Vat leaching
The simplest and certainly the cheapest method of leach processing gold ores is by vat leaching.
There are many opinions on how to design and operate vats, most of these operate satisfactorily for small scale operators.
The usual flaw in most of these vat designs is that the ore is not fully wetted and thus some of the ore is not leached. Any ore zones which are not fully wetted at the start of the leaching process will not miraculously become wetted during the leaching cycle.
The following is the only method I know of how to build and operate a vat and recover all of the cyanide soluble gold.
Note that the dimensions and weights given are directly scaleable for smaller vats, it is usually only the pipe diameters and the pump sump dimensions which are retained where possible.
The vats I have run are a maximum of 20,000 tons as a single cell.
Vats for small scale mining are usually configured around a 5,000 ton maximum ore parcel per cell.
A large, say 50,000 ton vat will be subdivided into 10 x 5,000 ton cells.
The reason for this is twofold, it is easier to control the process requirements in a cell of that size and it also simplifies the material handling aspects.
Most leaching vats are around 100 metres long by 50 metres wide and divided into 6 cells
Generally the cells are 25 metres wide by 2.5 to 3.5 metres high by 30 metres long. Fall of the length of the base is dependent on the ores being treated, a coarse free wetting ore may be in a cell with fall of 1 cm ( 1/2") per metre, a less permeable ore can have a fall of half this but the 1:100 fall is pretty standard.
There is a fall across the cell, generally around 1 in 25 so a 25 metre wide cell will have a 1 metre fall across it.
These sizes have proven optimal both in leaching performance and in materials handling.
The vats are usually dozer formed with a bobcat finish.
A pumping sump is dug at the lowest corner of the cell, it is usually configured as a 1metre cubed hole .
An access hole is maintained to the pump sump, usually a series of cyanide drums welded end to end for short lived projects or steel pipe 500mm (1.5') or greater diameter for extended use, 200 litre (55 gallon) drums are also often used. This pipe must be bedded on a series of large flat stones in the pump sump to avoid the pipe rupturing the liner. The top of the pipe projects at least to above the wall height of the cell.
The cells are plastic film lined, usually 1.5mm HDPE which is heat welded. Glueing is viewed with suspicion due to failures attributed to salts in the process water.
A cheaper alternative is to use 0.2mm PVC film as a one piece installation.
This does require a lot more bed preparation to flatten out all spiky bits of rock with a bobcat or loader with a lot of weight in the bucket and constant wetting of the working surface.
Overall it is the cheapest option even allowing for the extended bed preparation and if the bed preparation has been done thoroughly it will work as well as the heavier films.
Many of these PVC film vats have operated without any problems for several years.
These vats are usually closed down due to a gypsum build up in a layer just above the graded gravel bed rather than materials failure.
The type of liner used will most likely be mandated by local mines department regulations, the trend has been to the thicker liners.
Drainage inside the vat is provided by 50mm (2") slotted agricultural drain hose which is teed at 1.5 metre intervals off a 75mm (3") PVC pipe.
The 75 mm pipe runs from the pumping sump along the length of the cell at the base of the wall with the greatest depth.
The end of this 75mm pipe furtherest from the sump has an end cap glued on to it.
An elbow is fitted to the 75 mm pipe at the inside of the sump and the pipe then runs up the access hole to a tee piece at the top of the sump.
The horizontal leg of the tee leads to a diaphragm pump, usually diesel powered, the same units as are used on construction sites for dewatering.
This pump will deliver the pregnant cyanide solution to the carbon column, flow rates for the 30 x 25 metre cells are around 10 metres cubed per hour.
The vertical leg of the tee piece leads to a header tank which is at least 6 metres (20') above the level of the top of the vat.
A constant head arrangement is achieved by running the header tank in overflow mode from a separate water source.
A ball valve is installed adjacent to the tee piece on each of these two lines.
The 50mm ag pipes run across the bottom of the cell to the base of the side wall, note that these ag pipes are running uphill on the steep sloping base.
At the base of the side wall the ends of the 50mm ag pipes have a piece of 10 to 12mm poly pipe taped into them.
These lengths of 10mm poly pipe run up the wall to about 1 metre above the wall, ball valves are fitted to the upper ends of these pieces of pipe.
Filter fabric socks are wrapped or slid on all of the ag pipe lengths.
The entire floor is now covered with sized clean creek bed gravel, the usual sizing is 0.5mm (25 mesh) to 5 to 6 mm (1/4").
The gravel bed depth is at least 300 mm (1') and will cover the ag pipe by 250mm (10").
This gravel bed is crucial to the operation of the vat, if the gravel cannot be gotten in the required quantity then the vat will not operate properly.
Before the gravel is put into place a length of 50mm ag pipe is coiled inside the pump sump and connected to a length of 50 or 75 mm PVC pipe which rises up the access hole to tee into the pumping line just after the ball valve on the 75mm line adjacent to the tee on that line. This line is referred to as a scavenging line.
This pipe also has a ball valve fitted so that it can be isolated from the main pumping line.
This scavenging line is used only when a cell is being drained and recovers the liquor which does not report to the main pumping circuit.
The ag pipe part of this scavenge line is also fitted wjth a filter fabric sock and has the sump end capped.
None of the cells have drain pipes installed through the walls to empty out the ore after leaching. These drain pipes always leak solution along the outside of the pipe, despite a lot of trying no one has come up with a leak proof version.
The ore is loaded into the vat as a mixed material. It is vital that the charge is well blended and machine mixed. This ensures that the clay and other fines are evenly distributed and that zones of less permeable material are avoided.
You should never feed pulp directly into a vat from a mill, doing so will almost guarantee leaching problems due to the formation of fines zones.
The usual practice is to place the mill discharge into a dam and when the dam is full and the water has been siphoned off to use an excavator to mix the material well before placing it in the vat.
A large (40 ton+) excavator is used to place the ore into the vat, the boom length is such that the vat can be filled without having machinery drive over the surface of the placed ore
The vat is filled to about half a metre from the top of the walls.
When the vat has been filled the ore is covered with a cyanide solution which is allowed to slowly saturate the ore.
When the leach solution is present in the pump suction line the process of hydraulic lifting can start.
This consists of backflowing water from the header tank through the ag pipe drains to totally saturate the ore charge with water.
By selectively closing the valves on the pipework the water is limited to only flowing backwards through the ag drains.
At this stage the valves on the ends of the 10mm pipes on the ag pipes are all open.
The aim of the procedure is to have the water introduced as a slow moving front which saturates all of the ore.
It will take two or more days to totally fill the vat with water, the process is controlled by the constant head from the header tank.
The process will not work if a pressure pumping filling is used.
What you are trying to do is to remove all air from the ore solids in the vat and replace it with water.
This does not occur in the initial filling of the vat with cyanide solution, it can only be achieved by the hydraulic lifting process.
As the lifting water gradually moves through the cell, water will sequentially begin to come out of the 10mm pipes on the top of the walls.
These pipes act as air bleeds for the ag pipes.
If one of these pipes has no flow or a lesser flow than the other pipes then the valves on the adjacent pipes are closed so that the water flow is increased through the ag pipe and air bleed pipe in question.
When the bleed pipes show similar flows with no air bubbling they can be closed off at the valves as the water front has then passed them and there is no reason to have water discharging from them.
When all of the bleed valves are closed off then the ore parcel is totally saturated.
The lifting water is allowed to continue to be flowed into the vat until there is a layer of water about 300mm (1') deep over the surface of the vat.
The leaching stage can now proceed.
The valves on the pipework are adjusted so that the pump sucks only from the 75mm PVC pipe connected to the ag drain pipes.
The pregnant liquor is passed through 3 x 1 ton carbon columns in series.
These columns are often made from what materials are at hand but generally around a metre in diameter x 4 metres high
The reason for the three columns is that the first column does most of the gold adsorption, the second column acts as a scavenger and the third column is used when the first column is being changed over after loading so that there are always two columns in series doing the adsorption.
More importantly the third column is there to handle the high gold tenors in the leach liquor when a cell is either first started or restarted after resting.
These gold tenors can reach over 5 grams per cubic metre and will overwhelm a two column setup.
The barren liquor exiting the carbon circuit is returned to the rest of the liquor covering the ore, it is usual to run spray bars of some type to oxygenate the return liquor.
If the pump operates such that it can pump more liquor than the pump feed line can deliver, the pump product will include some air which has been sucked in through the ag lines.
This air will also help re-oxygenate the liquor but the carbon adsorption columns must be designed to accomodate this air, you do not want carbon floating out the columns.
Solution tenors are checked daily and the vat is run until the solution tenor going into the carbon circuit is the same as the exit tenor.
If the tenor is above 0.5 grams per cubic metre at equilibration this usually indicates that the carbon is loaded to the level where it requires stripping and this carbon is sent to the strip circuit.
It also is possible that the carbon has reached its equilibrium loading level for that tenor liquor and that the gold content of the cell is so low that the gold tenor will not increase without some other actions being taken.
This is generally around 0.2 grams per cubic metre and may take several months of leaching to achieve.
In this case the return water from the carbon circuit is sent to another cell and the first cell is allowed to completely drain, this is where the scavenge line comes into play.
After a couple of weeks drying out the cell has all of the ore dug and mixed by an excavator, this is referred to as fluffing up the ore.
The aim is to break up zones of fines which will have formed during the leach cycle.
The fluffed ore is allowed to aerate for at least a month before it is levelled out and the entire cycle is repeated.
This repetition of cycles is continued until it is no longer economical to do so.
It is not surprising to have initial liquor tenors of 2 to 3 grams per cubic metre on a restart for the first day or so.
When the vat is finally regarded as exhausted it is drained and after a couple of weeks drying out the ore is removed, replaced with fresh ore and the cycle started again.
Note that the exhausted ore is usually removed by trucking it to a tailings heap, vehicles can usually drive safely on the tailings after a couple of weeks drying.
Deano
