Formulation of Ceramic Crucibles for Fire Assays

The valorization of minerals requires physicochemical and thermal treatments, using various materials such as crucibles. They are refractory ceramics mainly based on clays. Crucibles must be resistant to chemical aggression by molten slag and metals at high temperature during repeated thermal cycles, to ensure the durability in use. Fire-assaying is extensively used in gold mining in Burkina Faso, needing a huge quantity of ceramic crucibles imported per year. In this study, we have characterized different clays to manufacture refractory ceramics for crucibles. Physical characteristics of laboratory made crucibles were compared to that of industrial crucibles. The behavior of an optimized composition with 25wt% of clay and 75wt% of chamotte (fired clay) was experimented to optimize the properties in use. Particularly, the adequate adjustment of all process parameters, as the paste plasticity, leads the control of the density and porosity of the fired ceramic. Fusion tests with copper and aluminum at temperature between 600°C and 1100°C proved the small penetration depth of the molten metal into the ceramic that reduce the corrosion phenomenon. Our work evidences the existence of both a scientific and a technological knowledge in the use of silico-aluminate mineral resources from Burkina Faso, for manufacturing refractory crucibles for the melting of both slag and precious metals. © 2019 Elixir All rights reserved © 2019 Elixir All rights rese ved. Elixir Appl. Chem. 127 (2019) 52601-52607 Applied Chemistry Available online at www.elixirpublishers.com (Elixir International Journal) Brahima sorgho et al./ Elixir Appl. Chem. 127 (2019) 52601-52607 52602 It is a key sector for industries related to gold mining in countries where gold resource is extensively available, as in Burkina Faso [8]. Fire assaying is a quantitative chemical analysis by which metals are separated and determined in ores and metallurgical products with the aid of heat and dry reagents. This technique is used since years to concentrate gold and other noble metals from ores or metallurgical products [9,10,11]. In a first step, a melt of at least two phases is formed by heating at about 1100°C a complex liquid borosilicate slag and a liquid lead phase of controlled quantity. The high degree of physical combination of gold and other noble metals in molten metallic lead and the great difference in specific gravity between the lead and slag allow the separation of the noble metals from the slag as a lead compound. In a second step, the removal of the lead as lead oxide in a porous magnesia cupel during a carefully controlled oxidizing fusion at about 950°C separates the lead from the noble metals. The remaining metallic bead is then quantitatively analyzed for the noble metals. The crucibles in service undergo failure mechanisms that must be checked: -a chemical attack leading to corrosion from the flux addition; -a mechanical damage by the high temperature and the thermal shocks. To ensure a high standard of quality, it is necessary to develop tests to simulate failure, during laboratory experiments. The article focuses on the optimization of clay refractory materials for specific crucibles that will be used for fire assay. The study also aims to realize tests in the metal contact region, where the most aggressive set of conditions occur. Materials and methods The clay raw materials for the study are from France (DAMREC Company). The two clays used are referenced RS 335 and RR 40. These clays were considered as reference clays since some laboratory studies at large scale were performed in France: from preliminary studies, we evidenced the very similar composition and behavior of RS 335 and RR 40 clays with clays mined in Burkina Faso [5]. Particularly plasticity of paste, as drying and firing behaviors were studied. The chamotte is a refractory clay that is calcined at a temperature between 1200 and 1400°C. It is milled and sieved to control grain size. The used chamotte is a commercial product from CERADEL Company, France. As for clays, the chamotte characteristics and thermal behavior is very similar to chamotte that have been obtained during small scale laboratory works in Ouagadougou University. The used chamotte is obtained from kaolinitic clay that is heat treated at 1300°C. It has controlled grain size and chemical analysis. The raw clays were crushed and sieved at 315μm, to remove larger grain that reduces the mixture homogeneity. The clays (RS 335 or RR 40) and the chamotte with particle size of 200 μm were mixed homogeneously in a planetary mixer, and water was added progressively (21-28 wt%) to obtain a plastic paste for the pressing process. Compositions of mixtures reported in Table 1, and water content for claychamotte mixtures are within common ranges for industrial uses [3]. Pellets of 13 mm in diameter and 15 mm height were obtained by die pressing at 20 MPa in a metal mold. They are dried at temperature varying progressively from 40°C to 110°C during 8h. Firing is at 1250°C with a 30 minutes plateau. Table 1. Compositions of clay-chamotte mixtures.