The methods employed in dressing the tin ore up to the required percentage of purity depend mainly on the type of ore crushed. These may be briefly divided into two distinct classes—viz., a free milling ore and a complex milling ore. In the former the ore is practically free from the common metallic sulphides, such as. pyrites, mispickel, blende, galena, and a clean concentrate of 70 per cent. metallic tin can be directly obtained in one continuous- act of concentration; in the latter, however, in order to get a good extraction, it is only advisable to primarily concentrate up to 30 to 10 per cent, tin oxide, the rest being the sulphides of the metals. This concentrate is then roasted, reground, and re-concentrated up to 70 per cent, metallic tin on a separate set of tables.
In the complex milling ore the tin is intermixed very closely with the sulphides, so that it becomes necessary to collect all the mineral present in order to obtain clean tailings. Stamp mills are the most popular machines for the primary crushing in the district, but whether they are invariably the best for the purpose is rather an open question. No doubt for some of the very tough chloritic ores they are the most efficient, but for a brittle quartzose ore, such as the free milling proposition usually is, the writer thinks Cornish rolls would well hold their own, especially when the tin occurs coarse, as is so often the case with this class of ore. Of course, the great drawback to stamps is the excessive quantity of tin slimed—and slime tin is one of the problems of tin-dressing—while rolls give perhaps the minimum quantity of any crushing machine. In certain cases Huntingdon mills are used where the ore is soft and clayey in character, and they have given eminently satisfactory results.
Most of the concentrators on the market have been tried with various degrees of success in collecting tin, of these, jigs, Wilfleys, Frue, Luhrig and Krupp vanners and buddies (three types) have proved the best. All the above tables are too well known to need any description, except a few passing remarks on the buddies. These are of three types—first, the old Cornish buddle, with moving arms and cement concrete bed; secondly, the type in which the face of the table revolves and passes under clean water sprays, which wash the material off; and, thirdly, that in which the table is stationary and the feed launder and sprays revolve. There is no doubt that the Cornish buddle is a splendid machine for dressing up a dirty concentrate to the required percentage of purity which other tables refuse to clean efficiently, or for a slime that is too fine to stay on them : the only drawback, however, is the high working cost, due partly to the amount of manual labour required, and partly to the intermittency of the process. It is essential, in order to work the Cornish buddle as efficiently as possible, to first classify the feed so that only one grade of material passes on to the buddles at a time. If this is not done, the coarse tin will be carried down into the tail, which will have to be re-treated instead of going to the dump, thus materially increasing the cost of operation. Cornish buddies can only be recommended as the principal concentrator in a small-capitalized company, or for testing a property. Combined with other tables, they are very useful for the treatment of fine slimes that are carried over from settling boxes.
A description of the Stannary Hills Mines and Tramway Co. Mill will perhaps give a good general idea of the subject.
The ore is conveyed to the mill over a distance of seven miles by a 2-foot gauge railway. The mill is situated thus far from the mines in order to obtain a plentiful supply of water without recourse to pumping or an elaborate scheme for the conservation of water.
Each truck contains approximately 2½ tons of ore; these are disconnected from the locomotive at the top of a gravity tramway, and lowered 500 feet to the mill. The slope is such that the full trucks pull the empties up, and no other power than gravity is required. The speed of lowering is regulated by means of drum and brake, operated by the man on top. The trucks, on arrival at the bottom, are weighed and tipped into a bin of eight hours capacity when full, feeding directly to a jaw crusher, which makes 204 strokes per minute, and will crush 200 to 300 tons of crude ore from the mines through a 2½-inch ring in 24 hours. The crusher discharges to a travelling belt conveyor, which distributes the ore over the whole length of the secondary bins; from these, which have a capacity of 120 tons, or roughly, 20 hours’ supply of hard ore, the material is fed automatically by shaking feeders to the stamps. A pair of Cornish rolls are being erected to reduce the feed from the Blake crusher from 2½-inch to ½-inch before feeding to the stamps. By this means it is expected that the capacity of the battery will be increased 20 to 30 per cent.
The battery consists of 30 head of 1,320-lb. stamps, making 104 drops of 6 inches per minute. Originally the battery consisted of 10-head units, but so much time was lost in having to hang up an effective 5-head section along with one out of action, that the present management instituted 5-head units instead. The advantages outweigh the disadvantages of increased cost of belting and increased wear of central brasses. On the institution of the 5-head units the same brasses and plummer blocks were used for the central bearing as previously, but considerable trouble was caused by the brasses breaking in halves, due to the unequal bearing of the two ends of shafts. There was also so much jarring that if a cap was put on the bearing, 1-inch bolts that held it down were sheared off. To obviate these difficulties a bigger plummer block was cast to carry two separate brasses, one for each end of the shaft, and two separate caps with stronger bolts. In the drawings of the battery (Figs. 1 and 2), this central bearing will be noticed; it will also be noticed that the pulley ends of the shafts are running open.
The difficulty of obtaining large-sized timber for battery guides suggested the idea of cast-iron bushes for these. In both top and bottom guides they were tried, but it was found the wear was prohibitive on the bottom set, owing to the quantity of grit and sand that was unavoidably splashed up by the stampers, and although the bushes worked fairly well in the top guide, they have been replaced again by plain wooden ones. A ½-inch liner of wood is placed between the two halves of the guide, which is reduced as wear takes place, so that a good fit is always maintained. The standards of the battery are of cast iron, and are manufactured by Walker & Co., Maryborough.
Another improvement that has lately been effected is the replacement of keyed cams and pulleys by those of Blanton design. Some difficulty was experienced at first with the Blanton’s, as the taper of the wedges was rather too fine, and cams gripped too tight, but, on being enlarged to the present size, little trouble was experienced in removing a pulley or cam when desired. The method employed to loosen a pulley of Blanton design is as follows:—A hole is bored in the outer edge of the pulley, and a rope passed through and connected with the travelling crane overhead; the pulley is then adjusted so that the biggest leverage possible is obtained on it; the cams are then blocked against the stamps stems and standards, and the rope wound up till a tension is obtained on it; a few blows, with a hammer on the box of the pulley will then loosen it. The united resistance due to the grip of the 10 cams on the shaft is sufficient to withstand the strain produced by the pulley, provided they are not jarred by hammering.
A shaft for Blanton cams is set out in this way: A centre line is scribed the whole length of the shaft, and the position and face side of the cams marked on it. A collar is then slipped on, which may be clamped in any position by a set screw. This collar or “ jig ” is of the same size and the same width as the boss of the cams, and has two holes bored in it of the same size and the same distance apart as the studs of the “cod” pieces. The semi-circumference is divided into five equal divisions, and taking for instance an order of drop of 1, 3, 5, 2, 4 the first division will be marked. 1, the second 3, third 5; and so on. To bore the stud holes of No: 1 cam, mark No. 1 on the collar is placed coincident with the centre line and in the position of the cam, the set screw tightened up, and the two holes for the studs bored out through those in the collar. For cam No. 2, mark 2 on the collar is placed along the centre line at the right distance, and the same proceeding as above gone through, and so on for all five. If it is a 10-head shaft then the next set of 5 holes must be respectively 18° in advance of the others to evenly distribute the load over the whole length of shafting. To do this off the same centre line the numbers 6, 8, 10, 7, 9 must be marked on the collar in advance of 1, 3, 5, 2, 4 to the above amount, and the same set of operations repeated.
Care must be taken in designing cams for Blanton codpieces that the blade will withstand a blow in the reverse direction to that of lifting the stamps. One type at the mill under review was of cast steel, very hard, and wore splendidly; but a tap with the hammer on the reverse side when removing would break several in every unit. Cast steel cam shafts were tried at one time, but gave a great deal of bother through breaking, in some instances only running six weeks in 10-head units, and even when used with 5-head their life was only doubled. Fagoted iron shafting has proved the most satisfactory. The stamp duty varies from 4 to 7 tons per 24 hours, the lower value being for very hard and tough chloritic stone, and the higher when there has been a good admixture of soft clayey ore.
The battery uses 9 to 10 tons of water per ton of ore crushed, and the dressing floor an equal amount. This is conveyed from the Walsh River to the mill storage tanks over a distance of one mile by a 15-inch x 12-inch wooden fluming.
During the last few months numerous tests have been made with regard to the advisability of crushing coarser, Originally the battery crushed through 25-mesh screens, but the high percentage of tin in slimes was always a considerable problem, giving extra work for the huddles and lowering the final extraction. The size of the apertures in the screens has been gradually increased till the area reached .00303 of an inch (i.e., 12-mesh screen of 22 S.W.G. wire). So far as these tests have gone, they prove conclusively that the coarser crushing has not affected deleteriously the extraction, and has increased the capacity of the mill 6 to 10 per cent., and lessened the slimes. It was thought perhaps, that the increase in the size of the particles would increase the time of roasting, but such was not the case, due to the fact, no doubt, that the coarser particles would admit the air more freely, and so more readily oxidize. These experiments are still in process, and coarser screens are yet to be tried till the economic limit is reached.
From the battery the pulp passes to hydraulic classifiers, where the coarse sands are separated out and go to jigs of the Hartz type, while the fines flow through a series of spitzkasten, which feed to Wilfley tables, and the overflow from these, to big settlers, which in turn feed 3 revolving buddies. Only hutches 3 and 4 of the jigs are in use, the product from, these going to the calciner pits and yielding a 30 to 40 per cent, tin oxide concentrate, while the tail of the jig feeds a Wilfley, which catches any mineral and fine tin that has escaped the jigs. Stream tin is commonly used for ragging, although cast-iron shot is employed whenever there is any shortage of the former. The plungers make 155 strokes per minute. Five Wilfleys treat the finer products from the battery, and the middlings of these, after a slight classification, pass on to two other Wilfleys. These tables make 220 ¾-inch strokes per minute, and treat 20 to 30 tons each per 24 hours. The revolving buddles make one revolution per minute, and yield a head, middle, and tail. They are only efficient, if not over loaded, for slime tin; the cost for repairs is exceedingly small.
It may here be stated that the battery pulp, as it leaves the boxes, and the final tails, before they pass out of the mill, are automatically sampled and their value tested every shift. At one time these samples were taken by means of a scoop from the launders, but they never proved satisfactory, so the management decided on an automatic device whereby the personal equation was completely eliminated. The machines so work that a small complete sample of the pulp and tails is taken every 2 to 5 minutes, the gross weight of which in 8 hours is about 20 to 30 lbs., or roughly ½lb. per ton of ore crushed.
The heads of the Wilfleys, which contain 90 per cent, of the tin oxide and the greater proportion of the mineral, are collected in cement settling pits alongside the Merton furnace. These pits are emptied by shovelling, and the concentrates are allowed to drain and partially dry by being placed on sloping platforms over the flues of the furnace. From 50 to 80 tons of concentrates are collected per week of six days. The Merton furnace, under existing conditions, has only a capacity of about 80 tons per week; each rabble makes 1½ revolutions per minute. An extension of the furnace is in process whereby the finishing hearth will be elongated sufficiently to carry two more water-cooled rabbles; by means of this it is expected that the capacity of the furnace will be increased 30 to 40 per cent. It is essential that the roast be of such a character that all the pyrites present is converted into the red oxide and the lead volatilized as much as possible. If the furnace is roasting badly the black magnetic oxide is produced, which is with difficulty separated from the tin, and it becomes well-high prohibitive under such conditions, due to increased labour, to dress the tin concentrates above 65 per cent, metallic tin. With a dead roast, and thus the conversion of the iron to the red oxide, the tin separates very readily, and a clean concentrate of over 70 per cent, purity can be easily obtained. The importance of having a furnace of ample capacity, so that there will be no occasion to rush the material through and only partially cook it, is thus manifest.
The question might be asked—“ Why not take a cleaner head off your primary concentrators, and so lessen the quantity of material roasted ? ” If this were done it would become necessary to re-grind the middlings. Increasing the capacity of the furnace, as above, will not increase the cost of roasting—in fact, will decrease it, as more will go through; and there is practically enough sulphur in the concentrate to do the roasting without the addition of more fuel. Besides, roasted material is decidedly more brittle and more easily re-ground than a raw product, so that, after roasting, the cost of re-grinding will be less than if performed before.
The Merton discharges into the sump of a plunger sand pump, and the product is elevated by this means to a settler, from which the coarse feeds three grinding pans, and the fine passes over to settlers that feed six Krupp van tiers. Of the three grinding pans, one is of later design than the others, having the compensating attachment at the top of the yoke instead of at the bottom, where very considerable wear takes place, due to the sands that are always scouring it. Cast-iron shoes for these machines last only six to eight weeks, and grind approximately 140 tons ; Krupp steel shoes last six months, grinding 560 tons of roasted, concentrates. The machines are not shod with a new set of shoes till one breaks or they cease to be efficient. This begins to be apparent when the thickness of the shoe is diminished to about ½-inch.
The material from the pans passes to a series of three classifiers and the product from each one becomes the feed to a pair of Luhrig vanners. These tables yield a head assaying 70 to 72 per cent, metal, a middle 50 to 60 per cent, metal, and a tail containing very fine and coarse tin, and assaying 7 per, cent. These tails discharge to a sand pump and are elevated to a series of four classifiers, which feed four Krupp vanners. These tables make 83 strokes per minute. The tails pass to the race unless they are dirty, when they are returned and repeat the above cycle. The middles pass over the same tables again. The heads from the Luhrig vanners are collected in settling boxes, dried, and bagged. The middles from the Luhrigs, termed “ seconds,” and the’ heads from the Krupp vanners, called thirds, are collected separately and tubbed in Cornish kieves, the iron skimmed off, and the clean concentrates dried and bagged. These kieves are wooden tubs 4 feet diameter and 4 feet deep, and the dirty concentrate as it is added, is briskly stirred in water by means of a shaft with radial arms. On the completion of the charge—about two-thirds water and one-third concentrate—the shaft and arms are removed, and a knock on the side of the tub substituted, produced by means of a cam shaft and lever arm and weight. This aids the heaviest particles to settle first and so grade upwards to iron, the lightest. The uppermost layer is skimmed off till the tin shows sufficiently clean for bagging. These skimmings are then treated in a small Cornish dressing buddle of 12 feet diameter, the heads re-tubbed, and the tails returned to the calciner for re-roasting.
The drier consists of a vertical shaft, around which the wet concentrates are placed, with a fire-box at the bottom. As the tin dries it is collected in a hopper, so arranged as to prevent as much dusting as possible. The dried concentrate is then placed in bags, a spear sample being taken of each. The bags are made of sail canvas with calico “ inset ” inside, and approximately hold 1 cwt. each when full. The overflow water from the calciner pits, which contains fine tin in suspension, passes through a series of settling boxes, and the product from these is intermittently discharged to Cornish buddles. The overflow water from the tin settling boxes on the dressing floor passes through a series of boxes and pits, and the settled products pass to Cornish buddies. All slime water is treated similarly, and considerable quantities of slime tin are thereby saved.
Wooden ties or boxes, which practically represent a section of a Cornish buddle in action, are being placed in the tail race. It is expected by this means to materially increase the extraction.
The labour question is rather important in the district, and the supply of good mill hands is very limited, so that it is essential to use gold tables with as simple a construction as possible and as nearly “ fool proof ” as you can get them. For this reason Frue vanners have been replaced by machines of other make, as the cost of keeping the former in good working order was too great compared with other types.