As well as recovery from ore bearing rock by smelting, copper was also recovered from the waters of the mountain by a process called precipitation. This method was carried out from before the end of the 18th century as it was described by Bingley in 1800.
...But a far richer produce of copper is obtained from the water lodged in the bottom of the bed of ore, which is highly saturated with the precious metal. This is drawn up, either by means of whimsies or windmills, to the surface, and then distributed into a number of rectangular pits thirty six feet long, some pits more and some less, twelve to fifteen feet broad and twenty inches deep.
To speak in the language of the adept, Venus must make an assignation with Mars, or this solution will have no effect. In plain English a quantity of iron must be immersed in the water. the kind of iron is of no moment; old pots, hoops, anchors or any refuse will suffice... These they immerse into the pits; the particles of copper instantly are precipitated by the iron and the iron is gradually dissolved into the yellow ochre; great parts of it float off by the water and sinks to the bottom. The old iron is frequently taken out, and the copper scraped off; and this is repeated till the whole of the iron is consumed. the copper thus procure differs little from native copper and is prized accordingly and sold for prices of £25 to £45 a ton.
This mode of precipitation is nor new; it has been practised long in the Wicklow mines in Ireland, and above a century in those of Hern-grudnt in Hungary where the precipitate is called Ziment copper. The water of the Hungerian mines are much stronger impregnated with copper than those of Parys mountain. The first effects it's operation in twelve or about twenty days, the last requires two months. horse shoes, iron made in shapes of hearts and other forms are put in the foreign waters and when apparently transmuted, are given as presents to curious strangers.
The ore of Parys mine abounds with sulphurous acid which, united to water percolates through the fissures of the vein, combines with the copper and holds it in solution. The water thus impregnated is conveyed into pits in which iron has been put. The acid having a greater affinity for iron than copper combines with the iron and leave copper at liberty to be precipitated in a metallic form.
CuSO4 + Fe(Metal) = FeSO4 + Cu (Metal)
This precipitated copper is a congeries of minute granules closely united and is nearly pure metal. To expedite the process of precipitation, the surface of the iron is repeatedly scraped and cleared to give the acid fresh surface to act upon, by which some of the decomposed iron in mixed with the precipitate which impairs it’s qualities.
The copper is taken from the pits in the form of mud and when dried is sent to the furnace to be smelted. This precipitate holds from ten to twenty four per cent. But if wrought iron is put in the mineral water and left undisturbed, that is without cleaning it to give a fresh surface ,till it be wholly dissolved it will precipitate nearly its weight in pure copper.
The pits in which the copper is precipitated from the mineral water ,are in ranks, one row beneath another, accordingly and the declivity and the extent of the ground admit; the water is let off from one set of pits into another, till the water has let go all the copper it held in solution. The water that runs from the lowest or last row of precipitation pits is conveyed into reservoirs where the decomposed iron subsides. The ferruginous ochre is useful as paint. The dimensions of the pits are commonly thirty six feet by twelve and about two feet deep ,with a space six or seven feet between each of them.
The mines were also visited by a famous Germany scientist called Lentin 1800. he also described the precipitation process.
... Today, my valued Friend, I will try to describe to you a process which, as far as I know, is used only in Anglesey. It is one, however, which you yourself will concede deserves to be used at other copper works because it not only shortens the most tedious of all metallurgical processes, but also produces particularly good copper at the same time. According to Schlüter this process was already in use in the old days in the Lower Harz Mountains but was subsequently discarded. Whether this was the correct thing to do can be decided from the following description
After complete roasting only the richest ores are, as you yourself still recall, kept aside and roasted alone, then sent without further treatment to the smelting works; whereas the others undergo the following operation.
Up by the ravine which divides the mountain into two ridges eastward from the peak several small sumps have been made which are approximately 12 feet long, eight feet wide and three feet deep. As much roasted ore in thrown in which can be covered with water in the sump. After the ore has remained in this condition for 12 - 16 hours, the water is drained off and led into a container in order to deposit the fine particles of mud which it carries with it. The ore, which remains behind undissolved, is washed in settling tanks, and the big pieces are broken into pieces the size of nuts on pounding benches by women and children, washed again and thereafter taken to the smelting works.
Because a considerable part of Sulphur contained in the ore unites with the oxygen in the atmosphere to form sulphuric acid and this dissolves the iron and - at this temperature - also a part of the copper, there is produced in the ore a mixed copper and iron vitriol which the water in which the ore is being softened, dissolves. Copper is extracted from this solution via precipitation with iron in the following way.
On the lower end of the large containers in which the vitriol water has collected there are several completely horizontal tanks 24 feet long, 12 feet wide and three feet deep and lined with bricks.
These are filled with old forging iron or, when not available in the required quantity, plates of cast iron, which are three feet long, two feet wide, one and a half to two inches thick.
The containers are also furnished with a four inch wide comb on one side which serves to ensure that the plates which one places vertically in the tank stand a suitable distance from each other and leave space for a sufficient quantity of vitriol water. Then a quantity of vitriol water sufficient to cover the iron is introduced into the tanks from the large reservoir. After standing like this for a few days, the iron is coated with a crust of metallic copper. This must be removed or the iron will not extract the copper still present from the vitriol water and can bind itself with sulphuric acid instead.
This happens with the forging iron which exists in smaller pieces. When it is worked around with a rake, pieces of iron rub against each other and scrape off the copper coating, making the iron suitable for a new deposit. The copper deposited on the iron plates is scraped off using a copper scraping instrument which is four feet wide and fastened on a handle.
After the copper water has been in contact with the iron for eight days, it is drained off and replaced by fresh copper water. Meanwhile the first lot of copper water still contains a considerable amount of copper, so one does not release it into the stream but sends it into other tanks set up like the first ones and likewise full of iron plates. One does not let the water stand in the first tanks until all the copper has been deposited, because just as the sulphuric acid loses its copper and binds itself with the iron, so a part of the iron precipitates freely as a yellow carbonate from the solution, and this would contaminate the copper first deposited. Therefore, to keep a part of this copper in the purest condition possible, the copper water from the first set of tanks is sent into the second set of tanks and then into a third series of tanks until finally all the copper has been extracted. These deposits are, of course, increasingly contaminated in each stage with iron carbonate, but it is easier to separate it from this by smelting than when both metals are bound with each other with Sulphur.
When a moderate quantity of copper has collected in the tanks, the iron - after it has been cleaned of all adhering copper - is removed from the tanks, and the water is gradually drained off so that the copper is not carried off with the liquid one is removing. In order to dry it more quickly, it is cut into pieces the size of bricks, then first dried in the air and then in a drying furnace. It is not necessary for me to describe this furnace to you because it does not differ much from the calcining furnace about which I will soon have more to say.
The large container in which the water, which has stood on the smelted ore, collects must be cleaned from time to time because the water deposits quite a lot of mud particles which it carries with it. However, because these still contain a lot of copper vitriol, one places the mud on the edge of the container or on the slope where the rain dissolves the vitriol and takes it back to the container.
This simple process yields four to six thousand hundredweight of precipitate - or cement copper, which is likewise sent to the smelting works where it is smelted and refined. The famous Wilkinson tried to make use of a part of the precipitate without first smelting and refining it, at his gun foundry, and the experiment was successful. In fact, one wonders why this application was not followed up further, for I do not doubt that it can also be applied profitably in the production of brass and for other metal compositions.
Copper exists, if not in a completely metallic state, then in one that is very close to this, so that it is completely reduced by the carbon which is mixed with the calamine; and because the copper here is in a very finely dispersed state, then in the production of brass both metals can unite with each other more uniformly because of the more exact mixing of the copper with the calamine, resulting in a better metal and a greater gain.
You see that I was not wrong when I said in the beginning that this process deserved to be used at several German copper works, in particular where Sulphur-rich, copper pyrites which is not rich in copper is processed and where iron is available at a low price to cause precipitation, such as for example, the Goslar Copper Works, where everything makes it so suitable for this process and where it would contribute exceptionally to the improvement of the copper.
Faraday also describe the process at Mynydd Parys in 1814.
... From hence we went to the precipitating pits. I have already said that the water which gathers in some of the workings is a very strong solution of sulphate of copper from its action on the sulphuret. This water is pumped up by a steam engine into large reservoirs and it is let down by sluices from there into small tanks placed side by side each about I2 feet long, 8 wide and 18 inches deep. Into these tanks is thrown old iron of all sorts, hoops, nails, saucepans, etc., and they frequently procure what they call iron from the iron works, but it is generally a mixture of slag and iron containing about half its weight of the latter. In this state the iron and water remain in contact for some time being turned now and then to expose fresh surfaces to their mutual action and then the water is drawn off and fresh let in. The waters are not thrown away after having been once over the iron but that which has been acted on in the highest tank is let down into a second where there is more iron and then again into a third, fourth and fifth in all of which there is iron until it is so poor as not to be worth working any longer. The result of this arrangement is the production of copper in these tanks occasioned by the play of affinities which takes place between the substances. The water contains sulphate of copper or blue vitriol to which iron is added and iron having a stronger attraction for oxygen and sulphuric acid than copper has, it takes both these substances from the blue vitriol uniting to them and forming a soluble salt and consequently the copper is thrown out and remains as a sediment in the tank. This sediment is never pure copper but always a mixture with the rust or oxide of iron a part of which comes from the dirty state of the iron when thrown in, and another part from the spontaneous decomposition of the salt of iron which is produced, for you must understand My Dear Girl that the combination first made by the Iron and Sulphuric acid is what is commonly called green vitriol or copperas. Now when the salt is dissolved and exposed to air it absorbs a portion of the oxygen of the air and the Iron becomes more oxidised. In this state as it is not so soluble in the acid as before and therefore a part is deposited as a red powder mixed with the copper rendering it impure, consequently the sediment is always copper mixed with oxide of iron and it is richer in copper from the first tank or the strong water and poorer when obtained from the last tank. It is found from experience that if the sediment yield less than 5 per cent of copper the expense of the iron is more than the worth of the copper obtained so that waters reduced until they yield the mixture of only 5 per cent copper are thrown away. In the first tanks the sediments are so rich in copper as to yield 80 or 90 per cent. These tanks are emptied of their sediments once a quarter. When the substance is dry it is taken down to the refineries and soon rendered fit for market. From 40 to 50 tons of copper are produced annually in this way.
When the water first runs from the tank it is of a fine red colour from the per-sulphate of iron it contains. The pools which receive it and the rivers it forms in passing to the harbour, look as if filled with blood. In the harbour it soon becomes diluted by the sea but the rocks to a great distance are stained by it.
All the early writers describe stone or bricked line pits. Some of the later pits were made of wood and production of copper by the precipitation process in these pits continued into the 20th century way after mining had been abandoned. This picture shows the precipitation pits at Dyffryn Adda on the mountain in 1900.