The Grondal Process: Magnetic Separation

Posted By Tom Feltham on 07 November 2014

Posted in The Vintage Machinery Almanac

This article was originally published in the April 1907 edition of “Electrochemical and Metallurgical Industry” magazine. Publication of this article aims to provide historical insight on The Grondal Process and its role in iron production. For more information about The Grondal Process, click here.

Following the crushing and wet treatment of iron ore, non-magnetic particles are removed from the pulp using a Grondal magnetic separator.

The diagram of Fig. 1 shows the principle of the separator proper. S representing the slimes, M the magnet, C the concentrates, T the tailings. The pulp feed from slimes passes to this part of the apparatus. This consists of a series of magnets with flat pole pieces arranged with their N and S poles alternately. The magnets are enclosed in a plain brass drum. The drum is made to rotate at a speed of from 80 to 100 revolutions per minute, and about I inch above the surface of the pulp, which traverses a pyramidal box divided  into two compartments by a partition reaching nearly to the top of the tax. The pulp enters at the top of the box, and a stream of water altering at the bottom and rising up on the same side of the partition carried all the pulp well over the edge of the latter and thus immediately under the drum.


The magnets within the drum powerfully attract and bring the magnetic particles out of the stream of pulp and against the revolving brass drum. The remainder of the pulp drops down on the other side lie partition, and is carried off by the water through the waste outlet.   The magnetic particles are lifted by the drum to j the very edge of the magnetic field, where they are thrown off centrifugally.    The capacity   of   the   double-drum   separator (which is the form commonly used)   is from 70 to 100 tons of ore per 24 hours; requires about 6 amps, at 120 volts, or, say 1 hp., while less than 1/4 hp. will drive the drums.

A plant consisting of four ball mills, four double-drum separators, and having a capacity of fully 200 tons of concentrates per 24 hours, would require approximately 180 gallons of water per minute, and a power plant generating 150 electric horse-power would be ample for all the motive power required.

The concentration process reduces the percentage of phosphorus more or less, depending upon the mineral form in which it occurs in the ore. In most magnetites it is found in the form of apatite, in which case the phosphorus can be almost entirely eliminated. For example, at Gellivare, Sweden, the phosphorus is reduced from 1.29 in the crude ore to 0.005 in the concentrates, and 0.006 in the briquets. The percentage of iron is raised to a high figure, and at the same time the amount of slag-forming minerals is lowered. As a result the ore is more easily reduced in the furnace, and requires less fuel per ton of iron.

The quality of the pig iron will be quite materially improved when employing purer ore and less fuel than ordinarily.

Sometime the ore mined is a mixture of magnetite and hematite. The dry magnetic concentrating methods hitherto employed give rather unsatisfactory results as regards the iron content of the tailing. With The Grondal Process, where the ore is already slimed up in water, a re-treatment of the tailings on jigs or slime tables can be carried out with advantage, thus increasing the output of the plant without crushing or grinding any additional tonnage of ore.

The Grondal Process can be regarded as practically the only existing method for the economic treatment of low-grade ores in which the magnetite is so finely and intimately mixed with the gangue that the ore has to be crushed down to less than twenty mesh before it can be separated. Take the case of the banded jasper ore in the Temagami district, which would require very fine grinding to effect an economic recovery of iron: the Grondal process could do this, where no other could. But it is also obvious that such fine ores must be briquetted.