The Effect of Aeration on Chalcocite Heap Leaching

Abstract

The effect of aeration on chalcocite heap leaching was studied by analyzing the historical data from two chalcocite heap leach pads located in Chile, one with aeration and the other without aeration. The data showed that the copper leaching kinetics of the aerated heap was greater than that of the unaerated heap within a certain leaching period, after which there was no effect of aeration on the copper leaching kinetics. The total acid consumption per ton of ore processed was higher in the case of aeration, which was supported by the higher pH of the PLS. But the net acid consumption per ton of copper produced was the same in the two cases. Approximately 5% of the total iron added to the unaerated heap was precipitated inside the heap. In contrast, there was no net iron precipitation or generation inside the aerated heap. Bacteria activities were thought to occur even in the unaerated heap, but the extent was much less than in the aerated heap. Given the importance of forced aeration in chalcocite heap bioleaching, mathematical modelling will be applied in the future work to assist the optimization of the aeration system design.

Introduction

Aeration provides oxygen and carbon dioxide for microbial activities to occur, a fundamental process occurring in chalcocite heap bioleaching. The role of bacteria in copper heap bioleaching is to oxidize ferrous to ferric in the presence of oxygen, which acts as the oxidant to solubilize copper from copper sulfides [1]. It is reported that copper leached is directly correlated with oxygen consumed and that oxygen levels are maximum near the bottom of the heap [2]. Chalcocite oxidation in heap bioleaching is reported to occur according to Eqs. 1 and 2 [3]. The oxidant, ferric ions, is regenerated from the oxidation of ferrous by oxygen assisted by bacteria according to Eq. 3.

${\text{Cu}}_{ 2} {\text{S + 1}} . 6 {\text{Fe}}^{{ 3 { + }}} \to 0. 8 {\text{Cu}}^{{ 2 { + }}} { + 1} . 6 {\text{Fe}}^{{ 2 { + }}} { + }\left( {{\text{Cu}}_{ 1. 2} {\text{S}}} \right)$

(1)

$({\text{Cu}}_{1.2} {\text{S}}) + 2.4{\text{Fe}}^{3 + } \to 1.2{\text{Cu}}^{2 + } + 2.4{\text{Fe}}^{2 + } + {\text{S}}^{0}$

(2)

$4{\text{Fe}}^{2 + } + {\text{O}}_{2} + 4{\text{H}}^{ + } \to 4{\text{Fe}}^{3 + } + 2{\text{H}}_{2} {\text{O}}$

(3)

Aeration can occur passively where air is drawn into the heap by natural convection, or actively where air is forced into the heap through aeration system installed near the bottom [4]. The aeration rate and aeration system design are critical for achieving optimum copper extraction. This paper reports findings on the copper extraction, acid consumption, and iron behaviour in two chalcocite heap leach pads, one with aeration and the other without aeration, at the case study mine located in Chile.

Case Study Site

The copper grades of the unaerated and aerated heaps were determined by sequential copper analysis, or diagnostic leach, which involves a sequential digestion of the mineral sample with acid, cyanide, and aqua regia. The initial acid digestion dissolves the oxide copper minerals, such as cuprite (Cu₂O) and chrysocolla ([Cu,Al]₂H₂Si₂O₅[OH]₄ · nH₂O). The cyanide digestion dissolves copper from the secondary sulphides, such as chalcocite (Cu₂S) and covellite (CuS). The final aqua regia digestion releases all of the remaining insoluble copper, associated with the primary sulphides such as chalcopyrite (CuFeS₂) and bornite (Cu₅FeS₄). This insoluble copper (CuI) is not generally recoverable by heap leaching. The results of the sequential digestion are shown in Table 1.

Table 1

Copper grades of the ore in the unaerated and aerated heaps

	No aeration	With aeration
Acid soluble Cu, %	0.134	0.135
Cyanide soluble Cu, %	0.408	0.279
Total soluble Cu, %	0.542	0.414
Total Cu, %	0.596	0.536

The particle size distributions after agglomeration are shown in Fig. 1a. The P80 of the unaerated heap was slightly lower than that of the aerated heap. The height of the unaerated heap was 7.5 m with a surface area equal to 21,877 m². The other heap has a height of 8.7 m and a surface area of 24,247 m². The heaps were irrigated with raffinate solution containing sulfuric acid and iron as ferrous and ferric. Figure 1b shows the irrigation rate of the two heaps over 300 days of leaching. The aeration rate of the aerated heap was less than 0.23 m³/m²/h, but the actual value was not monitored by the mine. Copper, iron, and acid concentrations in the raffinate and PLS were monitored in the course of leaching. Copper and total iron concentrations were measured by ICP and ferrous concentration was measured by UV/Vis.

../images/468727_1_En_110_Chapter/468727_1_En_110_Fig1_HTML.gif — Fig. 1
a Particle size distribution of the agglomerated ore; b irrigation rate of the unaerated and aerated heaps

Results and Discussion

The Effect of Aeration on Copper Leaching Kinetics

Figure 2 shows the copper concentration in the PLS and the cumulative extraction with and without aeration. Despite the lower cyanide soluble copper grade and larger particle size in the heap with aeration, which are unfavorable conditions for leaching, the aerated heap yielded PLS with a higher copper concentration in the initial 100 days of leaching, suggesting the beneficial effect of aeration. From 100 days onwards, the PLS concentration in the two cases were similar. Furthermore, copper was extracted with a faster kinetics from the aerated heap over a 200-day leaching period, after which the leaching kinetics in the two cases were similar.

../images/468727_1_En_110_Chapter/468727_1_En_110_Fig2_HTML.gif — Fig. 2
Copper concentration in the PLS and the cumulative extractions: a unaerated heap; b aerated heap

Despite the unfavourable leaching conditions such as lower cyanide soluble copper grade and larger particle size, the final copper extraction in the case of aeration was 81%, which was 20% higher than the case without aeration. These results indicate that aeration was likely to significantly enhance copper extraction. But the enhancement is only effective within a certain leaching period, after which aeration no longer has a beneficial effect. This may correspond to the transition of chalcocite leaching from a diffusion controlled first stage of leaching to a chemical reaction controlled second stage of leaching.

Total and Net Acid Consumption

Figure 3 shows the total acid consumption of the unaerated and aerated heaps and the pH of the PLS of the two heaps. The total acid consumed per tonne of ore processed was calculated by subtracting the cumulative acid in the PLS from the cumulative acid in the raffinate, divided by the total tonnage of ore in the heap. The acid consumed by copper was calculated as the stoichiometric amount of acid required for the observed copper dissolution. In this calculation, the weight ratio was taken to be 1.54, meaning that production of 1 kg of copper requires 1.54 kg of sulfuric acid. The total acid consumed in the case of aeration (8.48 kg/tonne) was higher than the case without aeration (7.49 kg/tonne).

../images/468727_1_En_110_Chapter/468727_1_En_110_Fig3_HTML.gif — Fig. 3
Total acid consumption per ton of ore processed and pH of the PLS: a without aeration; b with aeration

Another calculation was the net acid consumption, which represents the difference between the total acid consumption and the amount of acid consumed by copper dissolution. The latter is recovered from the SX/EW plant. Despite the higher total acid consumption of the aerated heap, the net acid consumption per tonne of copper produced was the same in the two cases (1 kg of net acid per kg of copper produced). The same net acid consumption suggested that the acid consumption behaviour of the gangue minerals was similar in the two heaps and was not affected by aeration, as supported by the similar gangue mineral compositions in the two heaps. The gangue minerals consist of 59% kaolinite and 41% sericite in the unaerated heap, and 57% kaolinite, 41% sericite and 2% biotite in the aerated heap. The greater total acid consumption in the case of aeration corresponded to a higher pH in the PLS of the aerated heap.

The Behavior of Iron in the Two Heaps

Figure 4 shows the concentrations of total iron in the raffinate and PLS and the total iron loss in the heaps. The total iron loss was calculated by subtracting the mass of total iron in the PLS from that in the raffinate, divided by the mass of total iron in the raffinate. The positive total iron loss is an indication of ferric precipitation. The total iron loss in the unaerated heap decreased from 70 to 10% in the initial 10 days of leaching, then gradually decreased to 5% on day 40, and stabilized at 5% in the remaining time of leaching. In other words, approximately 5% of the total iron added (total iron concentration in raffinate is shown in Fig. 4) to the heap was precipitated inside the heap. In contrast, the total iron loss of the aerated heap was stabilized at zero, suggesting that there was no net precipitation or generation inside the aerated heap.

../images/468727_1_En_110_Chapter/468727_1_En_110_Fig4_HTML.gif — Fig. 4
Total iron concentration in the raffinate and PLS and the total iron loss in the heaps: a unaerated heap; b aerated heap

The Role of Aeration

The total copper extracted in practice comprised the complete acid dissolution of copper oxides and the partial oxidative leaching by ferric of copper secondary sulfides. An assumption was made that only ferric added to the heaps via irrigation was responsible for the oxidative leaching and that no ferric was generated by the bacterial oxidation of ferrous to ferric. Figure 5a shows that under this assumption, the predicted copper extraction per tonne of ore processed was lower than the actual copper extraction obtained. This result suggested that even in the unaerated heap, ferric was generated inside the heap by bacterial activities, which must have been supported by natural convection. The beneficial effect of forced aeration in improving copper extraction was shown in Fig. 5b.

../images/468727_1_En_110_Chapter/468727_1_En_110_Fig5_HTML.gif — Fig. 5
Actual copper extraction and copper extraction predicted by ferric added via irrigation: a unaerated heap; b aerated heap

Conclusion

The study assessed the effect of aeration on chalcocite heap leaching by analyzing the monitoring data from two chalcocite heap leach pads located in Chile, one with aeration and the other without aeration. The data showed that the copper leaching kinetics of the aerated heap was greater than that of the unaerated heap, but the beneficial effect was only observed in the initial 200 days of leaching, after which forced aeration did not affect copper leaching kinetics. The total acid consumption per ton of ore processed was higher in the case of aeration, which was supported by the higher pH of the PLS. But the net acid consumption per ton of copper produced was the same in the two cases. Around 5% of the total iron was precipitated in the unaerated heap. In contrast, there was no total iron loss in the aerated heap. Bacteria activities were thought to occur even in the unaerated heap, but the extent was much less than in the aerated heap. Given that the aeration rate and aeration system design are critical for achieving optimum copper extraction, mathematical modelling will be applied in the future work to assist the design and optimization of the aeration system.

Acknowledgements

The authors would like to thank the heap leach operation to provide the monitoring data.