The main obstacle in directly reducing zinc sulfide concentrate in a commercial vertical retort is the difficulty to produce strong briquettes. The authors have successfully applied a new method to the preparation of strong briquettes which have satisfactory strength even after almost all the zinc sulfide in the concentrate has been reduced to zinc vapor. The method consists of mixing zinc sulfide concentrate with pulverized coal and calcium oxide, preheating and briquetting the mixture into a cylindrical briquette of 1.3cm in diameter, and then coking the briquette. Effects of mixing ratio, preheating temperature and time, particle sizes of concentrate, coal and calcium oxide on the strength of the briquette have been axamined. Sinze almost no sulfur has been observe to come out of the briquettes and also since they have considerably low specific resistance of 5.1Ω·cm, it has been suggested that the briquettes may be fed not only into a vertical retort but also into an electrothermic furnace without additional coke.

In a former report4) the authors have proposed a new method to prepare strong briquettes for the vertical retort zinc smelting process by mixing zinc sulfide concentrate with pulverized coal and calcium oxide followed by preheating, briquetting, and coking. In the present study, calcium carbonate has been substituted for calcium oxide because the latter quickly reacts with moisture in the ambient atmosphere and causes the briquette to collapse in a day or two. The coked briquette with calcium carbonate is as strong as the one with calcium oxide, does not collapse after being left in the laboratory for weeks, and is faster for zinc sulfide to be reduced. Fixation of sulfur during the reduction of zinc sulfide is excellent and substantially all the sulfur is fixed in the form of CaS. Although the specific resistance at room temperature, 29.2Ω·cm, of the coked briquette in this study in abtut six times as large as that of the former on with calcium oxide, it easily decreased to about 23Ω·cm when when heated to about 100℃, which suggests the possibility of feeding the briquette into not only a vertical retort but also an electrothermic furnace without additional coke.