Prime Group considers investments in mining as one of its important activities the company is focused on value creation in the mining sector, the focus is on finding new mineral reserves as it is critical to the success of our company, locating, extracting and processing these natural resources is a multiyear process that involves complex scientific, environmental and social planning.
Prime Group today mines Gold and Copper, as well as some other metals and minerals.
Africa is now profiting as never before from its natural resource wealth, and this is a key driver of its current success.
Arica is rich in resource wealth. It is estimated to contain 30 percent of the world’s mineral resources, including Bauxite, Cobalt, Copper, Gold, Graphite, Iron ore, Manganese, Nickel, Phosphate, Platinum and many other rare metals and elements.
Unlocking the potential of mining in Africa means investing in local capacity so that a flourishing mining industry can be led by local professionals.
Copper minerals and Ore are found throughout the Earth's crust. They occur in both Sedimentary and Igneous rocks. The outer 10km of the Earth’s Crust contains 33g of Copper for every ton of rock. This is not enough to make it commercially viable to extract the rock. Copper mines are only set up where there is more than 5kg of Copper per tons of rock (0.5% by mass). Ideally, the figure should be closer to 2%.
Roll over diagram of sequence the flow chart shows you how Copper is extracted from its Ore and converted into pure metal.
Mining & Processing of Copper Ore:
PROPERTIES OF COPPER
• It is ductile & a good conductor of Heat and Electricity thus used in making many things
• Copper is used for Water pipes due to the fact that it doesn’t corrode easily.
• Copper is also used in Currency, Computer chips, Integrated and Printed Circuit Boards.
• Electrical Wires, Utensils etc……
• It is easily mixed with other metals to form alloys such as Bronze (Copper and Tin) and Brass (Copper and Zinc)
Major Copper Producing Countries of the world:
Copper can be extracted from its ore by:
1. Traditional mining:
Underground: Sinking a vertical shaft into the Earth to an appropriate depth and driving horizontal tunnels into the Ore.
Open Pit: 90% of Ore is mined by this method. Ores near the surface can be quarried after removal of the surface layers.
2. Leaching
The ore is treated with dilute Sulphuric acid. This trickles slowly through the ore dissolving copper to form Copper Sulphate. The Copper is recovered by Electrolytic refining. Advantages of this process are: much less energy is use than in traditional mining, no waste gases are given off, it can be used on Ores with as little as 0.1% Copper - for this reason, leaching extraction is growing in importance.
What's in an Ore?
There are many types of Copper Ore found throughout the world. About 80% of all Copper extracted comes from Sulphide Ores.
A typical Ore contains only 0.5% to 2.0% Copper. It is a measure of the value of Copper that it is worth extracting it from such small concentrations. The Ore is a mixture of minerals and rock (called Gangue). The most common Copper minerals are:
|
Mineral |
Formula |
Appearance |
|
|
Cuprite |
Cu2O |
Red, earthy |
|
|
Chalcocite |
Cu2S |
Dark grey, metallic |
|
|
Bornite |
Cu5FeS4 |
Golden brown, metallic |
|
|
Malachite |
CuCO3Cu(OH)4 |
Bright green, earthy |
|
|
Azurite |
2CuCO3Cu(OH)4 |
Blue, glassy |
|
|
Chalcopyrite |
CuFeS2 |
Golden yellow, metallic |
|
The first step is to physically remove the Gangue.
Concentrating the ore
The ore is crushed, ground and then enriched (concentrated) by Froth Flotation. The powdered ore is mixed with a special Paraffin Oil which makes the Copper mineral particles Water repellent. It is then fed into a bath of Water containing a Foaming agent which produces a kind of bubble bath. When jets of air are forced up through the bath, the water repellent Copper mineral particles are picked up by the bubbles of foam. They float to the surface making froth. The unwanted waste rock (Gangue) falls to the bottom and is removed. The froth is skimmed off the surface and the enriched Ore (mainly the Copper mineral) is taken away for roasting. The mixture of Water, Foaming agent and Paraffin is recycled.
After this stage the enriched Ore now contains about 25% Copper by mass.
Conversion of Matte to Copper Blister
The liquid Matte is oxidised with air to form Blister Copper in a converter.
Formation of Blister Copper by reduction of Copper Sulphide: The Blister Copper produced by this process is 99% pure Copper. The name 'Blister' Copper comes from the fact that this final process produces bubbles of Sulphur dioxide on the surface of the Copper. The Blister Copper is cast into Anodes ready for electrolytic refining.
Blister Copper to Copper Cathodes:
The Blister Copper is already virtually pure (in excess of 99% copper). But for today's market a lot of industry needs it to be purified further using Electrolysis. This is known as Electrolytic refining.
The Blister Copper is cast into large slabs which will be used as the Anodes in the Electrolysis apparatus. The Blister Copper Anodes are immersed in an Electrolyte containing Copper Sulphate and Sulphuric acid, pure Copper Cathodes are arranged as a current of over 200A passes through the solution.
The Electrolytic refining of Copper produces the high quality, high purity Copper Cathodes required by industry.
The Gold & Silver Mining Process
Geological sampling
To separate Ore from waste rock, drill samples are sent to an assay laboratory for analysis. The Assay results are used to mark out zones of ore and waste rock, which are mined separately. The definition of ore and waste rock depends on the cut-off grade. If the cut-off grade is one gram per tonne (1.0g/t) all rock with more than 1.0g/t of gold will be marked out as Ore while all Rock which contains less than 1.0g/t of gold will be marked out as Waste Rock. The cutoff grade can be as low as 0.75g/t.
Assaying – Analysis of Ore Samples
Assay is the analysis of an Ore to find out the nature and proportion of the ingredients, the report of such analysis is called the Assay Report.
Types of Assaying
Fire assay
Fire assay involves adding a ground portion of the solid (or carbon) to a crucible containing a flux. Flux is used to lower the melting point of the samples to ensure the entire sample becomes liquid during firing. The flux contains Borax, Soda Ash, Silica Flour, Litharge (lead oxide) and Silver Nitrate in various quantities. In the firing stage everything turns liquid and the Lead oxide forms into small globules of metallic lead. The globules fall through the liquid and form an amalgam with any other metals it comes into contact with, including gold and silver. This all collects in the bottom of the crucible. After about an hour at 1,000 degrees Celsius the contents of the crucible are poured into a conical mould to cool. Again the lead, being more dense than other material, quickly settles to the tip of the conical mould and solidifies along with the glass-like rock remains.
When cooled, the lead is separated from the glass and then Cupelled. Cupellation involves placing the lead ‘button’ into a pre-heated Cupel at 1,000 degrees C. A Cupel is a small cup made of Magnesium Sulphate which is a material that is capable of absorbing the lead. At this temperature the metals liquefy and Lead is absorbed into the Cupel. Gold and Silver having a higher surface tension than Lead, are not absorbed and remain as a small ‘Prill’ in the bottom of the Cupel. The Cupel and Prill are removed from the furnace and allowed to cool. The Prill is then dissolved in Aqua Regia. The resulting solution is aspirated into an AA Spectrophotometer to determine the Gold content.
Aqua Regia
Silver in the solids and Carbons is determined by dissolving the sample in Aqua Regia, a mixture of Hydrochloric and Nitric acids. The sample has specific volumes of each acid added, then is placed on a heating block to digest. When completed the resulting solution is diluted and then aspirated into an AA Spectrophotometer.
1. Mining – open pit and underground
After sampling and mapping, some areas require blasting to loosen the rock prior to excavation. Other areas can be mined using a Bulldozer. Ore and Waste Rock are mined by excavators loading onto Trucks. In the open pit the Trucks take their loads to the primary crushers or to stockpiles. In the underground mine the Ore is taken to stockpiles at the processing plant, while waste rock is stockpiled for reuse as backfill once mined areas are completed.
2. Crushing
The primary crushers located at the open pit mine site, receive ore and waste at separate times. They break the larger rocks down to a size suitable for transport on the conveyor belt.
The crushers
The crushing system at the Mine open pit consists of a combination of two types of crushers, a Jaw crusher and two Stamler feeder breakers. The Jaw crusher is capable of crushing material with strength of over 150 megapascals (MPa). By way of comparison, concrete has strength of 20-30 MPa. The Stamlers deal with the softer material. The crushers are located below ground level, in a slot to minimise noise effects.
The Jaw crusher
The Jaw crusher operates by the mechanism of two large plates of Steel moving towards each other on a cyclic basis to break rock from a large size to a smaller size. The plates are positioned so that the gap at the top is larger than the gap at the bottom.
Jaw-Crusher
(1) Material is loaded into the crusher bin) by Trucks or loaders.
(2) Prior to entering the Jaw crusher the rock moves across a Grizzly
(3) The Grizzly has a series of slots in it allowing the smaller pieces of rock to fall through
(4) The remaining oversized material passes through the Jaw crusher and is sized down to less than 200mm.
(5) An electromagnet removes tramp Steel from the old workings.
(6) The material is transported to the main overland conveyor via a transfer station.
The Stamler feeder breakers
The Stamler feeder breakers are used to break moderately hard and soft material. A rotating drum covered with replaceable carbide-tipped picks serves to break the rocks, and feed the material along the conveyor underneath.
Stamler-Crusher
(1) Material is loaded into the crusher bins by trucks or loaders.
(2) The material travels along a conveyor underneath the rotating drum
(3) The picks fracture any material that is larger than the gap between the drum and the belt. Electromagnet removes any Steel.
(4) The material is transported to the main overland conveyor via a transfer station.
3. Transport
A Rubber belted conveyor transports the ore and waste rock from the open pit, to the mill and waste rock embankments. Large electromagnets remove any steel debris excavated from the mine. There are a number of conveyors required to ensure that following crushing, rock is transported to its correct destination.
4. Grinding and sizing
Ore from both open pit and underground is stockpiled separately at the mill before being fed into a S.A.G. mill with Lime, Water and Steel balls. The larger particles from this mill are returned to the S.A.G. mill for more grinding. The finer particles receive more grinding in a Ball mill, and are size classified to give a final product of 80% < microns.
5. Leaching and adsorption
Slurry of ground Ore, Water and a weak Cyanide solution is fed into large Steel leach tanks where the Gold and Silver are dissolved. Following this leaching process the slurry passes through six adsorption tanks containing Carbon granules which adsorb the Gold and Silver. This process removes 93% of the Gold and 70% of the Silver.
(1) The slurry of ground Ore and Lime moves into a series of six leach tanks
(2) Where a Sodium Cyanide solution is added. The tanks provide sufficient retention time to allow the Gold and Silver to be dissolved by the cyanide solution, Oxygen is added to assist in this process.
(3) The slurry then moves through a series of Carbon adsorption tanks. While the leaching process continues in these tanks, the primary objective is to remove the Gold and Silver from the solution. To achieve this, Carbon is fed through the tanks
(4) The Gold and Silver adsorbs in to the Carbon granules. Carbon is heated to 600 degrees C and fed into the circuit in the opposite direction to the slurry flow, moving from the last adsorption tank to the first. This is because the Gold and Silver move towards the Carbon via a diffusive process. It is necessary to have ‘fresh’ Carbon at the end of the chain, to scalp the Gold and Silver that has not been removed in the previous tanks. By the time the slurry reaches the final adsorption tank, most of the precious metals have been removed.
(5) The barren slurry, now known as Tailings, is pumped to the tailings storage facility.
(6) In contrast, by the time the carbon reaches the first adsorption tank it has recovered most of the gold and silver from solution. This ‘loaded’ carbon is then pumped to the elution circuit.
(7) Where the Gold and Silver is washed off with superheated water. The washed solution, called pregnant Eluate, is passed to the Electro winning circuit.
(8)The remaining barren Carbon is reactivated by acid washing and kilning and returned to the adsorption tanks.
6. Elution and Electro winning
The loaded Carbon is fed into an Elution column where the bullion is washed off. The barren Carbon is recycled. The wash solution—pregnant electrolyte—is passed through Electro winning cells where Gold and Silver is won onto Stainless Steel Cathodes.
7. Bullion production
The loaded Cathodes are rinsed to yield a Gold and Silver bearing sludge which is dried, mixed with fluxes and put into the furnace. After several hours the molten material is poured into a cascade of moulds producing bars of Doré bullion.
8. Water treatment
Some water from dewatering the mine, from the embankment under drains and decant water from the tailings pond is recycled for use in the grinding circuit. Excess water is pumped to the water treatment plant and treated to the required standards before discharge into the River.
9. Tailings disposal
Waste rock from the open pit mine is used to build the embankment structures. The embankments retain the tailings slurry in a pond where solids settle and compact. Water is decanted off and used in the process plant.
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