Recovery of Copper, Gold, Silver, and Iron from Copper Tailings

A. Anqing Copper Mine: Recovery of Copper and Iron

The ore types at Anqing Copper Mine are divided into four categories: diorite-type copper ore, skarn-type copper ore, magnetite-type copper ore, and skarn-type iron ore. The constituent minerals of the ores are all endogenous. The main metallic minerals are chalcopyrite, magnetite, pyrrhotite, and pyrite. After flotation and magnetic separation to recover copper, iron, and sulfur, a small amount of chalcopyrite that has not been fully liberated remains in the total tailings. Pyrrhotite contains iron and sulfur, and its magnetism is second only to magnetite. During desulfurization of the magnetic rougher concentrate by flotation, due to its strong magnetism, some fine magnetite is inevitably carried into the tailings.

The total tailings from the concentrator are classified: the +20 μm fraction is sent underground to fill sand storage bins; the -20 μm fraction is discharged into the tailings pond. The chemical analysis of the tailings is shown in Table 4-28.

Table 4-28 Chemical analysis results of tailings (%)

To comprehensively recover copper and iron resources from the tailings, Anqing Copper Mine made full use of idle equipment and built a tailings comprehensive recovery copper plant and iron plant based on local conditions. Copper minerals are mainly enriched in the coarse tailings, so the recovery of copper from coarse tailings is the primary focus. The concentrator tailings carry residual reagents, resulting in the natural accumulation of Cu- and S-bearing foam on top of the ore storage bins. The copper plant homemade an industrial forced-air flotation machine on top of the storage bin. The flotation rough concentrate was reground and then subjected to a cleaning circuit consisting of one roughing, two cleaning, and three scavenging stages, finally producing a qualified copper concentrate with a grade of 16.94% Cu. With an investment of 300,000 yuan, a 25 t/d copper plant was built on site near the backfill mixing station.

The data in Table 4-28 also indicate that iron is mainly concentrated in the fine tailings. Laboratory studies show that the iron in the fine tailings is mainly fine-grained magnetite and pyrrhotite. The iron plant targets the fine-grained magnetite and pyrrhotite in the fine tailings. Using three CTB718 low-intensity magnetic separators replaced from the main system, with an investment of 100,000 yuan, an iron recovery plant was established at the location before the fine tailings enter the thickener, taking advantage of the terrain elevation difference. A one-roughing, one-cleaning magnetic separation process was adopted to recover iron. To further recover iron resources lost from the concentrator, various iron-bearing wastewater, sludge, and the cleaning tailings from the tailings copper plant were all collected into the comprehensive iron plant. Finally, an iron concentrate with a grade of 63.00% Fe was obtained. The production flowsheets of the copper plant and iron plant are shown in Figure 4-19. The two plants achieved an annual output value of 4.9195 million yuan, with an estimated annual profit and tax of 4.2145 million yuan, yielding good economic and social benefits.

B. Tonglüshan Copper Mine: Recovery of Copper, Gold, Silver, and Iron

Tonglüshan Copper Mine is a large skarn-type copper-iron共生 deposit, with high copper and iron grades, large reserves, and associated gold and silver. The ores are divided into oxidized copper-iron ore and sulfide copper-iron ore. Both ore types are processed in the concentrator through two separate systems. The concentrator uses a flotation – low-intensity magnetic separation – high-intensity magnetic separation process to produce copper concentrate and iron concentrate. The high-intensity magnetic tailings total approximately 3 million tons, containing 25,000 tons of copper metal and 1.32 million tons of iron. The copper minerals in the high-intensity magnetic tailings include malachite, pseudomalachite, chalcopyrite,少量 native copper, chalcocite, bornite, and very少量 azurite and covellite; iron minerals mainly include magnetite, hematite, limonite, and siderite; non-metallic minerals mainly include calcite, chalcedony, quartz, mica, and sericite, with少量 garnet, epidote, diopside, apatite, and topaz. The multi-element and phase analyses of the tailings are shown in Tables 4-29 to 4-32.

Figure 4-19 Production flow sheet of the comprehensive tailings recovery copper plant and iron plant

Table 4-29 Multi-element analysis results of high-intensity magnetic tailings (%)

Table 4-30 Copper phase analysis (%)

Table 4-31 Iron phase analysis (%)

Table 4-32 Gold and silver phase analysis (%)

Based on experimental results, a comprehensive utilization plant treating 1000 t/d of high-intensity magnetic tailings was designed and built at the concentrator. A conventional flotation-gravity-magnetic combined process is used to recover copper, gold, silver, and iron. After grinding, the high-intensity magnetic tailings are subjected to sulfidization flotation using sodium sulfide as a sulfidizing agent, butyl xanthate and hydroxamic acid as collectors, and No. 2 oil as a frother to recover copper, gold, and silver. The flotation tailings are then treated with a spiral chute for iron rougher concentration, and the iron rougher concentrate is cleaned by magnetic separation to produce an iron concentrate (see Figure 4-20). The process conditions are: grinding fineness of 60% passing 0.074 mm, Na₂S 2000 g/t, butyl xanthate 175 g/t, hydroxamic acid 36 g/t, and No. 2 oil 20 g/t. Finally, a copper concentrate with 15.4% Cu, 18.5% Au, and 109 g/t Ag, and an iron concentrate with 55.24% Fe are obtained. The recoveries of copper, gold, silver, and iron are 70.56%, 79.33%, 69.34%, and 56.68%, respectively. Based on a treatment capacity of 900 t/d of high-intensity magnetic tailings and 300 operating days per year, the annual comprehensive recovery is 1435.75 t of copper, 171.26 kg of gold, 1055.92 kg of silver, and 33,757 t of iron. Preliminary economic estimates indicate an annual output value of 10.82 million yuan and an annual profit of approximately 10 million yuan, demonstrating significant economic and social benefits.

C. Xinjiang Ashele Copper Mine: Recovery of Copper

The Xinjiang Ashele Copper Mine is a pyrite-type copper-zinc polymetallic deposit of volcanic eruption‑sedimentary origin. Since its commissioning in September 2004, the beneficiation process has been continuously optimized, increasing the copper recovery from 79% in the early stage of production to 91.93%, with the copper concentrate grade reaching over 18%. To fully tap technical potential and maximize the recovery of mineral resources, an industrial-scale re‑flotation of copper from the zinc‑sulfur separation tailings of the current production process has been implemented.

In the current separation process, the copper content in the tailings of the zinc‑sulfur separation circuit remains relatively high. The copper is mainly present as tennantite, followed by chalcopyrite. Most of the copper minerals are liberated; among the unliberated ones, the vast majority form simple intergrowths with pyrite or are enclosed by pyrite. The mineral composition of this tailings sample is shown in Table 4-33, and the multi‑element chemical analysis and copper phase analysis of the tailings are shown in Tables 4-34 and 4-35, respectively.

Table 4-33 Mineral composition of zinc‑sulfur separation tailings

Table 4-34 Multi‑element chemical analysis of tailings (%)

Table 4-35 Copper phase analysis (%)

Based on active experimental research on recovering copper from the zinc‑sulfur separation tailings, the mine carried out industrial test research on tailings re‑processing. The industrial test was conducted from January to July 2010 in the 650 independent beneficiation system of Ashele Copper Mine. The test results are shown in Table 4-36, and the test flow sheet is shown in Figure 4-21.

Table 4-36 Industrial test results of re-selecting copper from zinc‑sulfur separation tailings              (%)

Figure 4-21 Industrial test flow sheet for re‑selecting copper from zinc‑sulfur separation tailings

The results of the seven‑month industrial test showed that for the re‑selection of copper from zinc‑sulfur separation tailings, the average feed grade was 0.90% Cu and 1.05% Zn. The copper concentrate grade was 7.56% Cu and 9.55% Zn, with a copper recovery of 54.95%. The recovery of zinc in the copper concentrate reached 59.34%. During the re‑selection of copper from zinc‑sulfur separation tailings, the enrichment ratio of zinc was high, resulting in excessive zinc content in the copper concentrate. Even with the addition of large amounts of zinc depressants during the industrial test, it was difficult to reduce the zinc content in the copper concentrate, and this also affected copper recovery. However, an appropriate addition of Na₂S helped to lower the zinc content. During the industrial test, based on the actual quality of the copper concentrate produced, the copper concentrate from the re‑selection of zinc‑sulfur separation tailings was mixed with the copper concentrate from the raw ore beneficiation in a φ38 m thickener. This ensured that the final copper concentrate for sale contained 18% or more Cu, with zinc content generally between 3% and 4%. This not only increased the overall copper recovery but also met sales requirements.

According to statistics, from January to July 2010, the 650 independent beneficiation system processed zinc‑sulfur separation tailings for copper recovery. Since the start of the industrial test, the average monthly recovery of copper metal was 62.17 t, generating an additional monthly profit of over 1.76 million yuan.

D. Fengshan Copper Mine: Recovery of Copper

Fengshan Copper Mine, under Daye Nonferrous Metals Group Holding Co., Ltd., is located in Yangxin County, Hubei Province. By the end of 2010, proven copper metal reserves were nearly 180,000 t. Based on the current annual copper production of approximately 5,000 t, the mine can continue to be mined for more than 30 years. The concentrator originally adopted a flotation circuit consisting of one rougher, two scavengers, one cleaner, regrinding of the rougher concentrate, and a copper‑sulfur separation stage.

To improve copper recovery, the process was optimized by adding a scavenging stage for the original tailings, i.e., the original process was changed to include three scavenging stages. In actual production, a combination of xanthate and dithiophosphate (black agent) was used as collectors. For the additional scavenging of tailings, xanthate was selected as the collector and pine oil as the frother. The pulp pH was 8.5, with no modifiers added. The original on‑site scavenging circuit used 8.0 m³ flotation cells. Therefore, flotation cells of the same model were selected. Based on an additional flotation time of 6 minutes, three flotation cells were added per series. For each series, three KYF‑8.0 m³ flotation cells were added as the third scavenging stage. According to the principle of sequential return, the froth was transported by a froth pump to the first cell of the second scavenging stage, while the tailings flowed by gravity into the tailings launder. After the process optimization, the flotation flow sheet is shown in Figure 4-22.

Figure 4-22 New process flow sheet of Fengshan Copper Mine after adding scavenging stages

After the process optimization, the beneficiation process became more reasonable, producing a copper concentrate with a grade of 22.15% and a recovery of 91.06%, while the tailings grade dropped to 0.058%. Compared with before the modification, the recovery increased by 1.8 percentage points, and the annual output value increased by more than 4 million yuan.

E. Dexing Copper Mine: Recovery of Copper and Gold

Dexing Copper Mine is a large-scale porphyry open-pit copper mine in China. In addition to the main metal copper, the ore is associated with valuable components such as gold, silver, molybdenum, and sulfur. The main ore minerals are chalcopyrite, pyrite, and molybdenite; the gangue minerals are mainly quartz, sericite, chlorite, and biotite. The daily mining and processing capacity reaches 100,000 tons, with a tailings yield of 97%. To recover valuable components from the tailings, the concentrator uses gravity separation (hydrocyclones) to recover pyrite. The tailings are pumped directly to φ350 mm hydrocyclones. Pyrite, being dense and relatively coarse, is enriched in the underflow and becomes a sulfur concentrate. Under the condition of a feed grade exceeding 25%, a sulfur concentrate grade of 35%–40% and an operational recovery of about 60% can be achieved. The annual recovery of sulfur concentrate equivalent exceeds 300,000 tons, reducing solid waste emissions by more than 1 million tons per year. To recover copper concentrate, Dexing Copper Mine has installed foam collection plates in the open tailings channel. The coarse copper-bearing mineralized foam from the tailings is scraped off, then subjected to classification, grinding, and multiple cleaning stages. With appropriate addition of xanthate and No. 111 oil, a low-grade copper concentrate with a grade of over 13% can be obtained. A processing capacity of 20,000 t/d has been established, producing 5,000 t of low-grade copper concentrate annually, containing 650 t of copper and 30 kg of gold.

F. A Copper Mine in Guangdong Province: Recovery of Copper and Iron

Due to historical and technical reasons, a copper mine in Guangdong Province had a simple utilization method. Only copper was developed and utilized from the ore, and even the copper recovery was incomplete, leaving recoverable valuable elements such as Cu and Fe in the tailings. With the development of mineral processing and metallurgical science and technology, especially the innovation in modern copper hydrometallurgy, previously unavailable copper resources have been transformed from dead ore into great wealth. Currently, a process combining leaching – solvent extraction – electrowinning (L-SX-EW) for copper extraction, followed by magnetic separation of the leaching residue for iron recovery, can effectively recover valuable metals from the tailings, achieving recoveries of 55% for Cu and 40% for Fe.

The tailings are from a chlorination segregation – flotation process from the 20th century. They are powdery, gray-black in color, and appear wet when containing heavy clay. Due to the clay content, the raw material is brown, brown-red, or brown-yellow. Before segregation, the ore was a copper-iron contained ore. After segregation, most of the copper was recovered by flotation, while iron and other minerals remained mainly in the flotation tailings. The metallic minerals in the tailings include magnetite, pyrite, limonite, native copper, cuprite, tenorite, and chalcopyrite. The gangue minerals are mainly feldspar, quartz, iron-stained feldspar, quartz, and clay minerals, along with small amounts of coke residue. The multi-element analysis and copper phase analysis of the tailings sample are shown in Tables 4-37 and 4-38.

Table 4-37 Multi-element chemical analysis results of tailings (%)

Table 4-38 Copper phase analysis results (%)

The analysis results show that the copper content is 0.75%, of which acid-soluble copper is 0.42%, accounting for 56.00% of the total copper, mainly occurring as copper oxides. The acid-insoluble copper content is 0.33%, accounting for 44.00% of the total copper. This acid-insoluble copper mainly consists of fine-grained chalcopyrite enclosed in gangue, as well as cuprite and tenorite mixed with limonite and percolated/dyed into gangue minerals (i.e., the so-called combined copper oxide in gangue), plus a small amount of native copper. After the leaching process, some copper remains, mainly as combined copper oxide and chalcopyrite, along with a small amount of unleached native copper.

Based on the principle process flow recommended by experimental research and drawing on practical experience in hydrometallurgical copper production at home and abroad, the recovery of valuable metals from the tailings adopts a process of leaching – solvent extraction – electrowinning (L-SX-EW) to extract copper, followed by magnetic separation of the leaching residue to recover iron. The copper production process mainly consists of agitation leaching, solid-liquid separation, solvent extraction, stripping, and electrowinning. The production process flow sheet is shown in Figure 4-23.

(1) Agitation leaching. During screening, the raw material is washed with water and slurried. The pulp then enters an agitation tank for leaching.

The leaching process is divided into two parallel lines, each consisting of three agitated tanks, using three-stage co-current leaching. The leaching is carried out under normal temperature and pressure, with a liquid-to-solid ratio of 2:1 and a leachant concentration of 35–40 g/L, for 2 hours.

(2) Solid-liquid separation. The agitated pulp is fed to a thickener. A two-stage washing and counter-current solid-liquid classification circuit, connecting the thickener and pumps, is employed. The pregnant leach solution is pumped to the solvent extraction plant, while the leach residue is sent to a magnetic separation process to recover iron concentrate.

(3) Solvent extraction. The leach solution enters the extraction equipment. A two-stage counter-current extraction is carried out with an extractant. The extract is washed, and the raffinate flows by gravity to a raffinate pond for use as make-up water in feed slurrying and thickener washing.

(4) Stripping. The washed organic phase enters the stripping equipment. The strip solution is sent to the electrowinning cells. After stripping, the organic phase is pre-equilibrated, and the regenerated organic phase is returned to the extraction circuit for recycling.

(5) Electrowinning. The strip solution is fed into electrowinning cells. The anodes are made of insoluble alloy materials. Electric current is passed between the electrodes, and copper ions gain electrons and deposit on the cathodes, producing electrowon copper. The spent electrolyte is recycled as the stripping agent.

Figure 4-23 Principle flow sheet of the copper and gold recovery process

Figure 4-24 Process flow sheet for iron recovery

The iron recovery process uses a single magnetic separation method. The process flow consists of one rougher stage, one scavenger stage, and after regrinding the rough concentrate, two cleaner stages. Because the tailings sand has a high clay content, which significantly affects iron recovery, a counter-current washing operation is added before rougher concentration. According to particle size analysis of the raw tailings, iron is mainly distributed in the -0.038 mm size fraction. Based on actual production conditions, the regrinding fineness of the rough concentrate is determined to be 90% passing 0.074 mm. The process flow for iron recovery is shown in Figure 4-24.

The results show that the grade of electrowon copper can reach 99.9%, with a recovery of 55%. For iron recovered from the leach residue by magnetic separation, the iron grade is 55% and the recovery is 40%.

G. Foreign Cases of Copper Tailings Re‑processing

The Morenci copper plant in Arizona, USA, uses sulfuric acid to treat stockpiled copper oxide tailings, achieving a copper recovery of 73.8% and producing 50,000 tons of cathode copper per year, accounting for 13% of the plant’s copper output.

Chuquicamata in Chile uses a large‑tank leaching process with sulfuric acid followed by electrolysis. At a rate of 52,500 tons of copper per year, a total of 900,000 tons of copper have been recovered from large volumes of old tailings stockpiled for many years.

Russia and Spain have also achieved good results in recovering copper from tailings using bacterial leaching technology.

Figure 4-24 Process flow sheet for iron recovery

Abroad, combined beneficiation‑metallurgy processes are also used for re‑treating copper tailings. In Michigan, USA, copper tailings are reground and subjected to flotation (or ammonia leaching). With a processing volume of 82 million tons, 338,000 tons of copper are produced. Another process similar to carbon‑in‑leach (CIL) for gold recovery is also used in the USA: carbon particles impregnated with an extractant are added to the copper tailings pulp to recover copper. The key is that the extractant must be inexpensive. At the Almalyk concentrator in Russia, tailings are ground to approximately 50% passing 74 μm and then floated, achieving an additional copper recovery of 80% from the tailings. At the Balkhash concentrator in Kazakhstan, copper and molybdenum are recovered from low‑grade bornite tailings through flotation, regrinding, and cleaning stages.