Mackie Continuous Vat Gold Leaching

Mackie Continuous Vat Gold Leaching was introduced by Dan Mackie in 1985 and is also kno wn as Mackie Vat Leaching Jig. In 1990, Innovated Limited began the commercialization.

The Mackie Vat Leaching Jig tries to employ a leaching solution with enough energy which must be able to fluidize the auriferous ore. For this, the solution is fed from an elevated tank until a grating built with holed pipes so that the solution can be discharge for the holes. The ore is fluidized in each cycle and try to get hydraulic equilibria. Once, got this state, the empty space formed at the top, must be filled with fresh ore. The leaching time is in the range 12 to 72 hours and depends on the particle size and porosity offered by the ore. Normally, the particle size is less than ¼ “.

Figure 4 shows schematically this vat system

The fluidization is influenced by the following factors:

- Bed expansion. There is uniform separation between particles during each pulsation. At the beginning of the first cycle, particles try to take a position according to its size so that the equilibrium can be obtained in the first moments. In a bed at onset of fluidization the voidage is a little larger than in a packed bed, and it actually corresponds to the loosest state of a packed bed.

- Percolation. Fine particles can cause problems because they can behave like a mud absorbing the cyanide solution and doing difficult the movement of the bed.

- Internal friction between particles. Repose angle can be modified in each pulsation and coarse particles can be affected. Thus, the movement of coarse particles is directly influenced by gravity forces. Particle size distribution as a function of height is closely related to the voidage distribution in beds.

- Bubbling. The solution under the ore trends to create discontinuity during each pulsation producing a movement sometimes no continuous.

- Hydraulic elevation. It necessary appropriate pressures which allow expand the ore and consequently mix the solution and the ore uniformly. This pressure is influence by the solution tank elevation.

- Exposition rate. Coarse materials require more time and then, it is necessary repeat several times each cycle (i.e. solution tank is discharged continuously). Other factor to be considered is the depth bed.

Continuous vat leaching offers the following advantages:

• High and fast recovery due to the high rate of leaching.
• Agglomeration is not necessary.
• Flexibility for plant location.
• Low requirement of energy.
• Maintenance has a low cost.
• Coarse gold particles can be recovered by gravimetry before leaching.
• Residue is washed and detoxified efficiently.
• No problems with tailings disposal.

Capital cost is 25 to 37% lower than a cyanidation plant by agitation. The operational cost is lower than conventional cyanidation but lightly higher than heap leaching.