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but by a direct catalytic reduction. This has been done in many modern plants, e.g., in that of the Consolidated Mining and Smelting Co., Trail, B. C.

For this recovery of sulfur, the present reduction method of SO₂, gas to elemental sulfur proves far superior to the old Chance-Claus process described above. Briefly, there are at present two main processes for the recovery of sulfur from the waste gas. These are the I.C.I. and the Boliden processes.^[1] The I.C.I process involves first concentrating the SO, gas from the waste gas, purifying the gas, and then liquefying it to liquid sulfur dioxide; or, alternatively, reducing it lo elemental sulfur by the water-gas method with the help of a catalyst. Concentration of the gas is achieved by the absorption of the weak gas in a basic aluminum sulfate solution, having a pH value of 3.5,.or 40 per cent basicity, the solution containing a small percentage of phosphoric acid as a stabilizer to help keep the Al₂O₃, in solution. Regeneration of SO₂, gas is then effected by the liberation of SO, from the basic aluminum sulfate solution by boiling at about 100 °C. As the SO₂ gas lies a tendency to be oxidized to SO, in the presence of oxygen, pulverized limestone or lime is used to neutralize the acid formed so as to maintain the proper pH value in the solution.

Gaseous sulfur dioxide is reduced by passing it through a water-gas generator at 1200°C., whereby many complex products are formed, as shown by the following equations:

1. $4C+2SO_{2}\rightarrow 4CO+S_{2}$
2. $2C+2SO_{2}\rightarrow 2CO_{2}+S_{2}$
3. $C+H_{2}O\rightarrow CO+H_{2}$
4. $2H_{2}+S_{2}\rightarrow 2H_{2}S$
5. $2CO_{2}+S_{2}\rightarrow 2COS$
6. $C+S_{2}\rightarrow CS_{2}$
7. $2SO_{2}+2CS_{2}\rightarrow 4H_{2}O+3S_{2}$
8. $2SO_{2}+2CS_{2}\rightarrow 2CO_{2}+3S_{2}$
9. $4CO+2SO_{2}\rightarrow 4CO_{2}+S_{2}$

Catalytic action will convert all these complex products to elemental sulfur. The catalyst consists chiefly of ferric oxide and aluminum oxide. By this process either liquid sulfur dioxide or elemental sulfur can be obtained.

The Boliden process, independently worked out by Swedish research chemists, is a direct reduction of the waste gas into sulfur in one step without concentration or purification of the SO₂ gas. This process is particularly suitable for the manufacture of sulfur directly from the waste gas, where reasonable control of the smelter gas is possible to keep the SO, content above 5 per cent.

These two processes of recovering sulfur from waste SO₂, gases are so efficient nowadays that it is possible to recover sulfur of high purity from the waste gases from roasters or smelters, comparable in quality with Louisiana and Texas sulfur.

Other processes, such as McCluskey's process,^[2] etc., are all based on</reflist>

1. See article by M. P. Applebey entitled "The Recovery of Sulphur from Smelter Cases," Journal of Society of Chemical Industry, May, 1937.
2. McCluskey, S. B., B. P. 421,289; 421,290; 417,276.