PYRITES 677 



other cases muffle-furnaces are employed, whilst in many establishments open reverbc- 

 ratory furnaces, heated by gas, are made use of; the flame being first taken beneath 

 the tile-bottom forming the hearth, and afterwards through the body of the furnace. 

 Whatever may be the form of furnace adopted, the flues in connection therewith are in 

 communication with a condensing tower, packed with crenulated brickwork, through 

 which a spray of water is constantly descending, which thus acquires a sufficient degree 

 of acidity to render it of value during the succeeding operation of washing. 



The acid waters from the condenser are also found to contain traces of copper, which 

 is thus retained, and a certain amount of loss of that metal thereby prevented. When 

 open furnaces are made use of, the amount of copper recovered is exceedingly small, 

 and of little or no commercial value ; it is however stated that when muffle-furnaces 

 are employed, the quantity of that metal caught in the condensers is much more con- 

 siderable. It must, however, be remembered that close furnaces consume a much 

 larger amount of fuel than open ones, and that the time required for the operation is 

 when they are employed much longer. It is therefore believed that any advantage to 

 be derived from the recovery of a somewhat larger amount of volatilised copper is, in 

 the case of the muffle-furnace, more than counterbalanced by the loss of time expe- 

 rienced and the increased expenditure of fuel. 



In the close furnace the calcination of each charge of the mixture of ground burnt 

 ore and common salt occupies twelve hours. In the open furnace heated by gas a 

 charge of three tons is drawn every six hours. On being withdrawn from the furnace 

 the calcined ore, whilst still warm, is taken in iron waggons to lixiviating vats. 



These vary considerably in their dimensions, but a very convenient size is 11 feet 

 square and 3 feet 6 inches deep. Each tank is provided with a false bottom of per- 

 forated tiles, supported on bricks placed on-edge, these are covered, to a depth of five 

 inches, with sifted cinders, which form a filter, and on this is placed the ore to be 

 lixiviated. The charge of a tank of the dimensions above given is about 15 tons. 

 Each charge of the lixiviating vats generally receives some eight or ten successive 

 washings, either with hot water, with weak copper-liquors, or with water acidulated 

 by hydrochloric acid ; the necessary amount of the latter is usually obtained from the 

 condensing towers attached to the establishment, but in the case of very refractory ores 

 this is not always found to be sufficient. 



The washing-tanks require to be placed at a sufficient height, above the floor to allow 

 of the copper-liquors being tapped directly into wooden spouts, by which they are 

 conveyed into a series of tanks of similar dimensions, where the copper is precipitated 

 in the metallic state, by means of scrap-iron. This operation is facilitated by heating 

 the solutions by a jet of steam conveyed into them through a leaden pipe. 



In order to separate the cement-copper produced from the fragments of undissolved 

 iron with which it is mixed, it is placed on a platform composed of plates of perforated 

 cast iron, which forms the head of a box or tank filled with hot water to a depth of 

 about four inches over the surface of the plates. It is here stirred briskly with an 

 iron rake, by which the copper is made to pass through the holes in the ' grid,' whilst 

 the fragments of iron, remaining on its surface, are raked off into shallow baskets, in 

 which they are again taken to the precipitating vats. 



The precipitated copper, which thus collects in the washing-box, is subsequently 

 allowed to drain, and, after being dried in a properly-constructed stove, is either sold 

 to the smelters, or is fused and refined in the establishment in which it is produced. 

 The precipitate thus obtained from Spanish and Portuguese pyrites usually contains 

 from 75 to 80 per cent, of copper, and a little over 20 oz. of silver per ton. 



The spent ore, remaining in the lixiviating tanks, is washed until it retains only 

 0'20 per cent, of copper, when it is removed for the reception of a fresh charge of 

 calcined ore. This substance, which is known as 'purple ore,' or ' blue billy,' meets with 

 a ready sale to the various iron works, where it is chiefly employed for the fettling of 

 puddling furnaces, although it is also extensively employed in the blast-furnace. It 

 has the following average composition : 



Analysis of Purple Ore from Portuguese Pyrites. 



Ferric oxide 96'00 



Lead, as sulphate 0'75 



Copper 0-20 



Sulphur 0-36 



Lime 0'40 



Cobalt, Arsenic, and Chlorine trace 



Phosphorus trace 



Soda . . 0-10 



Insoluble residue . . . . , . .2-11 



Total . 99-92 



