinc oxide, sulfide, and ferrite are contained. Zinc extraction from these compounds consists of using a single aqueous medium, where oxidant alkaline leaching with NaOH and NaClO was used. The parameters evaluated during the experiment were as follows: percentage of solids, NaOH/NaClO ratio, and temperature. The maximum amount of recovered Zn obtained during the leaching was 58%. This percentage was achieved by using the following optimal conditions: temperature of 60 °C, 0.22 of NaOH/NaClO ratio, 10% of solids, and a reaction time of 40 min. The calculated values of activation energy confirmed that the rate-limiting step of the reaction using the decreasing particle model is diffusion. The maximum percentage of zinc obtained could only have been achieved if the zinc oxide and part of the zinc sulfide (both present in almost equal proportions in the sample) were leached into the alkaline aqueous medium in the presence of NaClO.

This paper reports the effect of the components of a direct leaching residue (jarosite and elemental sulfur), on the recovery of valuable metals such as gold and silver. Leaching media such as cyanide and mixtures of cyanide with glycine were used to recover the gold and silver from the residue; however, a low recovery of these metals was obtained. This occurred due to the negative effect of its components which caused problems in the extraction process such as encapsulation of silver (due to jarosite) and the formation of thiocyanate and re-precipitation of silver (due to sulfur). Various treatments prior to leaching were tested, finding that when the residue was desulfurized with perchlorethylene and subjected to an oxidizing alkaline hydrothermal treatment, the gold extraction increased from 39.73 to 88% and the silver extraction of 64.76 to 94.29%. Additionally, it was determined that when cyanide was assisted by glycine, the latter decreased the cyanide consumption by inhibition of the dissolution of iron and sulfur in cyanide.