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COPPER LEACHING IN THE LATE 19th CENTURY
This diagram shows the process in use in 1865 when it was described by Mr Henderson, the company's chemist, in the Mining Journal. The Mining Journal description has been transcribed and can be downloaded HERE. The whole process took about 3 days to process one vat of material and there were 16 vats so that about 1000 tons could be processed each month. If you want more information about a part of the process, click the appropriate box. You may need to scroll the page, depending upon your screen size.
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The picture (left) is a plan of the treatment works in the 19th century
(click for enlargement). The plan shows White Barn Farm on the left, the
entrance gorge leading to West Mine and the buildings of the works (in black).
Wood Mine is on the right hand margin with the engine house outside the
entrance. The main sandhill is just below the centre of the map.
Mining
The processes for extraction of rock from the mines are described elsewhere.
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Crushing
The crushing plant to reduce the rock to sand was steam driven. Within the mine, the ore was roughly spalled into manageable pieces. The crushing plant reduced the ore to a size which would fit through a 1/2 inch screen. No screen was however required as the ore was comparatively soft and crushed easily.
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Loading leaching vats
Crushed sand was placed in a wooden or slate vat. The ore dropped from the crushing rolls into a wagon on a light tramway which extended over the 16 leaching tanks. The tanks were 11 feet long by 8 feet wide and a little over 4 feet deep. Ideally, they were made of stone but wood was also used. Each tank had a false bottom drilled with 1/2 inch holes and covered with brushwood to retain the sand but allow the leachate to pass through. The tanks were filled to within 2 inches of the top and held about 9 tons of ore.
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First leaching
Fresh acid was used on the first leaching. Enough acid was added to dissolve, in theory, 75% of the copper contained in the ore as calculated by the chemist from an assay of the ore. The acid was allowed to percolate downwards until it disappeared from sight.
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Second leaching
Wash water was used on the second leaching. The wash water contained residual acid and some copper and was added to within 2 inches of the top of the tank.
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Recirculating
The acid and wash water was pumped around through the sand to leach out the copper salts. The pumping was carried out at 2 hourly intervals. The description does not say how many pumps were used but it is possible that there were only one or two to serve all the tanks.
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Draining
Before the copper in the rock was all used up, a saturation solution was transferred out. The transfer was started when the copper concentration in the liquor stops increasing. The idea was to leave some copper in the sand so as to ensure that the solution drained off at this stage was saturated. The residual copper was removed in later stages.
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Rinsing
To make sure all copper was removed, the sand was washed with fresh acid. At the end of this stage, the bin should contain only sand and traces of acid.
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Water rinsing
Finally, the sand was washed with fresh water to remove the last acid traces.
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Emptying of sand
Clean sand was removed from the vats in trucks. Emptying and the final water rinse were not separate operations as the process involved taking off about 9 inches of sand at a time before adding more water and repeating the rinse until the vat was empty.
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Disposal of sand
Waste sand was tipped on a pile 100 feet high which was removed in the 1950s. The engine was used to haul away the wagons containing waste sand. There is reference in a newspaper advertisement of 1875 to attempts to sell the sand as a raw material.
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Treatment of copper solution
The copper solution was put into one of five or six vats (the number is unclear from the description) which were of similar size to the leaching vats. One precipitating tank was found to suffice for three leaching tanks. Light scrap iron is put into the vat which precipitates copper through ion exchange. The iron dissolves into iron chloride which is kept in solution.
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Drying and packing copper
Once a month, the precipitation tanks were cleaned out. The copper lay in the bottom in fine particles and it was washed with water before being dried and packed into barrels. The dried copper (about 75% pure) was sent away for smelting and purifying.
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Acid
Fresh hydrochloric (muriatic) acid was brought in by rail in carboys. The company had its own carboys and a wagon by which the acid was brought to the mines from Chorley (Alderley Edge) railway station. At one time, recycled acid was tried (see below) but this was found to be more expensive than buying fresh acid (shades of the present!).
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Water
Fresh water was used for washing. There are a number of artificial ponds in the woods around Alderley Edge and it is presumed that these were used to store water for the boiler and the process. It is possible that water was extracted from the mines as a drawlift is referred to in the sale particulars when the mine closed.
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Washings
The wash water was retained for re-use as it contained a weak copper solution and some acidity. By recycling the washings, disposal problems were reduced and the efficiency of the process was increased.
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Scrap iron
Scrap iron came from the tinplate and other local industries.
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Disposal of iron residues
The waste product from the ion exchange was ferric chloride. This was dumped most of the time although an attempt was made at one stage to recover the hydrochloric acid. The iron chloride solution was heated and then sprayed into a furnace surmounted by a coke packed column 60 feet high. In the furnace, hydrogen chloride gas was released which was scrubbed in the column and collected as the acid. The residues in the furnace were rich in iron oxide but also contained cobalt, arsenic and manganese. Cobalt was obtained as a by-product but the dangerous nature of the process and the low cost of fresh acid meant that the process was short-lived. In 1874, the iron liquors were offered to sale as a potential source of iron oxide used in purifying town gas.
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