Industrial roasting plant of sorption tailings after commissioning
by: Engineering Dobersek GmbH: Ph.D. A. Dobersek, N. Wotzka, Ph.D. O. Mishina
At present we are observing a growth trend in the world gold prices along with a noticeable decrease in reserves of rich and easily processed raw materials and consequently an increasing interest in involving of particularly refractory gold-bearing ores into the processing. The extraction of gold from such ores using standard operations does not meet modern requirements. The most used way to improve the gold recovery rates from the refractory gold-bearing ores is their pretreatment using one of the conditioning methods before processing via standard cyanidation process.
The best-known conditioning methods are pressure and biological oxidation, ultrafine grinding followed by oxidative leaching of sulfides in atmospheric conditions, as well as oxidative roasting. Most of the above methods ensure the releasing of primarily gold, which is finely disseminated in sulfide minerals, for example, in pyrite or arsenopyrite, due to their more or less complete oxidation.
However, there is another important reason for the refractoriness of the gold ores: the high sorption activity of the ore minerals, which causes significant losses of metal with solids of cyanidation process tailings. In most of cases, this sorption activity can be attributed to the presence of organic carbon in the ore. Oxidative roasting ensures almost complete oxidation of the both “problem-causing” components: organic carbon and sulfidic sulfur.
In case of especially high refractoriness of the ores , the use of combined oxidative technologies for the processing of particularly refractory gold-sulfide ores to achieve the satisfactory gold extraction rates could be necessary . Sulfide ores of the Kokpatas and Daugiztau deposits (Navoi Mining and Metallurgical Combine, Uzbekistan) belong to this type of ores.
The sequence of the treatment process of the Kokpatas and Daugiztau sulfidic ores is as follows: First, the process of biological oxidation of the flotation concentrate takes place. Then the gold associated with sulfide minerals is released, which is recovered at the subsequent stage of sorption cyanidation. The main part of arsenic dissolves and then becomes solid in the form of insoluble scorodite (FeAsO4). Further, the solid product of sorption leaching with low arsenic content is subjected to oxidative roasting, whereby organic carbon and residual concentrations of sulfide sulfur and (partially) arsenic are removed; as a result, at the final stage of sorption cyanidation from the roasted material (calcine), a significant amount of gold is extracted.
2007-2011 the biological oxidation plant was implemented at the Mining and Metallurgical Plant -3 (GMZ-3) of the Navoi Mining and Metallurgical Combine (NMMC) with the involvement of company Engineering Dobersek GmbH (Germany).
Before deciding on the construction of a roasting plant, a multi-stage research works was carried out, including both laboratory and semi-industrial tests. After the numerous investigations, the circulating fluidized bed furnace (CFB) for the roasting stage was favouritedd. This type of equipment provides a high capacity of the roasting process with high removal degree of the above-mentioned elements, which are the reason for the resistance of the material to the cyanidation process.
In addition to the roasting tests, all the preparation steps before roasting were investigated, such as dewatering of the tailings with washing the filter cake from cyanides, drying the filter cake, granulating the dried filter cake under conditions as close as possible to the actual conditions of the industrial plant. Based on the data obtained during the test works, the components of the main technological equipment of the industrial plant for the oxidative roasting of sorption tailings were calculated. The results of the above-mentioned works are provided by extensive patent protection.
The roasting plant – sorption tailing roasting system (STR System) – consists of the following main units and facilities:
• Filtration station of input slurry
• Intermediate filter cake storage
• Filter cake drying unit
• Filter cake granulation unit
• Roasting unit of granulated material
• Dry dust collection unit
• Wet gas cleaning unit
• Calcine grinding unit
• Water cooling system
- Process and instrument air
•Electrical equipment, instrumentation and controls, automated control system
The wet cleaning unit also deserves special attention, whereby a multi-stage gas cooling and washing process was implemented, ensuring compliance with the requirements for the remaining content of harmful impurities in the exhaust gas, such as arsenic and sulfur, at the outlet of the plant.
Commissioning of the roasting plant took place in August 2019. The implementation of the oxidative roasting process in the circulating fluidized bed furnace (CFB furnace) at the Mining and Metallurgical Plant -3 (GMZ-3) of the Navoi Mining and Metallurgical Combine ensures additional increasing capacity at the plant by about 10%.
The article also appeared in the First Mining Drc-Zambia Nov/Dec 2020