ROASTING ORES. 



707 



1714 



the sulphur. In the Lower Hartz, a heap of this kind coatinuos roasting during four 

 months 



2. The second method. The difficulty of 

 managing the fire in tho roasting of substances 

 containing little sulphur, with the greater 

 difficulty of arranging and supporting in their 

 place the fine ores to be roasted, and last of all, 

 the necessity of giving successive fires to the 

 same ores, or to inconsiderable quantities at a 

 time, have led to the contrivance of surround- 

 ing the area on which the roasting takes place 

 with three or four little walls, leaving a door 

 in the one in front. This is what is called a 

 walled area, and sometimes, improperly enough, 

 a roasting-fnrnace. Inside of these walls, 

 about 3 feet high, there are often vertical con- 

 duits or chimneys made to correspond with an 

 opening on the ground level, in order to excite 

 a draught of air in the adjacent parts. When 



the roasting is once set going, these chimneys can be opened or shut at their upper 

 ends, according to the necessities of the process. 



Several such furnaces are usually erected in connection with each other by their 

 lateral walls, and all terminated by a common wall, which forms their posterior part ; 

 sometimes they are covered with a shed supported partly by the back wall, built 

 sufficiently high for this purpose. These dispositions are suitable for the roasting 

 of schlichs, or pyritic sands, and in general of all matters which are to have 

 several fires ; a circumstance indispensable to a due separation of the sulphur, 

 arsenic, &c. 



3. The furnaces employed for roasting the ores and mattes differ much, according to 

 the nature of the ores, and the size of the lumps. We shall content ourselves with 

 referring to the principal forms. 



When iron ores are to be roasted, which require but a simple calcination to disen- 

 gage the combined water and carbonic acid, egg-shaped furnaces, similar to those in 

 which limestone is burned in contact with fuel, may be conveniently employed ; and 

 they present the advantage of an operation which is continuous with a never-cooling 

 apparatus. See IKON. 



It has been attempted to employ the same method a little modified, for the roasting 

 of ores of sulphide of copper and pyrites, with a Anew of extracting a part of the 

 sulphur. More or less success has ensued, but without ever surmounting all the ob- 

 stacles arising from the great fusibility of the sulphide of iron. For it sometimes 

 runs 4 into one mass, or at least into lumps agglutinated together in certain parts of 

 the furnace, and the operation is either altogether stopped, or becomes more or less 

 languid ; the air not being able to penetrate into all the parts, the roasting becomes 

 consequently imperfect. This inconvenience is even more serious than might at first 

 sight appear ; for, as the ill-roasted ores now contain too little sulphur to support 

 their combustion, and as they sometimes fall into small fragments in the cooling, they 

 cannot be passed again through the same furnace, and it becomes necessary to finish 

 the roasting in a reverberatory hearth, which is much more expensive. 



In the Pyrenees, the roasting of iron ores is executed in a circular furnace, so 

 disposed that the fuel is contained and burned in a kind of interior oven, above which 

 lie the pieces of ore to be calcined. Sometimes the vault of this oven which sustains 

 the ore, is formed of bricks, leaving between them openings for the passage of the 

 flame and smoke, and the apparatus then resembles certain pottery-kilns : at 

 other times the vault is formed of large lumps of ore, carefully arranged as to tho 

 intervals requisite to bo left for draught over the arch. The broken ore is then dis- 

 tributed above this arch, care being taken to place the larger pieces undermost. This 

 process is simple in the construction of the furnace, and economical, as branches of 

 trees, without value in the forests, may be employed in the roasting. See KUN. 



In some other countries, the ores are roasted in furnaces very like those in which 

 porcelain is baked ; that is to say, the fuel is placed exteriorly to the body of the fur- 

 mice in a kind of brick shafts, and the flame traverses the broken ore with which the 

 furnace is filled. In such an apparatus the calcination is continuous. 



When it is proposed to extract the sulphur from iron pyrites, or from pyritous 

 minerals, different furnaces may be employed, among which that used in Hungary de- 

 serves notice. It is a. rectangular parallelepiped of four walls, each of them being 

 perforated with holes and vertical conduits which lead into chambers of condensation, 

 where the sulphur is collected. The ore placed between the four walls on billets of 



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