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fireplaces, and is said to roast 15 tons of blende to 1 per cent. sulphur in 24 hours.

65. The roaster shown in Fig. 20 is known as the Herreshoff, although it is merely

a

d

5"x3" Opening

FIG. 20

a modification of the McDougall. It has a central shaft d, with a number of shelves which answer for hearths placed at right angles to it. Attached to the shaft are two arms e above each hearth, which carry teeth SO arranged that one set moves the ore from the center to the circumference of the furnace, where it falls on to the next lower hearth. The set of arms on the latter hearth have the teeth arranged so that the ore is moved from the circumference of the

furnace to the center and discharged on to the next lower hearth, and so on to the ore discharge at the bottom of the furnace, as shown by the arrows.

Power is transmitted to the shaft d and arms e by the shaft hand gear-wheels ƒ and g. The ore is fed on to the top hearth from the hopper a and the gaseous products of combustion pass out of the furnace through pipe b. The hollow shaft d is made large (14 inches in diameter) so that a large quantity of air is drawn up through it, this amount being increased by the sheet-iron stack c extended above the

top of the furnace. Between the shelves there are cross channels passing directly through the shaft at right angles, as shown in the vertical section drawing. These cross channels are about 4 inches wide and 5 inches high and allow ample space around them for the passage of the ascending air. Into these channels or sockets the arms e are inserted, and by a groove and rib are arranged to lock when horizontal and unlock when raised at their outer ends.

By raising the outer end of the arm e about 3 inches, the rib can be pulled out. Practice has shown that these arms, weighing about 100 pounds, can be unlocked, removed from the furnace, and new ones put in and locked in place in about one minute. Each furnace requires about horsepower. The arms are the most costly part of the running expenses in regard to wear and tear; they are said to cost about $30 per year for each furnace.

COST OF ROASTING

66. The cost of roasting varies according to the degree of roasting demanded, together with the price of fuel and labor. The cost of roasting to eliminate sulphur to 2 per cent. should not, with present mechanical roasters, exceed 50 cents per ton; however, to reduce the last traces of sulphur, the cost will probably be four times this amount.

According to Doctor Phillips, sweet-roasting cost $2.18 at the Haile mine in North Carolina. The cost of roasting at the Globe smelters in Denver was $3.975 in 1887 and $2.75 in 1898 for hand-rabbled reverberatory furnaces. The cost of roasting in Brown-O'Hara mechanical roasters was $2.21 per ton of ore. The cost of roasting at the Guggenheim furnaces, Monterey, Mexico, is given as $2.43 per ton of ore in hand-rabbled furnaces.

While mechanical roasters give at first glance a cheaper product, the cost of roasting is higher when repairs and the quality of the product are taken into account.

THE CYANIDE PROCESS

(PART 1)

HISTORICAL

EVOLUTION OF THE CYANIDE PROCESS

1. Cassell Process.-Records of gold being dissolved in solutions of potassium cyanide and water extend back to 1806. The resulting solutions were first applied to gilding metallic surfaces. That gold could be dissolved from some ores by weak solutions of potassium cyanide was known in 1867, and applied in that year, but without much success from a commercial standpoint. Doctor Cassell perfected a process which he sold to Messrs. MacArthur and Forrest, who were granted patents in 1890 for what is known as the MacArthur-Forrest cyanide process. Messrs. MacArthur and Forrest demonstrated the practicability of the cyanide process, although since its introduction many useful improvements have been added to make this branch of hydrometallurgy a success. The only patentable feature in the MacArthur-Forrest process was the method of precipitation by zinc shavings, and even this is denied by some.

2. Electrolytic Process.-The application of mechanical agitation to assist in dissolving gold from ores was early recognized by J. H. Rae, of Syracuse, New York, who made a test of his process in 1867. He applied to his

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cyanide solution the electric current and precipitated the gold electrolytically in a bath of mercury.

Many years afterwards, Messrs. Pelatan and Clerici patented a similar process, which was more successful, however, for where Rae used an alternating current, they used a direct current. Pelatan and Clerici introduced sodium chloride into their cyanide solutions, but this was also done by J. W. Simpson, of Newark, New Jersey, in 1885. There is probably nothing in the Pelatan-Clerici process that has not been patented or used prior to its introduction in this country.

The pneumatic-cyanide process was originated about the same time in New Zealand and the United States. In this process, the agitation of the ore and aqueous potassium-cyanide mixture is accomplished by a blast of compressed air.

SCOPE OF THE PROCESS

3. Cyanide Solutions Defined. Whenever cyanide solutions are mentioned in this work, it is to be understood that they are those made by dissolving cyanide of potassium KCN in water. The cyanide process refers to dissolving gold and silver from ore by means of cyanide solutions and afterwards precipitating the gold by some one of the methods hereinafter described. While the chemical. reactions that take place between gold and cyanide solutions are known, the reactions between cyanogen and some other elements are as yet but slightly known, for which reason the cyanide process has not been fully developed and its scope is at the present time limited.

4. Treatment of Free-Milling Ores.—Free-milling ores are most successfully treated by the cyanide process when the gold is in a fine state or subdivision. The cyanide process has a field of its own in working tailings and concentrates resulting from wet crushing and plate amalgamation and can generally be applied to those ores that have their gold particles encased in some substance or have the gold so

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