METALLURGY. The only ore of tin which is used for the extraction of the metal is tinstone or cassiterite. The extraction of tin is usually performed by dry methods—that is, by a process of calcining and reduction in furnaces, hut wet methods and electro-metallurgical methods are also employed to some extent. The ore when mined from veins has• to be stamped to a very fine powder before the valuable portion can be effectually separated. This separation is performed by an elaborate series of washings whose effect is to remove the lighter impurities and to leave the heavy par ticles of tin where they can be collected. Various forms of washing apparatus are employed (see ORE DRESSING) , but the most common appliance is the puddle. In this device the ore and earthy matters, in the state of a thick mud. are con veyed by square pipes or channels to the circum ference of a conical table on which. by the aid of water, the metallic portion separates, the lighter stony impurities flowing down toward the centre, and being carried away. There are also brushes for agitating the ore during the operation.
The tin ore thus far purified has next to be deprived of its sulphur and arsenic if these substances should be present: this is done in a reverberatory furnace, the flues of which are connected with large condensing chambers, in which the arsenic is deposited in a crystalline form. The sulphur which is present in the state of sulphide of iron is decomposed by the heat into sulphurous acid gas. and the remaining oxide of iron is removed by a subsequent washing. Sulphide of copper, when present, is converted by roasting, and afterwards exposing it to the air, into sulphate of copper, and is then easily dissolved out by lixiviation.
After this final washing the ore is ready for smelting in a reverberatory furnace. The charge consists of from 20 to 25 ewts. of ore mixed with one-sixth of its •weight of powdered anthracite or charcoal, and a small quantity of lime or fluor-spar, to serve as a flux for the siliceous im purities. Before being put into the furnace, the mixture is moistened with water, to prevent the finely powdered ore being carried away by the draught. When the charge is placed on the hearth of the furnace, the doors are closed, and the heat gradually raised for about six hours; the oxide is then reduced by the carbon of the coal. At this stage the furnace door is opened, and the mass worked with a paddle, to separate the slag, which is raked off, and the richer por tion of it melted over again. The reduced tin
subsides to the bottom, and is run off into a cast-iron pan, from which it is ladled into molds, to produce blocks o• ingots of a convenient size.
The tin has still to be purified, first by a proc ess of lignation, and afterwards by that of boiling. `Liquation' consists in moderately heat ing the blocks in a reverberatory furnace till the tin, owing to its comparatively easy fusibility, melts and flows into the refining basin, leaving on the hearth of the furnace a residuary alloy of tin with iron and other metals. More blocks are added and heated in the same way, till the re fining basin contains about five tons. The tin is then ready for 'boiling.' In this operation billets of green wood are plunged into the melted metal, the disengagement of gas from which produces a constant ebullition. and so causes a scum (chiefly oxide of tin) to rise to the sur face, which is then easily removed; at the same time, impure and dense parts fall to the bottom. When the agitation has gone on long enough, the bath is allowed to settle and cool. The tin then separates into zones—the upper consisting of the purest portion. the middle being slightly mixed with other metals, and the lower so much so that it requires to go through the refining process again. The residuary alloy of the liquation process has also its tin extracted and refined again.
In former times in Cornwall tin was smelted in a blast furnace (see Bios' AND STEEL) instead of a reverberatory one; and this is still the case on the Continent. By this method a pure tin is obtained, but the loss of metal in the process is greater. It works best where coal is scarce and wood abundant.
Tin ores which contain the mineral wolfram (tungstate of iron and manganese) are treated by a special process, patented by R. Oxland, of Plymouth. England. This mineral and tin ore are so nearly the same in specific gravity that no mechanical process of washing will separate them. Oxland's process consists in roasting the dressed tin ore with sulphate of soda, for the purpose of converting the insoluble tungstate of iron and manganese into the soluble tungstate of soda, which is easily removed by lixiviation. The oxides of iron and manganese, which are left in a finely divided state, can then, from their lower density, be readily got rid of by washing. The tungstate of soda procured in the operation has been found to be one of the most valuable substances for rendering cotton cloths non-inflam mable.