**3.3 Heap leaching**

Heap leaching (crushed and agglomerated) [18] is composed of air, acid and microorganism where commutation takes place on rock samples to turn it to smaller particles which increases the surface area for acid digestions and conditioning it to microorganisms, particle should not be very fine and should be piled allowing a simplifies aeration pipe placed to facilitate air flow. To improve drainage of the mineral containing solution from the bottom of the ore, conditioned ore is spread on specially engineered pads (lined with high-density polyethylene (HDPE)), which consist of perforated plastic drain lines and air also supplied to optimize the growth of microorganism [7]. Heap can be large up to kilometer long, but commonly less than 500 m wide and 10 m long, the size and height of a heap depends up on air (for bacterial to grow) water, acid, heat generated due to the process and its dissipation [31]. Heap surface should be permeable enough for the sulfuric acid to infiltrate and dissolves iron to ferric solution producing ferric ion that react with copper sulfide results ferrous ion and copper solutions. Acidithiobacillus Ferrooxidans oxidize iron where the bacterial can be inoculated and works by attaching itself to ore, with having free movement. After collecting PLS (pregnant leach solution), then solvent extraction is followed where the target mineral recovered and formed into cathodes.

This aerobic bacteria works only in the presence of oxygen in the heap, those bacteria consume it from the solution where oxygen is in liquid phase. This process enhances the conversion of ferrous to ferric ions as per the reaction below.

$$\text{Fe}^{2+} + \text{0.25O}\_2 + \text{H}^\* \rightarrow \text{Fe}^{3+} + \text{0.5H}\_2\text{O.} \tag{18}$$

Heap some time can be crushed 19 mm with rotating drum with acidified water [29] aeration can be conducted using low pressure fans those directing air through piping on the pad [26]. It is clear that heap leaching requires the preparation of the ore, primarily size reduction, so as to maximize mineral-lixiviant interaction and lay of an impermeable base to prevent lixiviant loss and pollution of water bodies. Heap leaching basically used to leach low-grade ore of copper and zinc, even in the case of copper grade level can be (0.2–2%). To have an effective heap leaching

process a mathematical model has been developed by taking heat, mass transfer, liquid, gas flow and chemical process in to consideration [31]. Heap also employed to bioleach silicate mineral, in a study where two microorganism were tested 'Ferroplasma acidarmanus or the common Acidithiobacillus ferrooxidans against the amenity of silicate minerals. Beside oxidation process energy was generated from flat plate solar energy collectors where heap is designed by HeapSim, heap bioleach simulation tool was used to simulate the heap and process occurring in the heap, even calculating the copper output [32].
