on the Continent of Europe, Pattinson plants have been made to serve for the Parkes process, with as little alteration as possible, and in these the lead frequently has to be pumped from one part of the plant to another, but in most English and American works the plant is arranged in terrace form, as shown in Fig. 129. This is a sketch section of a somewhat unusual but very effective type of "refinery" (as the whole plant is usually called) arranged like that of the Broken Hill Proprietary Company at Port Pirie, in which the softening is performed in two stages (v. Chapter xv.). In this figure and in Fig. 130, which is a sketch plan, a represents the receiving or unloading platform, b is the so-called "copper," or drossing furnace, cc are "antimony" or softening furnaces, dd the desilverisation pots, and e the liquation furnace for zinc crusts; ff are refining kettles, g is the casting pot, h the dross liquating furnace, and i the refinery blast furnace for drosses, skimmings, litharges, &c.; j and k are sidings for unloading and loading respectively. The retorting and cupellation rooms are situated beyond the crust liquating furnace, in what is practically a separate building. The pots must be perfectly smooth inside, and are always of cast-iron unless they have to serve for subsequent refining of the lead, when cast-steel is sometimes employed, as already mentioned.* Pots which have to serve for the whole process from drossing to refining, as at Lautenthal (Harz) and Tarnowitz (Silesia), are made small (12 to 15 tons); those for desilverisation pure and simple are made to hold from 30 to 50 tons, and may last for 150 to 300 charges. The form is usually a segment of a sphere, and the pot is supported either by a rib cast round it at half its depth, or simply by its flange as shown in Fig. 109. A 30-ton pot is frequently 8 feet 10 inches wide by 3 feet 4 inches deep (the radius being 4 feet 7 inches), and has a flange 9 inches wide. Larger pots are sometimes made elliptical, like those of the Omaha and Grant refining works at Omaha (Neb.), which are about 12 feet long by 7 feet wide and 3 feet 3 inches deep, holding 48 tons; but the 50-ton pots of the Balbach (N.J.) and Pittsburg works are round, like the 40-ton pots of the Howdon (Newcastle) and Pertusola (Italy) works. Amount of Zinc Required.-The quantity of zinc required to extract the silver varies according to the richness of the bullion treated, though, proportionately, poor lead takes more than rich. Every refinery has its own table of quantities, and Roswag has attempted to deduce general formulæ adapted to bullion of any given tenor, which do not, however, seem to have commended themselves to practical refiners. At Freiberg, Plattner found that the following quantities are required : * Chapter xv. + La désargentation des minerais de plomb, Paris, 1884, p. 241. But much less is found sufficient in most modern refineries by using the crust over again, so as more completely to saturate the zinc contained in it. A Reference has been made to the fact that the gold-copper crust is formed without saturating the lead-bath with zinc. very small proportion of zinc is, therefore, sufficient for the first or "gold-zine" addition, as little as 0.10 to 0·12 per cent. by weight being found sufficient for properly softened bullion containing not over 8 dwts. gold per ton,* and subsequent additions are directed solely to the removal of silver. Occasionally, when the bullion contains a large quantity of gold, and is practically free from copper, no separate gold crust is made, but, in that case, all the silver produced from the silver crust has to be parted for gold, and it is far preferable to extract the gold first concentrated a form as possible; this is the almost universal modern practice. in as When a gold crust is first taken off, the additional zinc, for bullion of 300 ozs. and upwards per ton, is occasionally added in three portions (viz., two-thirds, one-fourth, and one-twelfth respectively of the total); but with ordinary bullion of 150 to 300 ozs. per ton, the work is almost always done with two silver-zincings of about equal amount, say 0.8 to 10 per cent. each of the weight of lead under treatment, of which only the second is fresh zinc. Low-grade bullion of less than 125 ozs., if sufficiently softened, can be perfectly desilverised with only two zincings, yielding one gold and one silver crust. The first silver crust from rich bullion should contain from 2000 to 4000 ozs. per ton at least, and leave the lead with 30 to 90 ozs. per ton; the second crust will run from 200 to 700 ozs., and leave a very poor lead; while, if a third addition is required at all, the resulting crust will contain only an ounce or two of silver. are In any case only the first crust is liquated; those obtained from subsequent zincings contain much free zinc and returned to the pot as the first zincing of the next following. charge. *Private notes, Port Pirie, 1896.J Process of " Zincing."-The pot, having been whitewashed and heated up to the temperature at which a splinter of wood will ignite, is filled with lead by tapping from the softening furnace direct, or by syphoning from a "collecting pot" in which it has been drossed, if such is employed. When it is tapped direct from the furnace a gutter of angle-iron is employed, which empties into an iron pipe reaching to the bottom of the pot, so as to produce as little oxidised dross as possible. In American works the lead is usually drossed at once after addition of any liquate lead from gold crusts of previous charges. Unless, however, the collecting pot has been employed, it is much better to allow the lead to cool in the pot so as to skim off as much copper dross as possible before commencing desilverisation, as this plan permits the complete extraction of gold in an abnormally small weight of gold crust. After drossing, the bath is heated up to about 450° C., and the first addition of zinc is made. The zinc is usually employed in slabs which are simply melted down on top of the lead and stirred in by hand. At some works, however, it is put in a perforated iron box, which is held at the bottom of the pot by an iron rod until the zinc has melted out of it in little streams which can be easily mixed with the lead; at others, again, it is melted separately and poured into the lead, the object in both cases being to avoid oxidation. However it is melted down, the zinc must be very thoroughly stirred in, which takes thirty to forty minutes; this may be done in two ways: Hand Stirring is done by means of a circular perforated sheetiron disc ("paddle") attached to a 6-foot handle with a crosspiece. Two men on opposite sides of the pot lower their paddles down the sides and raise them in the middle, so producing an upward current in the centre of the lead-bath; the operation is most fatiguing and many attempts have been made to replace it by steam stirring, which, however, has the grave disadvantage of producing a more oxidised and poorer crust; on this account a majority of the best and most modern works still adhere to hand stirring. Steam Stirring is done by means of a steam pipe reaching down vertically some 2 feet into the centre of the pot. The steam must be perfectly dry in order to avoid explosions, and, therefore, steam traps for condensed water are provided; and the pipe must be thoroughly warmed up before lowering it into the pot. At, or a little above, the melting point of zinc (415° C.) there is comparatively little oxidation of that metal, though at temperatures much above 450° (at a cherry red for instance) oxidation is very rapid. As soon as the steam is turned on it produces a downward current in the centre of the bath, but the bubbles drive away the molten zinc together with the incipient crusts to the sides, whence they are continually pushed towards the centre by a wooden hoe. When the "stirring in" is completed, or somewhat before, the fire is damped down with wet slack and the doors opened so as to cool the pot, and, if steam has been used for stirring, it is blown through the lead a little longer with the same object. After about two or two and a-half hours the surface crust formed begins to adhere to the sides of the pot, and is then removed by means of a perforated skimmer with a long gaspipe handle worked upon the edges of the pot as a fulcrum, or suspended by a hook, which serves the same purpose. At the Pueblo Smelting and Refining works (Pueblo, Colo.) the Howard skimmer is used, which consists of two semi-circular perforated plates hinged in the centre, like a "butterfly valve," and folding downwards. In this condition they are lowered into the lead; they are then unfolded just under the crust, and raised with it above the level of the bath to drain. The crust must be very thoroughly drained and shaken free from lead before turning into the liquating kettle or into a cast-iron mould. That part which adheres to the sides of the pot is then barred off with a long steel chisel, and the operation of skimming repeated till no more crusts form. About eight samples of one A.T. are then taken and cupelled; the silver beads are added together and parted to see if all the gold has been removed. In the meantime the pot is heated up, liquated lead from the first silver crust of the preceding charge is added, and, as soon as hot enough, the first silver-zinc is stirred in. This consists of the second (and third, if any) silver crust from a previous operation which contains much free zinc, and a little fresh zinc, if necessary, to make up the amount required. The operation of collecting and removing the silver crust is exactly similar to that described for the gold crust; the crust is generally poured and pressed into moulds, so as to be conveniently handled and charged into the liquation furnace. The lead, now containing from 30 up to 90 ozs. silver, is again sampled and treated with zinc for a second silver crust, which is also generally moulded, or simply deposited on an iron plate close to the pot to wait for the next charge. If, after removing this second silver crust, the lead still contains oz. or so of silver per ton, the necessity for a third zincing can be avoided by blowing steam through the pot for half an hour or more, which causes a small additional formation of zinc crust carrying silver, and reduces the silver contents of the lead to 3 or 4 dwts. Time Required and Fuel Consumed. The time required for each melting and stirring averages half an hour to an hour, cooling two to three hours, removal of crusts half an hour to an hour, re-heating one and a-half hours to two hours, so that five * v. Hofman, Mineral Industry, vol. ii., 1893, p. 40. to six hours may be reckoned as the average time for each complete zincing. With three zincings giving one gold and two silver crusts, ordinary bullion can be desilverised in about fifteen hours, including running the lead into the pot and syphoning it off afterwards. The Port Pirie practice, subsequently described, of doing the work very slowly has many advantages. The consumption of coal for desilverising and liquating the crusts is about 54 lbs. per ton of original base bullion* (= 2·41 per cent. by weight). At some of the refining works in Chicago, crude petroleum oil, sprayed by means of compressed air, is used as fuel, and is said to be more economical than coal, as well as affording the convenience of being extinguished and relighted in a moment, which accelerates both the cooling and the re-heating. Natural gas has also been used at Pittsburg with the same objects and similar results. Examples of Desilverising Practice.-At Freiberg, † lead which has been already partially desilverised by the Pattinson process and still contains 0.1 per cent. Ag (32 ozs. per ton), is desilverised in 20-ton kettles with three additions of zinc, together amounting to 1.485 per cent. of the weight of lead. The first addition carries off the gold and brings down the silver contents to 8 ozs., while the second reduces it to below 13 dwts., and sometimes to as little as 6 dwts., in which case the third addition is dispensed with. At the Port Pirie refinery, † where 500 tons of lead per week are desilverised, the plant consists of four desilverising pots of 30 tons capacity, and the bullion treated averages 300 to 330 ozs. silver, with about 5 dwts. of gold per ton. Only three zincings are given, and the last of these leaves the lead with only about 6 dwts. silver per ton. The lead is tapped direct from the softening furnace into the zincing pot, where it is allowed to cool for three hours in order to admit of a further drossing which much facilitates the clean separation of the crusts, especially of the gold crust. The amount of "pot dross" removed averages about 200 lbs., containing 210 ozs. Ag, 96 per cent. Pb, and 0.2 per cent. Cu, which is treated together with that produced in the first drossing furnace. The temperature is then raised, and the gold-zinc stirred in. In the first instance, on starting a new campaign a single addition of 75 lbs. zinc is used and is found to extract all the gold. The crust obtained from this very thoroughly softened bullion is found to be still capable of withdrawing most of the gold from a second charge of lead, although the total amount of zinc added is not more than one-tenth the amount required to saturate the lead. It is, therefore, returned to the next potful of lead ready for treatment, from which nearly the whole of the gold is extracted, and the enriched (because twice used) gold crust is * Hofman, Metallurgy of Lead, p. 337. + Private notes, 1896. |