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SMELTING PROCESSES 



1) TUc raw matt from the load slag process : and 



2) The blendio and pyritic Durr and flux ores which come to the lead slag process. These cither in natural 

 lumps (or in agglutinated masses resembling lump ore), or stamped, or pulverized. 



The sulphur of the raw matt amounts to 32-25 per cent. ; that of the lump ore, 32 35 per cent. ; and that of the 

 powdered ores, 38-80 per cent. Ore lumps arc prepared artificially as follows : — 



Rich and infusible on- dust is well mixed in a large wooden taidc, and moistened with dilute sulphuric acid. The 

 mass is then worked over with a wooden rake, pressed into iron moulds, cut into small cubes, and dried on the iron 

 slabs over the condensation canal of the roasting furnace. In this manner these acquire such firmness that they can 

 be roasted in kilns just like natural lump ore. The cause of this solidity is to be found in the formation of sulphates 

 of zinc and alumina, compounds which are not easily decomposed by heat. 



All these lumps (natural and artificial) arc roasted in kilns, but the ore dust is desulphurized in Gerstciihofer's 

 dust roasting furnace. 



The kilns are a kind of cupola furnace in their main features. The inside is a space either like a parallclopipedon, 

 or like the frustrnm of a pyramid in form. They are of many different sizes. The high kilns are principally used in 

 the roasting of the raw matt, and the low kilns in roasting the lump oren and ore lumps. 



On the workman's side of these kilns are openings at different heights from the ground, through which the work- 

 man can reach all parts of the mass within, by means of suitable iron instruments, and loosen it. The kiln is put in 

 operation by the workman filling it two-thirds full of already ignited raw matt, then kindling a fire underneath, and 

 after the walls of the furnace are red hot, letting in the charge of raw matt through tho roof. 



The raw matt burns of itself by virtue of its percentage of sulphur. After the lapse of a few hours, a portion 

 of the charge is drawn out of the opening nearest to the sole of tho kiln, and this causes the whole superincumbent 

 ma-ss to sink, leaving place at the top for a new charge of raw matt. The new charge is distributed evenly over the 

 surface of the already ignited portion. 



About 18-20 centners of raw matt is introduced at once or a charge, and Ibis is repeated about every twelve hours.- 

 Tin; introduction of the charge always follows tho drawing out of a part from below. The interior of a Gerstenbofer 

 furnace is a space iu the form of a rectangular prism, across which from side wall to side wall a great number of 

 triangular prisms of burnt clay are inserted, with one of their faces upwards. 



There are two narrow slits in the roof of the furnace, through which tho ore dnst is fed through rollers. 

 Under these slits are two larger triangular prisms, having their edges up, so that tho pulverized ore, falling 

 upon these first, is diverted to the right and left, and falls through the entire height of the furnace, resting a short 

 time on each bar. These roast, bars must be previously boated to redness by a fire kindled in the furnace itself, but 

 gradually withdrawn as the ore itself ignites. 



The powdered on; which collects on the flat sides of tho roast bars is ignited by them, and once fairly ignited, 

 evolves such a heat that all other lire is unnecessary. The atmospheric air requisite for combustion is partially intro-i 

 duced through numerous ail- boles in the workman's side of the furnace, and (in ease more air is needed than these 

 can supply) by means of a ventilator. Willi ibis exception the furnace is closed, and the products of combustion, of 

 which sulphurous acid forms the greater part, are collected. 



The feed is continuous, and 40 cut. pulverized ore can be roasted in such a furnace in one day. 

 The gases are first conducted through a condensation chamber, in which the greater part of the arsenious acid 

 condenses, and from here through a subterranean canal (which also serves to condonse the still remaining arsenious 

 acid and small particles of ore carried forward by the draught) to the lead chamber, where, by the necessary nitric 

 acid and steam, sulphuric acid' is formed. The atmospheric air required to convert tho sulphur into sulphurous acid 

 Bows into the furnace, and from there it goes with the roast gases into the lead chamber. The amount of sulphurous 

 acid in the products of combustion does not exceed fl-7 per cent, by volume, and indeed in the roasting of the raw 

 mitt; it is not above i per cent. The load chambers are large rooms lined with sheet lead, in which the formation of 



sulphuric acid takes place. The lining sheets of lead hang from a strong scaffolding and are soldered together witli 

 lead. A "system" of chambers consists generally of ono entrance chamber, two main chambers, and two roof 

 chambers which are connected with each other by strong leaden pipes. The entrance chamber receives tho nitric acid 



