Effect of Cooling Rate on base Metals Recovery from Copper Matte Smelting Slags N. Tshiongo , R K.K. Mbaya , K Maweja, L.C. Tshabalala Abstract—Slag sample from copper smelting operation in a water jacket furnace from DRC plant was used. The study intends to determine the effect of cooling in the extraction of base metals. The cooling methods investigated were water quenching, air cooling and furnace cooling. The latter cooling ways were compared to the original as received slag. It was observed that, the cooling rate of the slag affected the leaching of base metals as it changed the phase distribution in the slag and the base metals distribution within the phases. It was also found that fast cooling of slag prevented crystallization and produced an amorphous phase that encloses the base metals. The amorphous slags from the slag dumps were more leachable in acidic medium (HNO3) which leached 46%Cu, 95% Co, 85% Zn, 92% Pb and 79% Fe with no selectivity at pH0, than in basic medium (NH4OH). The leachability was vice versa for the modified slags by quenching in water which leached 89%Cu with a high selectivity as metal extractions are less than 1% for Co, Zn, Pb and Fe at ambient temperature and pH12. For the crystallized slags, leaching of base metals increased with the increase of temperature from ambient temperature to 60°C and decreased at the higher temperature of 80°C due to the evaporation of the ammonia solution used for basic leaching, the total amounts of base metals that were leached in slow cooled slags were very low compared to the quenched slag samples. Keywords—copper slag, leaching, amorphous, cooling rate I. INTRODUCTION URING the production of copper, slag is produced as a waste product which was traditionally granulated and rapidly solidified in a high pressure high flow rate water jets during tapping from smelters. The historic and presently employed way of dealing with slag is dumping in the slag dumps or landfill sites. It has been estimated that for every tone of copper production, about 22ton of slag are generated each year from world copper production [5]. Slags are considered hazardous materials as they contain some base metals with concentrations higher than the required level in water and soil [2, 4,10 &12] in and around the dumping dams, some of these base metals have been proven to be toxic [9 & 10]. However, the composition of slag differs according to the source of generation, processing techniques employed and origin. N.Tshiongo :Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa R.K.K. Mbaya: Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa (+270123823597; fax: 303-555-5555; e-mail: mbayaR@ tut.ac.za). K. Maweja: Element Six Production (Pty) Ltd, DRL, PO Box 561, Springs 1560, South Africa When slags are leached with rain water [10], base metals are liberated from the slag and contaminate the soil and ground water system around the slag dumps.There are landfill site limitations and increasing disposal costs [4, 6 &7] involved in dumping slags that are untreated to recover toxic metals contents. So far slags have been used in the construction due to its high strength, durability and chemistry [13], and this has reduced the amounts of landfills. Rapid cooling of slag by water granulation results in an amorphous and glassy phased slag, encapsulating metal oxides and lowering the solubility of base metals [2, 5, 6 &13]. If the slag is cooled too fast, it passes from liquid state to a solid state without developing the crystal structure. Slow cooled slags displays chilled glassy outer margins and an increased crystallinity away from this margins towards the center of the mass [5,6 & 10]. Slags exhibited more homogenous matrix and lower bulk concentration of base metallic compounds [11]. Nevertheless, the formation of glassy matrix investigated by [13] was found insufficient to enclose the base metals and prevent them from being leached. There have been various studies on the recovery of base metals from slags and other materials that were previously considered as waste using simple physical or chemical beneficiation techniques like classification, magnetic separation, floatation, leaching and roasting. Slags are no longer classified as waste materials but as secondary resource of metals [12]. Flotation of copper slag could be limited as it is the same as floating sulfide ores with which only metallic copper and sulfide minerals can be floated effective [12]. However, high recoveries of 88% Cu, 87% Co, 93% Zn & 83% Fe were achieved using a pyro-hydrometallurgy route (roasting followed by leaching) as an alternative for flotation [1].Studies have been conducted to remove Pb, Cu, Co & Zn from dumped slags with direct reduction of copper matte smelting slag by using latent heat energy of molten slag [8], and It was proved that direct reduction using carbon is effective in simultaneous recoveries of Pb, Cu, Co & Zn from slag, with recoveries of 65-90% Cu, Co & Zn at coal to slag ratio of 5%. Higher base metals recoveries can be achieved in leaching using sulphuric acid, hydrochloric acid, ferric chloride, ammonia, cyanide etc [12]. Leaching of Ni/Co/Cu slags using sulphuric acid can result in recoveries as high as 85% Cu & 93% Zn with a large portion of Fe [3]. Silica gel has been shown to be a problem during leaching of slags with sulphuric acid. Although there are few studies on gel D World Academy of Science, Engineering and Technology International Journal of Materials and Metallurgical Engineering Vol:4, No:10, 2010 649 International Scholarly and Scientific Research & Innovation 4(10) 2010 scholar.waset.org/1307-6892/15585 International Science Index, Materials and Metallurgical Engineering Vol:4, No:10, 2010 waset.org/Publication/15585