Heap Leaching - Limiting Long-Term Environmental Liability

While heap leaching is widely used as part of normal copper recovery operations, the process can produce high volumes of sulphuric acid and lead to potential environmental liabilities.

The usual way to close an operation is to turn off the irrigation, wait until draining stops and rehabilitate the dump. The tailings reservoir should remain chemically stable, but this depends on the method chosen for the closing operation.

When the lixiviation loop, defined by the transport of solution amongst the Heap - PLS pond - SX plant - EW - Raffinate pond – Heap, has a high concentration of metals such as aluminium, magnesium and iron, there is a high probability that the sulphate salts will crystallize and remain in the tailings reservoir.

Sulphate salts that come into contact with water can become sulphuric acid, creating an environmental risk and a long-term liability for the operator. Processes to avoid this occurrence should be considered as part of the Mine Closure Plan.

For the purposes of our study, our focus is on the production of copper from oxide ore, which involves crushing, agglomeration, acid heap leaching lixiviation, solvent extraction and electrowinning.

A typical copper oxide ore may include several kinds of mineral species. One can find chlorates, iodated sulphates, silicates, carbonates, phosphates, arsenates, molibdenates, oxides, sulphur and clay of copper together with other mineral species not containing copper, or gangue.

The way minerals and gangue distribute their mass in a particle rock depends on geological events.

From a process point of view, it is important to know how the mineral containing copper is distributed both on the surface and inside the rock.

Sometimes copper minerals are homogeneously distributed all over and inside the particle. This type of distribution is not normal for oxide copper ore, but usual for sulphide copper ore. Oxide copper ore is commonly distributed as films of mineral over the surface of the rock and also inside veins or grains inside the particle. This is very important for the process engineers to know in advance when they design the processing strategy, as, if much of the mineral is associated with internal structures, the ore needs to be crushed down to a size small enough for liberation of the copper mineral particles.

Normally, the concentration of copper in oxide ore is low, and crushing to a very small size does not become economical for the project. The optimal operation choice is to crush down to 2.5-5 cm, followed by an agglomeration process in which fine particles fix to the larger ones by the addition of a mix of sulphuric acid and water. This process helps the solution to penetrate inside the particles. This kind of operation means that the final result depends on two types of reactions: the surface reactions and the inner reactions. The surface reaction happens quickly, (e.g. within the first few days there can be up to 60 % of recovery).

The inner reactions account for a further 20% and combined, the typical 80% recovery can take up to a month.

The production of pregnant leach solution (PLS) from a heap leaching operation must be constant and stable all over the year. The dynamics of such an operation means that at any one point in time, some piles will be in operation with intermediate leach solution (ILS), while others will be operating with raffinate solution, in draining time, in discharge or charge mode, or in the actual surface preparation stage.

There is a well-known process inside the leaching procedure; it is the copper process. The leaching subject is to recover copper. The acid required is supposed to be exclusively for copper recovery. The acid is not supposed to be used for the recovery of other elements. So, if there is any other element in the production (known or unknown), that production would be a parasite of the copper process.

To limit the possibility of a long-term environmental liability, it is imperative that the crystallization be done outside the heap, and totally under operational control. Rivera et al (2007a; 2007b) describe how crystallization from acid solutions can be done by a controlled process by creating a process unit for crystallization of raffinate solution before it enters the heap, thus avoiding the creation of problematic tails and the associated liability.

In terms of current tailings in reservoirs, the options of ‘washing’ and treating the washing water to recover the sulphate salts should be considered as a component of a mine closure plan to reduce the long-term risk and environmental liabilities.

There is a high probability that for those ores with high amounts of aluminium, magnesium and iron, the heap leaching run with parasite process produces a high volume of waste sulphuric acid and an environmental liability.

A heap leaching process of 21,000 DTD (dry tonnes per day) of mineral produces at least 6.9 million of DTD of solid tailings in one year. On an annual basis, the tailings could contain 494,250 tonnes of Al2(SO4)3-18H2O, 533,230 ton of MgSO4-7H2O and 331,497 ton of FeSO4-7H2O. This equates to 1.4 millions tonnes of metallic sulphate salts. If only 10 % actually end up in the tailings reservoir (because, for example, the loop was not always fully saturated) that would result in 136,000 tonnes of metallic sulphuric salts in the tailings, which in turn could become sulphuric acid, with heavy metals escaping into the environment.

We suggest that the tailings should be washed with fresh water before they are released to the reservoir, rather than with water containing low copper concentrations, or with water that contains low concentrations of aluminium, magnesium and iron.

It is further recommended that heap leaching loops have an extra unit for crystallization in order to avoid the potential environmental liabilities.
Ultimately, it is vital to determine the processing options in accordance with the Mine Closure Plan, in order to avoid creating a long term environmental hazard.