152 TRANSACTIONS OF THE 
into the atmosphere. Thallium has also been produced 
in small amounts from this source. The sulphur dioxide, 
however, is still a problem. In some cases sulphuric 
acid is now being produced from these gases as well as 
some liquid sulphur dioxide. These products find their 
way back into metallurgical and other industries and 
become a source of revenue instead of a menace. Con- 
siderable experimental work has been done with the 
production of elemental sulphur from these gases. Fur- 
ther advancement in the treatment of smelter gases may 
be expected. 
Turning now to the pyrometallurgy of ores which 
ds not lend themselves to direct smelting or concentration 
methods, we are confronted with two general types of 
ores, the complex sulphide and the oxidized ore. We 
can have a sulphide ore containing sufficient value in 
metals to ship if all of that value could be realized but 
having its value distributed in lead, zinc, copper and 
precious metals. Now such an ore would best be hand- 
led by a lead smelter, but its zinc content becomes a 
deficit rather than an asset. and the seller instead of 
receiving remuneration for the zinc is penalized for it, 
an amount dependant upon the quantity present. Two 
possibilities of solution of this problem present them- 
selves, the initial stage of which is pyrometallurgical. 
The first of these is the roasting of the ore followed 
by lixiviation and further hydrometallurgical treatment. 
The actual roasting of the ore, to accomplish high ex- 
tractions of zinc, is not an easy operation; but recent 
work, chiefly in connection with the electrolytic zinc 
industry, has established a promising practice which 
with some modifications for certain ores may offer a 
solution to this problem. The residue, after leaching 
the zinc, can be smelted or otherwise treated to recover 
the other values. 
A second line of attack is the chlorination of the 
metals of the sulphide by chlorine gas at elevated tem- 
peratures, producing a product which should be amenable 
to hydrometallurgical treatment. Chlorination should 
command more than a passing interest in the next few 
years. 
The second type of ore which offers particular dif- 
