Table of Contents
Here is a short account of the treatment of the base bullion for the recovery of lead, silver, and gold, and for the treatment of by-products. Appended is a short description of the chief facilities of plant which have been adopted to overcome the main difficulties in the refining of lead. The difficulty of obtaining fair samples of gold drosses, and the inability to obtain particulars of fuel consumptions, prevents the paper from being as complete as one would wish. The limited length precludes detail, except in more important parts. The base bullion is brought in carts by contract to the refinery from the blast furnaces, and dumped opposite the copper softener. The bullion bars, weighing 80 to 90 lbs. each, are run into the copper softener by means of a peel, similar to the baker’s implement.
The following is an analysis of base bullion from the smelters:
The furnace is of the reverberatory type, built of Gartcraig fire-brick and black New Zealand fire-clay. The whole hearth is cased in 7/8-in. iron plate tank, in case of collapse of the fire-bricks, when the whole charge may be caught. There are three charging doors on one side, and one skimming door at the end of the furnace, and above the latter is a horizontal flue, connected with the underground. refinery flue and stack. It is fitted with a hand-hole for cleaning (see Copper Softener, Plate XI.). There is a water-jacketed tap-hole, closed by a wooden plug, leading down to the antimony softener, and round the furnace at lead level runs a water-jacket 8 in. wide, fed in two sections, which the corrosive action of the litharge on fire-brick requires (fig. 3). Beneath this water-jacket, at the junction with the further wall, is a weak spot, and to protect this an apron is provided which overlaps the jacket on the outside. The joint is rammed tight with cupel material. The roof of the hearth is arched both longitudinally and laterally ; the bed is also an inverted arch in cross section (fig. 2), sloping to the tap-hole. The iron tank rests upon two rows of railway iron at right angles, which in turn rest upon a layer of bricks, which again rest upon a foundation of slag.
The sides and ends of the furnace are securely braced by buck-staves, united by tie rods at top and bottom, both length and crosswise. At the crown of the arch there are seven draught holes 4 in. x 4 in., which serve the purpose of cooling and preserving the roof. When new, the furnace will hold about 43 tons, but through wear it becomes somewhat enlarged. It requires chipping about once a month, owing to the formation of accretions in the hearth. These chippings go back to the smelters.
Between 1,000 and 1,100 bullion bars from the smelters form a charge for a copper softener, and charging takes four hours. The fire, previously kept low, is got fairly under way and regulated at a moderate temperature to avoid melting the copper dross which rises to the surface of the metal, and which in the molten state would
mix into the lead and spoil the silver. After heating for about three hours, the scum is skimmed off by means of an iron skimmer
(fig. 4) into liquidating pots—ordinary slag pots with a tap-hole near the bottom through which, by removal of the wooden plug, any contained lead may be drained off when the pot is full of dross.
In this way two tons of copper dross are taken from the furnace and re-treated in the smelters; the liquated lead is put back into the furnace, and, as soon as the antimony softener is free from a previous charge, the product of the copper softener is run in by a whitewashed gutter of channel iron. Both at the copper softener and here two men are required to work the furnace—one skimmer and a man to work the scum forward to the door.
The method of separating arsenic and antimony from the lead is to heat strongly enough in an oxidising atmosphere to form a layer of litharge on top which absorbs these impurities as arseniates and antimoniates. The higher temperature required necessitates more fuel and stronger bracings for the bed. The details of doors, water-jacketing, accessories, and dimensions are identical with those given for the copper softener (Plate XI.). The channel iron is supported by wooden trestles and the joints are made tight with clay. The charge is then run down in molten condition, and, together with the bullion from the antimony dross furnace, forms the complete charge, for the second softening.
Au and Ag same as Cu softener practically.
It is then fired hard for six hours, and if it contains much antimony will need to be poled with iron rakes to bring all the antimony into contact with the litharge. The rakes are worked down the sides and up in the centre. It is then left to cool back until the litharge crust has solidified, which is skimmed off and taken to the antimony dross furnace for treatment Fire clean and fire upon the charge for four hours hard. Take a sample of the litharge and allow the furnace to “ go back ” by raking out most of the fire and opening the working doors.
If when cold, the litharge is highly crystalline and yellow, it will not be necessary to completely skim again; that nearest the skimming door only is taken off, and the rest left in the furnace for the next charge. At times, in spite of the litharge test, the lead still contains too much antimony, and when this is run into the Parke’s pots it may need to be poled with wood and skimmed ; but, if the sample be black and not so crystalline, there is too much antimony still, and another skimming is necessary.
The life of the furnace is about four weeks, when the bricks which have been slagged (mostly at the litharge level) must be renewed.
The weight of skimming is about one ton, and will be dealt with under “ Treatment of By-Products.”
The zincking kettles, before receiving their charge, must be washed and heated up by means of fires underneath; the lead must also be cooled back in the antimony softener, so that, when run in, there is no appreciable difference of temperature between the two, which would cause the pan to strain, and perhaps crack. The kettle itself is a circular cast-iron pan of 9 ft. diameter and 3. ft. 9 in. depth. This will hold, approximately, 45 tons of metal. The 6-in. flange of the kettle forms the support on which it rests, being supported above the firebox by a circular brickwork structure capped with 1½-in. plate of cast-iron, and on this rests the shoulder of the kettle. The firing is done from the next floor, grate area being 2 ft. x 8 ft. A step hearth is used, so that the flames must go within a foot of all parts of the kettle when the hearth is clean. As the working
goes on, however, the hearth gets choked up with ash till this space is greatly diminished. If the space is not uniform all round the kettle this will produce unequal heating, and severe strains in the iron.
The principle upon which the separation of gold, silver, and lead is based is that these first two metals form an alloy with zinc more readily than with lead; this alloy is both more infusible and lighter than lead, consequently it rises as a scum, which may be removed on cooling the pot back. The gold and any copper present have a greater affinity for zinc than silver has, and may be separated before the lead is saturated, whereas silver does not alloy until after the saturation point of lead by zinc is reached. This forms a ready means of separation of gold and silver.
In former times these skimmings were treated first in a liquating furnace, and, after liquating the lead from the zinc, were treated in cupels for recovery of gold and silver ; but in this way such a lot of lead was lost as litharge, in getting the gold and silver, that the skimmings were again put back into a pan richer than that from which they came, to absorb more precious metal, being finally pressed off in a Howard press.
Considering the “ gold skims ” first: the pans are worked in pairs. The zinc is added in several small additions, thus giving a more perfect separation of gold and silver, and a more efficient use of zinc than one large addition. The gold is practically all taken out before the silver by returning the skims from one kettle to another which is richer in gold. In this way, zinc, which is added, passes through three or perhaps four different charges for gold, being finally pressed off as a “ first gold press.” Then 80-100 lbs. zinc is added to this kettle, and a “ second first ” is skimmed off, which in its turn is used again. Thus the clean zinc is only added to a charge after the “ first ” has been taken off. Starting with pure zinc and no returns: the first zinc has to be returned, and returned alternately through the two pots until the fifth time through, when it is pressed as a “ first.” To clean the first charge in A of gold (fig. 6), 80-100 lbs. zinc is added, skimmed, and stacked ready to pass through B as “ second first.” whence it is skimmed and stacked as a “ second times through.” To clean B of gold 80-100 lbs. zinc is added, and the skimming with 150 lbs. zinc passes through the third charge, which is in pot A—the first charge having been de-silverized and run down into the refining furnace—and again through a fresh charge in B, and finally stacked with the “ second times through.” Then both the “ second times through ” are together put into the fifth charge in A and pressed off as “ the first gold press.” The second firsts from the third, fourth, and fifth charges pass into the sixth charge in B, and are stored as before. The sixth charge in B is cleaned of gold as before, and the process goes on in a cycle.
The Silver Skims Form Another Cycle
A quantity of zinc calculated from assay of the silver in the lead charge is added— about 500 lbs. spelter—skimmed and pressed, forming the main silver press—“ the second skim.” Then an additional 734 lbs. is added to clean the lead of the last traces of silver, and is skimmed as “ the third skim.” This forms the “ second skim ” for the alternate kettle. In the cycle fresh zinc is only added for the “third skim,” passes through the other kettle as “ second skim,” and is then pressed off. If the lead now goes more than .3-.4 ozs. silver, another zincking is given, and is skimmed very cold.
The time of operation from charge to discharge in the zinc pans is about 32 hours, being roughly divided in two gold skims of eight hours each, and two silver skims of eight hours. The gold zinckings are heated up 1½ hours, stirred ½ hour, cooled back 4½ hours, and skimmed ½ hour, being left from an hour to 1½ hours for a sample to be taken. The silver zinckings are given more time for heating and cooling than this. The weight of skimmings taken from the kettles is about six tons. Two men are employed on each pair of pans—one right-handed and one left-handed skimmer. The life of the pans is very variable; they may crack after two or three charges. The lead is removed if possible before the pan collapses, and the pan is taken out and broken up, the scrap iron being used in fresh casting.
The first gold press, after being broken on the floor, is taken to the gold retorts and retorted for four hours, no attempt being made to condense any zinc, of which there is comparatively little. The copper has the effect of concentrating the gold irregularly in the bullion, making it almost impossible to get a fair sample and assay of it. Very roughly it would go about 12 ozs. to 1,000 ozs. dore bullion. A dross is skimmed from the retort; this is sweated on a bath of lead in a cupel. The gold retort bullion is stacked, and, when there is enough to fill a pot, both the sweat and the bullion are put into a “ special gold Parke’s pot ” to effect a still further separation of the gold from the silver contained in the bullion. This reduces the cost of parting by obtaining the gold freer from silver.
Analysis of lead for gold and silver:
When the “ special Parke’s pot ” is heated, a “ special gold dross ” rises to the top, which is pressed off and sweated on a cupel, whence a copper slag is removed and sent to the smelters. Then 700 lbs. of spelter is stirred in, and a “ special gold press ” skimmed off. This is retorted for 12 hours, and a certain amount of zinc recovered. There are two products—“ special gold retort bullion ” and a dross. The former joins the “ special gold dross ” in the sweating cupel; the latter is first separately sweated, and then joins the other two. This rich bullion is then concentrated up to special dore bullion, which goes roughly 30-40 ozs. gold per 1,000 ozs. dore.
When the pan goes .3-.4 ozs. silver it is clean, and the siphon (2 in. diam., ¼-inch thick) is put bodily in to get hot for 10-20 minutes. It is filled and placed in position, and the taps turned on. The lead runs by means of another channel iron gutter into the refining furnace—a reverberatory furnace in itself identical with the antimony and copper softeners. This furnace is braced, so as to resist very high temperatures, with buckstaves, each consisting of two lengths of railway iron. The refining operation is done to get rid of the zinc from the lead as a dry dross, which is skimmed like copper and antimony drosses. Here it is fired for five hours, so hard that the working doors get red hot; and, when cooled back, it is ready to skim It is then fired until clean—the time from charge to discharge being 12 to 14 hours.
The lead is clean when a sample of well-skimmed lead in an iron mould shows no frosting on the surface. If there is much zinc in the lead from the Parke’s pots the furnace is liable to crust up, owing to the formation of zinc accretions, which diminish the capacity, and require to be chipped occasionally. This is done soon after the charge is run down into the Merchant kettles, by lighting a fire in the hearth and hammering a crowbar into the crusts, which have formed mostly at the surface of the lead. Most of the lead is run into the Merchant kettle, where it is cooled to moulding temperature more economically than in the refining furnace. The small residue left behind becomes part of the next charge. The dross goes back to the smelters. The furnace requires the attention of one skilled labourer to fire and to skim.
The Merchant kettles are reproductions of the zinc pans on a small scale, and are capable of holding 35 tons. The charge is fired on and skimmed of dross, which goes back to the refining furnace, is cooled back and run into moulds by means of a 1½ in. siphon tap of ¼-in. metal.
The stream of lead may be regulated by a nut and is run out through a sieve at the mouth of the tap, just hot enough to show no iridescent colours on the bars, into moulds, holding 100 or 200 lbs., placed in a ring about 10 feet from the tap. The 100-lb. bars are “English bars,” and the 200-lb. are “China bars,” which may contain .5 ozs. of silver, which is paid for by the Celestials. One skilled labourer in the lead ring fills the moulds by manipulating an arm of the siphon tap, which is pivoted so as to turn about a spindle at the centre of the lead ring. With the other hand he skims the dross with a triangular skimmer. Another ordinary labourer stamps the number of the charge, and stacks the lead ready for removal to the weighing machine and wharf. He also scrapes off all loose particles of lead from the bars. The time taken to mould a charge is about six hours.
The retorts originally used were direct fired, but this method gave way to the more economic practice of gas-fired retorts, in which coal slack (an otherwise waste product) is made use of. A current of air is forced through the fine coal in A¹ by means of a steam jet introduced beneath the grate B¹, producing a water gas, which passes by means of one of two flues (8 in. diameter) into the retort area. The direction of draught is regulated by screw valves in the flues. These valves are changed every half-hour.
If the crust of coal forms a clinker, which is difficult for the steam to permeate, a long-pointed bar is put down through either of two ” observation holes ” to break the crust. The temperature of the grate is carefully regulated, the black mass only showing a glow here and there.
This condition gives the most economic use of the fuel. Air, previously heated by passing through regenerative flues filled with brickwork, E¹, is mixed with gas in the bottle area. The admission of air through a 15-in. iron pipe may be regulated to enter at the same part of the area as the gas by a butterfly valve. The furnace itself consists of the producer and bottle area, made to hold two retorts. Two furnaces built back to back constitute a “ nest.” This gives economy of heat used.
The retorts are built in at an angle of 30° with the horizontal, and are supported at the base and neck by brickwork, designed to give the maximum exposed area to the hot gases consistent with stability. Graphite retorts are used, made by Morgan, Battersea.
The condensers are cast-iron cylinders lined with composition, consisting of limestone and cement, to a depth of 2 in. These are supported by iron trestles on little four-wheeled trucks to enable them to be quickly and easily removed. There is a small outlet at the lower end for tapping condensed zinc.
Beneath the bottles, or retorts there is an opening, to be used when the retort breaks and the retort bullion runs in a cast-iron, brick-lined channel into moulds placed for the purpose, or onto the refinery floor. The bottles are now being used till they break. The final bullion charge is thus always caught. This ensures a greater length of use for the retort.
The retorts are first heated by lighting a small fire in the bottle area; the gas is slowly turned on, and when the bottles are hot the silver presses (previously broken up before removal to the retort furnace bin) are shovelled into the retorts with a special narrow shovel till full.
The bullion melts down, and more is added till the retort is full; the charge = 12 cwt. When all the charge is molten, a heated condenser is run into place and luted on with clay. The silver bullion is retorted until most of the zinc has sublimed (which takes about 12 hours) ; the condenser is then run back and the charge is skimmed. A trolley stacked with moulds is run into place, and the charge is dipped. The skimming and dipping take three-quarters of an hour, the whole silver retorting taking twelve hours (directly proportional to amount of zinc present).
There are three products—
bullion, dross, and blue powder. About half a ton of bullion goes to the concentrating cupels, 2 cwt. of dross to the retort dross furnace bin, and the blue powder, which is the first zinc condensed in powder form, is taken back to the blast furnaces, to recover the silver it contains. Old retorts are chipped, and if the chippings are rich they are sweated on a bath of lead in the refinery; if not, they go back to the smelters. A certain percentage of zinc used is recovered and used again in the Parke’s pots. The deficiency is filled up with German spelter when necessary. One skilled and one ordinary labourer are required for each nest of furnaces.
The bullion from the retorts is taken to the cupels for the concentration of the silver. The process consists of oxidizing the surface Pb to litharge, which overflows into a slag pot.
Plate XVII. is an illustration of the English cupel used at Port Pirie. The fire-box is situated on the left; the flames pass over a fire-bridge on to the surface of the metal, where they are met at right angles by a blast at 5 or 6 ozs. per square inch pressure, blown across to the door from the back of the cupel. The blast causes waves on the surface of the litharge, the level of the metal being just below the outlet channel, so that only the litharge is removed by the wave motion. For the escape of the gases down the flue there are three small openings to the right of the cupel. In front is the dipping door, and to the left of it the charging door, big enough to allow admission of a bullion bar. The test is replaceable, and when one is finished with a truck is run in beneath the supporting rails, and these are removed, and the test taken away. Re-lining a test consists in filling it with composition made up of—marble,. 300 lbs.; crushed limestone, 120 lbs.; Portland cement, 120 lbs. ; white fireclay, of putty-like consistency, 60 lbs.
A wooden mould is placed on it and weighted down. The sides are pressed in with the fingers to free from air, and the cupel is covered with wet bags and put in a warm part of the refinery to dry. The new test is run in on a truck and wedged up into place, when the rails are replaced. The cupelling operation is carried on in three stages in the concentrating cupel (concentrated to 16,000 ozs.), the refining cupel, and, lastly, the finishing cupel.
The concentrating cupel has an inch water pipe running round at the litharge level to protect the lining. This is dispensed with in the other two, owing to the thick coating of pure silver which always forms upon it. All the cupels have, however, water jacketed litharge taps. The brickwork of the furnace is supported by buck-staves and tie rods. The capacity of a test increases from 14 to 20 bars as the lining wears down
The test is gradually heated up, and a charge of bullion put in, the fire made up, and the blast turned on. The time of operation varies greatly with the richness of the bullion The amount of litharge run off is about five pots from the first charge, three and four from later charges; the reason for this is that the new test absorbs silver, and, therefore, the first operation takes much longer than succeeding ones to reach 16,000 ozs. The cupel litharge, above that required for the retort dross furnace, goes back to the smelters. Bars are continually added to keep the litharge at running level by the man who is required for the furnace. When the sample shows “half silver ” the charge is baled with a red-hot baler into moulds, and taken to the refining cupels.
The life of the lining is variable, being on an average 45 charges. Experiments are continually being made with new composition, but that mentioned above, consisting of 70 per cent, limestone, 20 per cent. Portland cement, and 10 per cent, fire-clay, has so far proved the best. Old linings are treated exactly like old retorts, generally in the blast furnace. The refining cupel is shallower than the concentrator, although the shape and lining is the same. The time of operation, and also the amount of coal used, depends upon the condition of the test, which may hold from 14 to 24 bars of bullion.
When fired clean the surface of the molten silver has a transparent appearance, which is wanting when lead is present. Towards the dip
the test is fixed fiercely upon, and the ladle put in to get red hot. The charge is dipped into brick-shaped moulds, holding 1,000 ozs. roughly, weighed, and put into the finishing cupel, where it is “ dried ” on a new test. Any impurity left sinks into the lining, or is absorbed by powdered “ compo,” which is sprinkled on the surface. This is removed when sticky by adhering to an iron rod. The charge is dipped into octagonal moulds, three of which weigh about 2,050 ozs. This silver is too pure for the market, and so its fineness is reduced by adding granulated copper, to reduce it to “ 997 fine,” and melting down in the graphite remoulding crucible—which is about 10 in. diameter and 18 in. deep, and holds 2,050 ozs. The fuel used is Australian coke, assisted by a low pressure blast. The crucibles are lifted from the furnace by an overhead bogey and pulley, and run on to an iron plate, whence the silver can be poured into a counterpoised mould on a balance.
Each mould must contain not less than 1,000 ozs., and not more than 1,050. While pouring, a piece of wood is kept burning in the crucible to prevent freezing. All loose pieces are filed off the bars, and, if overweight, chisel marks are taken out. One end is hammered flat, and the number of the bar, the weight and fineness, and “B. H. P. Co. Ld.” stamped on, and the bar is ready for shipment.
Every six months sufficient dore bullion has been stored to run the sulphuric acid parting plant economically. The acid is at present obtained from chemical works in South Australia in air-tight steel drums, at a strength 66° Beaume.
Granulated bullion, or in the form of grids, was tried to obtain the best solvent action of the acid, but these forms produced too violent a frothing, and were displaced in favour of the shipping bars, which dissolved just as quickly and without effervescence. The kettle, one of three (of white cast iron, free from carbon, but containing phosphorus), is 3 ft. diam. and 3 ft. deep. It is lead lined, and fitted with a lead-lined overflow to the settlers in case of frothing. A lead-lined conical hood of cast iron, with an escape for volatilized acid to coke condenser and concentrator, is used. Into the two outside kettles the bars, to the weight of four or five thousand ounces dore, are charged in, and the kettle, three-parts filled with acid, is heated by a small fire. Here the bullion is allowed to digest for eight hours, then the fire is withdrawn and the gold settled. The acid, containing most of the silver as sulphate in solution, plus a little mechanically suspended gold, is siphoned through a 1-in. lead pipe into the “ settling vats—lead-fined vats, 6 ft. x 3 ft. x 1 ft. 6 in.—where it is settled for 1 hour, passing thence by a water-cooled antimony tap into the crystallizing vats.
The tap (placed ½-in. from the bottom) is only in immediate contact with the acid during transference to the crystallizing vats, at other times being shut off by a lead strip in the settler, which covers the entrance. In the crystallizing vat the solution is brought to a density of 62° Beaume by injecting live steam—this density giving the best precipitation. The vats are cooled by the circulation of water round a 3-in. jacket, placed all round the tank. The silver sulphate crystallizes upon strips of sheet lead, 18 in. x 3 in., hung from wooden beams across the top, and is left to crystallize for 16 hours. The silver sulphate is scraped from these strips and ladled on to a leaden sieve of 1/16-in. mesh. The mother liquors are pumped up to the mother liquor circulating tank, whence they may be admitted to the dissolving kettles, and serve the purpose of cooling the acid from the centre or “ sweetening kettle ” ready for the next charge. The silver sulphate is washed with distilled water, dried, and cupelled with 6 per cent, of coke dust. The washings, containing 2½ per cent, silver sulphate, are run into the “ wash water vat,” 8 ft. x 8 ft. x 2 ft., steam heated, where the silver sulphate is precipitated on copper tiles. The gold from the outside is transferred to the “ sweetening kettle ” and treated with fresh acid. After boiling some time, the sweet acid is siphoned into the side kettles to be further used.
Analysis for gold:
Raw concentrates…………………………..About 6 grs.
Base bullion………………………………………1 dwt.
Dore bullion……………………………………40 ozs./1,000.
From the sweetening kettle the gold is removed by a copper shovel and bucket to a vat, where it is washed free of silver, sulphate and ferrous sulphate, being in contact with diluted sulphuric acid and heated with steam for three days. At this stage the gold contains .5 grs. silver per 1,000 ozs. The acid is drawn off and passed through flannel filters, which are subsequently burnt, to another vat where the remaining silver is deposited by a little common salt or zinc. This chloride is mixed with the silver sulphate on the cupel. Any copper in the solution, after precipitating the silver, is collected on scrap iron in another vat. The liquors from this vat are allowed to run to waste.
The gold still containing lead is put into a large bread crock, and treated with strong hydrochloric acid, and heated by steam. The solution, containing lead chloride, is drawn off and allowed to run to waste. The gold is dried, melted, and cast in 400-500-oz. ingots—a little sugar being put on the molten metal to give a bright face.
The gold is of good quality, and will bring £4 4s. 7½d. per oz. on the market. The plant only requires the labour of two men.
Analysis of products of antimony dross furnace :—
Treatment of Antimony Dross
Principle: partial reduction of litharge with carbon and absorption of antimony in the rest of lead oxide as antimoniate.
This is carried out in a reverberatory furnace of smaller dimensions than the copper softener, and differs in that it has no water-jacketed walls. Its capacity is 10 tons. It has three charging doors, but two tap-holes—one at the end at a higher level taps the slag first into a couple of cast-iron moulds, which catch any lead (the slag overflows into an enclosed space on the floor made tight with sand); the other at the side taps the lead.
A charge consists of 2,800 lbs. of broken-up skimmings, and requires 110 lbs. fine coal and 110 lbs. coke dust for reduction. It takes two hours to complete the operation—i.e., four charges per shift. The slag is tapped once in 24 hours. The lead is tapped into a kettle and thence ladled into moulds, and is finally returned to the antimony softener. Two men are required on the furnace. The furnace lining just about lasts a campaign, which is three weeks or a month.
The antimony dross furnace slag is taken to the antimony blast furnace Raschette type—with 10 tuyeres. To start the blast furnace, a wood fire is put in to warm the crucible and the water in the jackets is turned on. All the fire is raked out, and a 3-in. bridge of wood is placed over the crucible, and kindling wood put on to a height half-way up the jackets ; 200 lbs, coke is added, then 15 bars antimony lead, 200 lbs. more of coke, and another 15 bars of antimony lead; then four charges of 100 lbs. coke and 450 lbs. antimony slag. The regular charge is then 100 lbs. coke, 450 lbs. antimony slag, 250 lbs. ordinary slag, 15 lbs. scrap iron, and so on until full. The fire is lit through the tuyere holes, and, when caught, a light blast is turned on, and, as the process goes on, the regular charge is run in. The ultimate blast pressure = 3-4½ ozs. The lead will be ready to tap two hours after charging, and the slag four hours after charging, A fresh charge is run in every quarter of an hour.
The lead obtained is “ white metal,” and finds a ready market, being used for type metal. The slag obtained goes to the smelters for retreatment. When the furnace is being run it requires the labour of five men.
Analysis of products from antimony blast furnace:
Treatment of Silver Retort Dross
This is really a sweating operation on a large scale. The furnace is a reverberatory of about the same size as the antimony dross furnace. It is charged and skimmed like a copper softener. The lead is run into a kettle, whence it is ladled into moulds.
The bath consists of 200 bars of silver-clean lead (about 5 tons). When this is molten and beginning to oxidize, 750 lbs. dross is put in and roasted to free it of zinc. Then 750 lbs. from the cupels and 25 lbs. coal dust is added to reduce; the remainder collects the impurities from the lead and silver. A fresh charge is added every two hours, consisting of-dross, 750 lbs.; litharge, 500 lbs.; coal, 25 lbs., until full, and then a skimming is taken once a shift. When
skimmed clean the furnace is allowed to cool back, tapped into a kettle, and then baled into moulds. This bullion goes to the concentrating cupels, the slag to the smelters.
This consists of a cylinder through which pass two piston rods which rigidly connect this piston within to an iron basket below by two parallel rods, guided in grooves in the cylinder covers. The press is double-acting ; steam admitted at the bottom forces the cylinder and ram into the basket; admitted on top, it raises the press. The whole thing may be raised by another steam cylinder and piston at the top. From this it is suspended to an overhead bogey running upon rails above the line of Parke’s pots. Before the pan is cooled the press is raised (by means of the upper cylinder) and run over the pan. The steam in the upper cylinder now escaping lowers the cylinder B, and by admission of steam above the piston the basket is lowered into the pan to get hot, thus avoiding a thick coating of chilled lead. The pan is cooled back until a crust is formed, which is pushed by rakes over the basket. This is then raised just above the surface and is filled with skimmings with perforated ladles. The basket is then lifted by B, and the ram presses most of the lead out and compresses the skimmings. This operation proceeds again till the basket is full, when the press is lifted out and over the floor ; the nut C is knocked loose and the bottom of the basket, working on a pivot, E, is swung out of centre. The ram is forced down and the cake falls upon iron bars on the floor, is broken up, and transferred to the retort furnace bins. The area of the ram is about 12 in. diameter by 1½ in. thick. The dimensions of cylinder are B, diameter 2 ft., height 3 ft.; of basket outside, diameter 2 ft. and inside 15 in., with slots about 3 in. x ½ in. The bottom is about 2 in. thick. The gold press is larger in basket proportions than this, as it has to be skimmed hot to effect a separation of the gold and silver, and, as the gold alloy crystals are finer, if too tightly pressed might force the gold back into the pan. This difficulty is not experienced with silver skims.
This is another appliance lately introduced to overcome difficulties in stirring in the zinc. It consists of a hood which closes in the pan; beneath this a paddle revolves about a vertical axis worked by bevel gearing and rope belting connected to a shaft. The paddle works in a small open-ended cylinder to cause connection currents and prevent the charge moving round as a whole, which would cause spilling. C is a lever whereby the stirring apparatus, irrespective of the hood, may be raised or lowered accord-
ing as the pan is near the top (for gold zincs) or 3 or 4 in. down (for silver). These stirrers may be run along, like the presses, to any pan desired, and are counterpoised by lead weights. The stirrers are held down to the pans by extra lead bars. This is a great improvement on previous methods—hand stirring was very laborious, and steam stirring was liable to cause explosions unless the steam was absolutely dry. The modern method is just as efficient, requires much less manual labour and is not dangerous.