The development in late years of so many mining properties containing large quantities of comparatively low grade and refractory ores, has created a demand for a furnace of large capacity, in which such ores can be roasted preparatory to chemical or further igneous treatment.

Heap and stall roasting and the hand reverberatory roasting furnaces have, in turn, made way for furnaces in which the ore is stirred or rabbled by mechanical means. The designs of the first mechanical roasting furnaces followed closely along the lines of the hand reverberatories, but the later designs have been, in nearly every instance, furnaces having a round hearth and revolving rabbles.

In the Holthoff Revolving-Hearth Furnaces, the design of Mr. Henry C. Holthoff, of Milwaukee, Wis., this principle is reversed. The hearth, which is annular in form, is made to revolve about a centrally located gas producer. The wall of this producer forms the inner wall of the roasting chamber and support for its arched roof, while the outer wall is supported on cast-iron columns or structural steel.

The gas generated in the producer enters the roasting chamber through a number of pipe channels radially disposed in the walls of the producer. Air pipes are also located in the walls of the producer, the lower ends opening to the atmosphere through a regulating valve, the upper ends extending into the roasting chamber in close proximity to the gas exits.

The air, heated in its passage through the producer walls, mixes with the gas upon entrance to the roasting chamber, and combustion is spontaneous. The flame traveling radially across the hearth finally discharges through a series of tile pipes into a large circular smoke flue supported on steel frame work just above and around the outer edge of the furnace.

It will be noted that while the hearth resolves, the main furnace structure is stationary.

The rabble arms, of which there are four sets, extend down through the arched roof of the roasting chamber, and all cooling water connections and adjustment of the angularity of the rabble blades are accomplished outside of the furnace. The rabble arms consist each of two pieces of cast-iron pipe of different diameters, one within the other, the outer pipe being tapped into the hollow rabble blade.

This arrangement is a simple and effective means for water-cooling the rabbles.

The ore, fed through a feeder having an adjustable eccentric and a spiral conveyor extending through the outer wall of the furnace, drops on to the outer edge of the hearth, and, as the hearth revolves, is rabbled across same in a spiral path. Reaching the inner curb of the revolving hearth, it drops through discharge spouts on to the inner edge of the stationary cooling floor located directly under the revolving hearth.

It is then moved across the cooling hearth and finally discharges at the outer edge of the cooling floor into a revolving hopper or to a belt conveyor or elevator.

The hearth revolves upon equalizing roller bearings of conical form, and is revolved by means of a roller rack and a pinion. From the inside of the mantel upon which the outer wall rests, a steel metal apron is hung, extending down inside the outer curb of the hearth, and the top of the ore bed being higher than the bottom of this apron, the opening between the hearth and the wall of the roasting chamber is effectually sealed against the entrance of cold air.

By means of valves or dampers located in the air inlet and smoke discharge pipes, the temperature in the roasting chamber is under complete control of the operator at all times, and, as the angularity of the rabble blades can be changed quickly and easily, the time the ore remains in the furnace is also under complete control.

The angularity of all the rabbles of one set is changed by the movement of a lever conveniently located for the operator, and which slides in a quadrant on which are figures indicating the degree of angularity of the rabbles.

A View at the Portland Mill in Colorado Springs, along the Short Line R.R.

The furnace above described was designed especially for treatment of low sulphur ores, such as are found in the Cripple Creek district and was built about a year ago for the Portland Gold Mining Company's mill at Colorado City.

In a two months' test run of this furnace the following results were obtained, the figures given showing the average per day of twenty-four hours during the entire test.

These furnaces are also built in three other types, one similar to that described above, except that a plain fire-box is substituted for the gas producer. Another type designed for roasting sulphide ores preparatory to smelting, is built along the same general lines as that described above, except that the cooling floor is omitted and the roasted ore discharges automatically into a stationary receiving hopper from which the calcines are drawn off into cars or buggies and taken to the smelting furnace.

The fourth type of furnace is designed for roasting high sulphur pyritic ores. In this type of Holthoff furnace the cooling floor is omitted and a plain fire-box is substituted for the gas producer. This fire-box, however, is used only to ignite the sulphur in the charge. After ignition, the sulphur burns spontaneously, and the fire-box is used merely as an air flue.

The advantage claimed by the builders of these furnaces, the Power & Mining Machinery Co., at Cudahy, Wis., are:

First, great capacity.

Second, ability to control the temperature in the roasting chamber, as well as the rate at which the ore is moved across the hearth, thus giving the operator complete control of the roast during the entire operation.

Third, by using a gas producer incorporated in the furnace structure, no heat is wasted in piping, and the expense of installation and repairs for piping, found in furnaces in which the fire-box is separate from the furnace proper, is eliminated. Also, by generating gas in the producer, the combustion of which takes place in a roasting chamber, the greatest possible utility is made of the heating value of the fuel, and it will be noted from the figures given above that the fuel consumption per ton of ore charged is exceedingly low.

Fourth, the small amount of manual labor connected with the use of these furnaces is an important item. The only labor required is for lubricating and such general attention as is due any piece of machinery however simple, and one man can attend several of these furnaces.

Fifth, throughout the general construction of these furnaces, provision is made for expansion and contraction of the various parts, so that the users are saved the annoyance and expense of repairs from this cause, which are so frequent in other furnaces.

The Power & Mining Machinery Co., Cudahy, Wis., are issuing their Bulletin C-I, describing this furnace, which, we are informed, will be sent gratis on request. This bulletin contains complete detailed description of the design, construction and operation of these furnaces.