The latest form of these roasters is 30 ft. in height, and 17 ft. diameter at bottom and 14 ft. at top, with the central flue terminating in a draft-stack 48 in. in diameter. The ore is dumped into the top of the kiln and occupies the annular space between the two walls. De scending by gravity. it first meets the current of gas from the upper set of fire arches and gets a preliminary drying and warming. Passing thence before the next and lower arches it gradually reaches a red and even white heat, every part of the ore rolling and turning over in its passage, and being subjected while highly heated to drafts of air, the liberated sulphur passing directly off into the central stack. The annular space, being 14 in. at the top and gradually increasing to 29 in. at the bottom, gives opportunity for constant moving of the ore and decreases the chances of its adhering to the walls. The roasted ore is drawn through chutes directly into bins, barrows, or conveyers. The discharge of ore is regulated by draw ing from the chutes, and the heat by varying the amount of gas. The furnaces vary in ca pacity, according to the ore. At the Croton mines, Brewster. N. Y., from 200 to 300 tons per day are said to be run through each furnace. Mr. W. H. Hoffman (Trans. A.
E., October, 1891) thus describes the practice there : " A series of experiments was made to determine the best size for economical roasting (the ore containing '2 per cent sul phur), and at the end of three months a size that would pass through a 21-in. ring was adopted as giving the most rapid work for
the quantity of fuel consumed. Crude Lima-oil is used for roasting, the furnaces being remodeled for this purpose. Through experi ments conducted by our general foreman, Mr. T. Blass, we found the average consumption of fuel-oil to be 3.75 gals.; but by enlarg ing the combustion chambers we have reduced this amount to a little over 3.6 gals. per ton of raw ore. The cost of the oil is 21 cents per gal., making a fuel cost of 81 cents per ton of raw ore.
The labor of filling and discharging amounts to only 3 cents per ton, as this work is largely antomatie. The average temperature is 1250° F., and the ore is roasted down to about 0.5 per cent sulphur." A modification of this type is shown in Fig. 1, in which the draft-stack is cut off and surmounted by a bell, the draft being downward and outward at the bottom of the kiln. In this ease the ore is dropped from self-dumping cars directly on to the bell which distributes the charge, and falling by gravity is drawn directly into the furnace barrows, thus avoiding all handling of ore from the mine to the furnace-top.