Roasting or calcining of ores
The copper ore extracted from Mynydd Parys was high in sulphur. Initially the sulphur was thought of as an impurity that needed to be removed from the ore prior to smelting. Simple heaps of ore were burnt on the mountain side and the sulphurous gasses allow to blow away to sea.
However as the need for gunpowder and sulphuric acid increased thought out the land it was realised that the sulphur could be recovered and provide an additional source of income.
The structures built on the mountain and the methods used were described by various visitors. Two types are described which appear to have been operational concurrently. One was a horizontal development of the Champion roasting kiln the second was a vertical conical structure with a separate condenser. Many structures where built on the Northern side of the mountain in the area know as the Brimstone yard.
Mathew Boulton 1787
… I spent three days inspecting the Anglesey copper mine. It is not like a Cornish mine but is an open work like a quarry or gravel pit and worked by open daylight. The ore… is calcined in kilns built of brick in a conical form , and as it burns the sulphur arises to ye top and is condensed in the form of flowers of Brimstone in the Condenser which is a big empty space built with brick in the ground…The flowers is then put into a Cast Iron Cylindrical vessel and melted by a gentle heat into a solid form & ladles out and poured into moulds. This Brimstone is sold for the purpose of making oil of Vitriol of which Anglesey make 3000 tons per year.
…The more impure ore is also broken to the size of about hen's egg; but in order to clear it from the quantity of sulphur with which it abounds, as well as other adventitious matter , it must under go the operation of burning.
For that purpose it is placed between two parallel walls of vast length: some kilns are twenty, other forty and fifty yards in length; some ten others twenty feet wide, and above four feet in height. The space between is not only filled, but the ore is pilled many feet higher, in a convex form, from end to end: the whole is then covered with flat stones, closely luted with clay; and above is placed a general integument of clay, and small rubbish of the work, in order to prevent any fumes from evaporating.
Of late some of the kilns have been constructed with brick arches over the ore, which is found to be the best method of burning. Within these few years, attempts have been made to preserve the sulphur from escaping; and that is done by flues, made of bricks, who's tops are in the form of a Gothic arch, many scores of feet in length: one end of these opens into beds of copper which are to be burnt. Those beds are set on fire by a small quantity of coal, for all the rest is effected by its own phlogiston.
The volatile part is confined and directed to the flues; in its course the sulphurous particle strike against their roofs and fall to the bottom in the form of the finest brimstone; which is collected, and carried to adjacent houses, where it is melted into what is called the shop stone brimstone.
The beds of copper thus piled for burning are of vast extent. Some contain four hundred tons of ore others two thousand. The first require four months to be completely burnt, the last near ten. Thus burnt, it is carried to proper places to be dressed, or washed and made merchantable. By this process the ore is reduced to a forth part in quantity, but considerable improvement in quality: and by this means the water is strongly or richly impregnated with copper, which is dissolved by the sulphuric acid; and is collected or precipitated again by iron in the pits.
Lentin wrote a series of articles on the operations at the mine in 1800. He first described the horizontal roaster.
…Before the existing conical smelter was introduced, smelting took place in roasters which are quite similar to ours but which differ from them in several essentials which I will briefly describe to you
A rectangle 70 feet long, 20 feet wide and 8 feet high is erected from undressed stone and clay. This will serve as a roaster. Internally the walls are perpendicular, but on the outside they slope like a roof to such an extent that the slope is four feet off the total height of the roaster, so that the whole structure looks like part of an oblong pyramid. There are four vents on each side which are three feet wide and three feet seven inches high. A similar opening is also found in the middle of each end of the structure.
In order to charge such a roaster, which must be set alight with coal because of the lack of wood here, one sets a conduit of bricks from the vents at both ends of the structure, leading inside the structure, which are placed on edge with a space of four to six inches between each brick. The same is done with the vents on the sides, dividing the floor of the interior into ten rectangles. These conduits are filled with coal and covered with iron plates; the rectangles, however, and the other free space are filled full of ore.
This must be done with care initially, so that the conduits, which serve to maintain the draught, are not disturbed. One piles the ore up to four feet over the walls in the form of a pyramid, evens off the top surface, which has to remain two feet wide and sprinkles it with small ore tailings.
Now a channel of bricks is built along the length of the top surface to catch Sulphur. It is 11 inches wide internally and 11 inches high. The lower layer of bricks is positioned so that there is a space of four inches between each brick. Two other channels, at equal distances from the ends of one side of this channel, run down from the wall from whence they proceed horizontally into a building that stands close by. It is 60 feet long, six to eight feet wide internally and six feet from the crown, with which it is closed off, to the plastered base.
This building, called the condenser, can be closed at each end with a wooden door and has three or four vents in the vaulting through which the rising sulphuric acid vapour can escape. To ensure that all the Sulphur vapour that has developed is drawn into the conduits and, through these, led into the condenser, the ore which rises over the wall, in a pyramidal shape, is covered with slates which are placed so that they lean over the lower layer of bricks on the conduit. The joints are painted with clay.
The ignition of the pile of material to be smelted occurs through the side or end vents, depending on where the wind best serves the ignition of the bituminous coal. The first vapour produced will not be sent into the condenser because it is contaminated with bits of coal and, on account of the moisture in the ore, contains a good deal of free sulphuric acid. So the side conduits are not covered at first but left open until one sees pure flowers of Sulphur rise. This usually takes 12 - 14 days. The roaster does not need any further operator attendance now because the draught will be regulated at the condenser by the opening or closing of the openings in the vaulting as conditions require.
During the roasting the condenser must be cleaned out several times, in fact every 14 days in the beginning; later, however, only every four weeks. For this purpose the two side conduits are opened and a suitable piece of slate or an iron plate is set inside, so that enough openings are left in one conduit to ensure that the draught is not blocked off completely. One then removes both doors of the condenser, and after the vapour, which contains sulphuric acid, has drifted away, the Sulphur, which had collected in the form of flowers of Sulphur, is removed as quickly as possible so as not to disturb the roasting any longer than necessary.
So you see, my dear friend, that this method of roasting has very significant advantages compared with our own; because it ensures a continuous draught, which remains the same throughout the entire operation, the ore is roasted better, and the amount of Sulphur collected, which is not, as by us, completely dependent on the influence of atmospheric conditions, is incomparably greater. The costs of using this method are, therefore, soon recovered through the gain in Sulphur; one can minimise them by setting up a condenser between two roasters so that one can empty one and charge it up again, while the other is burning.
Lentin then goes on to describe the vertical continuous roasting method.
This method of roasting was first discovered here in Anglesey. Initially they used the same method of roasting as was used in calcining furnaces in other foundries in England. But the cost of bituminous coal and wages made things more and more expensive. Because of this they did not stay with this method but tried to find a better one, because the cost of bituminous coal and the iron plates, which were destroyed each time by the Sulphur containing vapour, were considerable. The search for a new method succeeded with the development of the conical furnace, which I will now try to describe to you, and to which the drawing on the second copper plate refers.
A conical furnace, which is made from bricks, is built onto a base of unfinished stone which is 8 feet high and so deep that a 12 feet long, six feet wide vaulted passageway, open at the front, can be fit inside. The height of the cone is 27 feet, the lower internal diameter sixteen feet and the upper internal diameter four feet. At the base, where the wall is two feet thick, there are, equidistant from each other, four draught holes, each two feet square, whose inner corners have to be severely rounded off in order to be able to reach inside everywhere with scrapers.
In order for the furnace to withstand the force with which the ore expands during roasting, it is reinforced by one set of iron rails running from top to bottom of the cone and another set around its circumference, each rail being four inches wide and 1/3 of an inch thick, so that they enclose the furnace down to the draught holes like a lattice whose openings are six to seven inches wide and seven to four inches high. The upper aperture of the cone is covered with an iron plate in such a way that a two-foot square opening remains in the centre. The ore is thrown in through this opening and afterwards it is closed with an iron cover.
The condenser, which is of the same shape and construction as that described in the first roasting method, has to be placed as close as possible to the furnace. In this respect, and in order to throw in the ore more conveniently, it is necessary to erect it near a hillock and to build a perpendicular wall near the furnace, filling up the space between it and the hillock, in order to produce a plateau on which the condenser can stand. This wall is connected to the furnace by an arch. A channel, which is 11 feet in internal diameter runs over this arch. It is covered with iron plates and runs into the condenser about two feet under the upper mouth of the furnace.
This is the simple construction of a smelting furnace which I hope I have described clearly enough. Now the process itself.
From the middle of the floor of the furnace there is an opening four feet in diameter which leads into the cellar. When the furnace is first being charged, this is closed with an iron plate which is supported from below by props so that one can open it again more easily. Now brick conduits are built from the draught holes to the middle of the furnace where they form an open rectangle. These channels are filled with bituminous coal and covered with plates.
In the meantime the furnace is half-filled with ore. Initially this is done with some care to avoid damaging the conduits. Afterwards the fire is ignited on the side of the furnace where it appears the wind will most effectively assist the spread of the flames.
When a considerable amount of smoke is produced which smells of Sulphur vapour and the ore is crackling, this indicates that the ore has ignited. At this point the furnace is filled up to a third of its height. The top half must always remain empty. One leaves the furnace in this condition for the first 14 days without shutting the top opening of the furnace or the doors of the condenser because the vapour during this period contains very little useful Sulphur.
After this period the appearance of pretty, yellow flowers of Sulphur in the upper opening of the furnace indicate that it is time to direct the vapour into the condenser. One adds more ore and closes the upper opening of the furnace and the doors of the condenser. In addition two or, according to conditions, three of the lower draught holes are covered with iron plates, and the joints are painted with clay in order to keep the fire from getting too intense and detrimental to the roasting.
After ignition a six week period is usually required to give the first part of the ore the requisite roasting. So after this time has passed, one removes the plates with which the opening in the vaulting of the cellar is sealed and pushes the lower layer of ore through the draught holes down into the cellar where it remains until it is cold and where it, at the same time, serves to close the opening so that one no longer needs the iron plates.
The upper part of the ore, which is now being smelted, drops to the floor of the furnace, and fresh ore is thrown down on top of it. This ignites itself from the heat of the roasting ore without requiring new fuel. The removal of the smelted ore and its replacement with fresh ore now occurs once or twice weekly. To this end the smelted ore is removed from the cellar, and that which has not yet been properly smelted is sorted out and afterwards set on top of the heap of smelting ore, the lower layer of ore is pushed from the oven into the cellar and fresh ore is added.
Eight weeks after the onset of smelting the Sulphur which has collected in the condenser is removed for the first time. Afterwards, however, this occurs every 14 days, and one chooses to do this when the oven is full of ore because it would otherwise contaminate the Sulphur with dust rising from the ore. For this reason each time the conduit, which directs the fumes into the condenser, is erected, access is closed off.
This is achieved by taking up a plate which serves to cover a conduit and setting a piece of slate or a board in front of the part of the conduit which leads to the furnace and painting it with clay. The doors of the condenser are then opened, and as soon as the gases have dissipated, the Sulphur is removed as quickly as possible, so that the furnace does not stay open too long, because this would heat up the ore too intensely.
With this method of smelting a lot depends on control of the fire, and this demands uninterrupted attention. The pre-eminent resource in controlling the fire are the openings in the vaulting of the condenser through whose opening and closing the draught can be minimised or increased; however, because flowers of Sulphur frequently settle on their openings and clog them, one has to clean them from time to time and so the process continues without stopping and demands little work. The quantity of smelted ore which is produced weekly by such a furnace is about 200 hundredweight of which 25 - 30 hundredweight is flowers of Sulphur.
At the time I was staying on the island there were 45 of these furnaces, and one found them so advantageous that more were to be introduced so that one could smelt all the ore in them.
It would be quite superfluous to discuss the merits of this method of smelting extensively because they are so obvious. I will, therefore, close this all the same rather long letter and am, etc.
Another famous scientist and author, Faraday visited in 1819 and recorded the method of operation in his diary.
… Mr. Treweek now took charge of us and showed us the work above ground. We went first to the kilns and in our way passed other mine workings belonging to the Mona Company.
At the kilns the following process is carried into effect. The ore is raised from the mine and broken by the women as described, is placed in heaps about 35 feet long, 10 wide and 10 high. Larger pieces of ore are used for the outside which is something like rough brickwork but the ore is wheeled in anyhow into the interior. Four or five large holes are made in the mass below like ash pits and when the heap contains enough ore flues are built across and along the top, the large pieces of ore which are connected with another flue running two or three feet from the kiln on the ground and this being done the whole heap is covered with earth and clay so as to prevent the entrance and exit air or vapour except by the holes before-mentioned and the flues.
A brick chamber is built a few feet from the kiln and connected with its flue at one end, the other having a small aperture. Some lighted coals are now thrown into the holes left at the bottom of the kilns and in the course of a day they heat and inflame the ore immediately about them and afterwards no further additional fuel is necessary but the combustion goes on with the ore itself one part roasting the other. This lasts five or six weeks and all the sulphur separated and sulphurous acid generated pass through the flues into the chamber and are there condensed. In this way very little vapour escapes and the process instead of being a general nuisance as at Swansea is a very magnificent and agreeable example of sublimation.
When the kiln goes out of itself and is cooled it is pulled down and the ore taken away in carts to the refineries near the port. Those parts which happen here and there to be only half burned being carefully selected and put into other kilns. The chamber is not disturbed for the first, second, even third kiln but after the sulphur of many kilns has been sublimed into it is opened the brimstone taken out, washed from the acid which adheres to it and is fused and then it goes to market.