The problem of protecting metal from corrosion is one of the most important technical problems. Among the large number of well-known methods of protection against corrosion is hot-dip galvanizing. The high price of zinc, which is a major cost in galvanizing, requires the economical use of zinc and the disposal of its waste, in particular, ash. The yield of burnt is from 0.5 to 3.5% of the mass of skipped products. It contains up to 30-40% of metallic zinc. The complex chemical composition of ash makes it difficult to further use it. When processing ash, it is of great importance to study the behavior of lead, as one of the main impurities that hinders its further use. The results of the work do not give an unambiguous picture of lead removal. There are no data on minimizing the lead content in the metallized and non-metallic part of the ash. The forms of occurrence of zinc, lead and other impurity metals in oxide-metallized melts, which form the basis of ash, have not been studied. The questions of the mechanism of distribution of lead between the products of smelting the metal part and the chlorinating processing of the non-metal part of the ashes remain open. Of great interest for practice is the development of a highly efficient technology for the processing of ash with the complex extraction of zinc and its oxide into commercial products.
Creation of a new resource-saving, environmentally friendly, waste-free technology for processing ash to produce commercial products with high added value: compact metal ingots of zinc and pure zinc oxide powder of high quality (“know-how”), suitable for the preparation of food additives for animal and bird feed.
- A new waste-free technology for the complex extraction of zinc and lead from hot-dip galvanizing waste will be developed by treating ash with a mixture of chlorine-containing reagents to obtain commercial products with high added value;
- Large-scale laboratory tests will be carried out on the processing of ash and zinc-containing oxidized materials from the non-ferrous metallurgy of the republic with a high extraction of valuable metals into commercial products;
- New scientific knowledge will be obtained in the field of chlorination reduction of zinc-, lead-containing solid products and optimal technological parameters for the implementation of the ash processing technology will be determined.
The results obtained for the 1st stage of the schedule
1) For the first time, new data have been obtained on the forms of metals in the waste from the hot-dip galvanizing process. It has been established that the presence of Si, Cu, As, Sb, Pb, Fe in the ashes leads to the formation of stable phases of complex composition, which must be taken into account when developing a method for its processing. It is shown that in all known processes of galvanizing products, the formation of a significant amount of zinc intermetallic compounds with metal impurities of the ZnxMey type (Me - Cu, As, Sb, Pb, Fe) is observed, the presence of which does not allow obtaining pure metallic zinc..
2) A comparative analysis of the existing methods of processing the metal part of the ash, obtained after galvanizing of products, was carried out and the choice of the optimal melting parameters was made: temperature - 460-520 ºС; chlorinating reagent - NH4Cl (consumption is 1.5 times more than the stoichiometry according to the reactions of chlorination of impurities). It has been established that in the temperature range of 460-520 ºС, the possibility of formation of intermetallic compounds is excluded (the melting point of intermetallic compounds is above 560 ºС).
3) New data have been obtained on the thermodynamics of the interaction of ash components with calcium and ammonium chlorides. Based on thermodynamic calculations, the values of the Gibbs free energy and the rate constants of the reactions of chlorination of metallic zinc and metal impurities with calcium and ammonium chlorides were determined in the temperature range of 973–1373 K.
4) It has been established that the chlorination of impurities and their deep sublimation in the form of chlorides is achieved by their interaction with ammonium chloride. New data on the behavior of impurity metals (Cu, Pb, Fe, As, Sb) during chlorination have been obtained. It is shown that the use of ammonium chloride provides better conditions for deep sublimation of impurities than calcium chloride..
5) Regularities of chlorination of lead from its oxide with calcium chloride have been established. It is shown that in the temperature range 1073…1373 K, the Gibbs free energy of the interaction reaction between lead oxide and calcium chloride varies from ∆G1073K = –100.6 kJ/mol to ∆G1373K = –83.0 kJ/mol.
6) It has been established that the optimal consumption of calcium chloride, necessary for the complete chlorination of lead oxide, is 1.2 - 1.7 times its consumption in terms of stoichiometry.
7) A preliminary general technological scheme for the processing of ash has been developed, including separate processing of the metal and oxide parts of the ash to obtain pure metallic zinc and pure zinc oxide powder with a minimum content of impurities.
List of performers
- Koishina Gulzada Myngyshkyzy - scientific adviser
- Zholdasbai Yerzhan Yesenbayuly - chief researcher
- Kaplan Arkady - leading Researcher
- Kurmanseitov Murat Bauyrzhanuly - senior researcher
- Tazhiev Yeleusiz Bolatovich - researcher
List of publications
- Койшина Г.М., Жолдасбай Е.Е., Курмансейтов М.Б., Тажиев Е.Б. Выбор и обоснование технологии комплексной переработки изгари с получением товарных продуктов // Вестник КарГТУ. – 2021. – № 3 (84). – С. 87-93. DOI: 10.52209/1609-1825_2021_3_87
- Koishina G.M., Zholdasbay E.E., Kurmanseitov M.B., Tazhiev E.B., Argyn A.A. Study on the behavior of zinc and associated metal-impurities in the process of chlorinating roasting of dross // KIMS. – 2021. – №3 (318). – P. 71-80. DOI: 10.31643/2021/6445.30