AT FREIBERG. 



411 



Tho charge enters the muffle by two boles in its roof, from the drying hearth above it. The charge is here brought to 

 a dark oherry-red glow, and remains perfectly in powder form without agglutinating. After 4 or S hours roasting 

 it is reached through the middle working door, and drawn by a rake down through a vertical shoot ending in Uie sole 

 of the muffle and generally closed by a slab. 



It is then roasted in direct contact with the dame of stone-coal. Hero the temperature of the hearth is raised 

 until it loses its powdery state and forms larger or smaller lumps. Meantime a fresh charge is brought upon the 

 muffle, receives its preliminary roasting and is raked down upon the hearth as before described. 



There are three periods distinguishable in the course of the process. 



1) The period of ignition. 



2) The period of desulphurization. 

 8) The period of complete roasting. 



In the period of ignition which takes place in the muffle, the ore loses its hygroscopic water. Tho charge 

 is allowed to lie quietly in tho muffle without turning or raking it, until the beginning of the desulphurization, which 

 a-rises from U to 2 hours after the commencement ("Burning-out" of the Ore). 



In the period of desulphurizaton the sulphur and arsenic present in the ore, commence to burn and keep the ore 

 in a state of ignition by their own combustion. During tins time it is necessary to turn and rake the ore diligently 

 with the roast rake, "crutch," and shovel, in order to keep always presenting new surfaces of tho glowing ore par- 

 tides to tho atmosphere, and thus to accelerate tho desulphurization. Quantities of vapor of sulphurous and arse 

 nious acids (so called Roast Oases), are developed. The greater part of tho period of desulphurization fakes place on 

 the lower hearth. This period lasts 8| to 4 hours, during which the Are must not be increased too much, lest a, pre- 

 mature agglutination of particles of ore fake place, which would interfere with a, proper roasting. 



During tho period of desulphurization the ore loses the powder form and becomes more and more iloeculent and 

 bulky. The last two hours of the period of roasting, constitute the period of complete roaUing. The charge ceases 

 to omit gases, and must, therefore, be strongly ignited in order to decompose the sulphides of the metals and the 

 sulphates of the raetalic oxides. After tho completion of the roasting (i. e. 8 to 10 hours), the charge is drawn out 

 of some convenient working aperture, at a vvA glow into a car which has been pushed under the furnace sole, and a, 

 sample is taken for assaying ; the remainder is dumped into the receptacle for the particular row of furnaces to which 

 it belongs, cooled oil.' with water and loft till it is needed for smelting. The charge which is drawn out, must have 

 been roasted down to 4-5 p. o. of sulphur, whereas its percentage was originally about 25 p. c. 



For the determination of the. percentage of sulphur, a, weighed quantity of the charge is pulverized with three 

 times its bulk of carbonate of soda, and saltpetre. This mixture is melted up, dissolved in hot water, and Altered 

 into a beaker glass, to which is added a little chemically pure chlorohydrio acid ; and is then titurated for sulphuric; 

 acid. The daily number of workmen to a furnace is six, two going on every eight hours. One looks out for the 

 muffle and the new oharge, the other finishes the roasting of a charge, and attends to the firing. 



As the inufiloina.il has less to do, they change about every four hours. 



The average consumption of fuel in 24 hours is about 7 bushels of hard slaty coal with a large percentage of ash. 



A bushel weighs two centners. 



Middle Products of the Lead Process. 



1) Slag, The aim is to make this a very fusible silicato of tho protoxide of iron, tho earthy oxides, and the 

 oxides of zinc and manganese with a little load (in reality about 5 p. c, containing 0.005 p. c. silver). Of (his slag a. 

 part is used in compounding the new charge, and the rest in extracting silver and lead in the English furnace. 



2) Lead Matt is a, combination of several sulphides of the metals, principally of lead, but containing also sulphides 

 of iron, copper, zinc and silver. The matt contains on the average 25 to 20 p. e. lead, 10 to 15 p. c. copper and 

 0.1 to 0.15 p. o. silver. In 24 bonis about 10 to 12 centners of matt are formed. 



The matt is stamped, and roasted in the hearth and mullle furnace, after which if is again mixed with slags and 

 various materials rich in lime, and brought to the cupola furnace, in order to extract as much as possible of tho silver 

 and tho lead from it, and to increase as much as possible its percentage of copper. It contains the larger quantity of 

 the copper in the compounded charge. The second time the matt is treated, it is called the "modification " of the 

 matt, and takes place after the conclusion of a run, in the same furnace in which the lead process itself is conducted. 



