Low-grade chalcopyrite ore, heap leaching or smelting recovery route?

Highlights

• A comparison in feasibility of the pyrometallurgical and hydrometallurgical treatment of low-grade chalcopyrite ore were made

• The economy profoundly affected the process selection such that the process with lower Cu recovery turned into a feasible one

• The interest rate impact in economic preference of a hydrometallurgical route over pyrometallurgical route is substantial

Abstract

Chalcopyrite, the major mineral source of copper, is commonly treated pyrometallurgically, using froth flotation, smelting, converting and electrorefining processes. The ore treatment however is limited to the cut-off grade concentration below which the extraction is not economically viable. In this study a comparison of feasibility of the pyrometallurgical and hydrometallurgical treatment of low grade (0.23%–0.35% Cu), high content chalcopyrite (>90% CuFeS2) ore were studied from both technical and economic viewpoints. On a technical stance, the results of a pilot test on froth flotation of 100 t of low-grade ore are presented. The pilot test results confirm the generation of a Cu concentrate of a grade range 23.3–19.6% Cu with a copper recovery in the range 90.9–78.1%. For the hydrometallurgical treatment, bio heap leaching of 40,000 t of low-grade ore using agglomeration and aeration was proven to achieve a copper recovery of 32.6% for the chalcopyrite portion of the ore over 326 days of operation.

On the economic side, the operating cost of the process per tonne of copper cathode for both processes were reported. The operating cost for the pyrometallurgical (PM) processing plant was calculated to be 1.4 times less than the hydrometallurgical (HM) processing plant. The sales revenue of the PM route was also higher than that of the HM route due to the byproducts credit. Capital cost estimates were made assuming that the current solvent extraction/electrowinning (SX/EW) facilities in a HM route and the current smelting and electrorefining plants in the PM route have enough free capacity to absorb the upstream outputs. Applying the copper extraction efficiencies of the pilot plants, the profitability indicators for both routes in treatment of 3000,000 t ore/year was studied. The impact of the interest rate was found significant in economic preference of the HM route over the PM route. This is due to the high capital cost of the mineral processing plant despite of the higher operating cost of the HM plant. The capital cost of the mineral processing plant can make the HM route feasible at interest rates higher than 27.5%. At discount rates lower than 27.5%, the PM route process is viable over the HM route at copper market price of US$9500/t. A drop in copper selling price to US$6000/t acts in favor of the HM route, making it feasible at interest rates higher than 22.4%. For a general scenario where the construction of SX/EW and smelter/refinery plants are required, the discount rate at which the net present value (NPV) of the two routes are equal has decreased from 27.5% to 17%. In this scenario, for discount rates higher than 17%, the HM route with 31% total copper recovery is more feasible than the PM route with 79% recovery.

Introduction

Chalcopyrite, the most common copper mineral, is typically concentrated through froth flotation followed by the extraction of copper metal pyrometallurgically, through smelting/converting and electrorefining processes. Higher copper recovery and byproduct credits of silver, gold and molybdenum result in lower operating costs and higher sales revenue (per tonne of copper cathode) in pyrometallurgical treatment of chalcopyrite concentrates compared to the hydrometallurgical route. However, high capital cost and low flexibility in pyrometallurgical extraction of low-grade ores limit the treatment of chalcopyrite to the cut-off grade concentration. The capital investment for a pyrometallurgical plant was reported 10 times higher than the capital cost required to build a hydrometallurgical plant. For example, the fixed capital investment of a pyrometallurgical plant producing copper cathode from 0.5% Cu ore was reported US$30,000 per annual tonne of copper production (Schlesinger et al., 2011). Peacey et al. (2004) reported the capital cost of a HM plant with capacities between 40,000 Mt./y to 200,000 Mt./y to be around US$4000 to US$5000 per annual tonne of copper cathode. For smaller size plants, the capital cost was found higher per tonne of copper cathode production. For example, the financial data published recently show a fixed capital investment to vary between US$6980 to US$4948 per annual tonne of copper cathode for HM plant capacities between 7000 and 3000 Mt./y (Mokmeli, 2020).

High capital cost of the larger PM plants has encouraged copper producers to invest building smelters with lower annual production capacities specially in China. In 2019, about 36% of the copper produced pyrometallurgically was produced in smelters with lower than 200 kt/y comparing to the 25% in 2003 (Wang et al., 2019; Watt and Kapusta, 2019). In this regard, flash smelting, a suspension smelting technology, despite being a dominant technology, has seen a slight decrease in its share of world copper production since flash smelters are not suited for smaller size operations. In contrast, the bath smelting technologies including Suikoushan Bottom Blowing Smelting (SKS-BBS) furnace, Isasmelt and Ausmelt Top Submerged Lance furnaces and Vanyukov technology are currently used in the leading majority of global primary copper smelting operations that are mostly in a capacity range of less than 200 kt/y. The wide adoption of bath smelting technologies is mostly due to their lower capital cost investment (Gonzales et al., 2019).

On the other hand, as was also confirmed in this study, the operating cost of the HM plants are usually higher than that of PM plants. For example, Peacey et al. (2004) showed that the operating cost of a large smelter and an electrorefining plant is less than US$0.15/lb. while the heap leach/SX/EW operating cost is US$0.4/lb-US$0.55/lb. (in 2004). The practice on how to estimate the operating cost for a bio heap/SX/EW plant as a function of copper extraction efficiency, ore grade and heap acid consumption has been recently published by the author of this work, Mokmeli, 2020 (see section 3.3.1 of this study). In this study, the direct operating cost of hydrometallurgical treatment of a chalcopyrite ore with 0.28% copper content is shown to be varying between US$2.02/kg Cu to US$3.89/kg Cu for copper extraction efficiencies between 70% to 30%, respectively at a sulfuric acid price of US$90/t. In addition, the construction of a bio heap/SX/EW plant with production capacity of 7000 t Cu cathode required a capital investment of US$38.1 million in 2018. This is equal to an investment of US$2300(70% Cu recovery) to US$5400 (30% Cu recovery) per annual tonne of Cu cathode production.

The economic viability of a process varies depending on the operating cost, capital cost and the revenue from sales. In this study, technical and economic feasibility of treating 3000,000 t/year low grade chalcopyrite ore (CuFeS₂ content >90%) through pyrometallurgical and hydrometallurgical routes were investigated. It is expected that at least a 100 million tonnes of low-grade run of mine ore, mostly chalcopyrite with the average analysis presented in Table 1, is available (rough estimate) at the waste dump areas at Sarcheshmeh mine, Kerman, Iran. For this amount, and with the treatment of 3000,000 t/year, 33 years of operation, in minimum, seems promising. At Sarcheshmeh copper mine the majority of the copper, 160,000 t copper cathode/year, is produced pyrometallurgically. The hydrometallurgy plant on the other hand, has a design capacity of 14,000 t copper cathode/year, but is operating below 50% of its capacity due to the issue related to depletion of oxide ores. Developing an economically efficient process to utilize the low-grade ores with the possible use of free capacity available at SX/EW plants or smelting/electrorefining plants, has been the motivation to this study. For this purpose, two scenarios were investigated. In the first scenario, construction of a new mineral processing plant to treat the low-grade ore was studied. In the second scenario, construction of a bio heap plant to treat the low-grade ore was considered. In both scenarios it was assumed that the downstream operations (SX/EW for the HM route and smelting/refining for the PM route) has the free capacity to absorb the upstream PLS or concentrate. A sensitivity analysis section discussed a more general scenario at which the construction of SX/EW and smelter/refinery plants are requiered.

The novelty of this work is the combination of the technical results based on a large pilot plant with economy of the process. In this research, the copper ore grade and its recovery (efficiency) corelated with the economy of the selected process. In this study, the importance of economy and factors such as discount rate were shown to profoundly affect the process selection such that the process with lower copper recovery can turn into a feasible process. The calculation methods employed in this study can easily enable interested researchers to follow the same approach using their own technical and financial data.