This invention relates to a method for dry concentration of metal in a metal containing ore, a concentrate sepa¬ rated therefrom or slag, in which method the ore, the ore concentrate or the slag batch is fed to a pressurized feeding device of a counterjet grinder and fluidized by means of a working gas flow, the fluidized gas-solids mixture is accelerated by means of pressure via two main¬ ly oppositely directed acceleration nozzles to a grinding chamber, whereby the fragile mineral or slag material portion of the solids particles colliding with each other decomposes and loosens from poorly grinding metal-con- taining particles, and the produced pulverized gas solids material mixture is immediately led by means of an over¬ pressure prevailing in the grinding chamber (12) of the counterjet grinder to a centrifugal classifier (13), in which the metallically poor fine fraction is removed from the classifier via an axial opening (13a) and the metal¬ lically rich coarse fraction is returned to the feeding device (3) of the counterjet grinder for recirculation.

In previous dry concentration prosesses, a ball mill or a corresponding mechanical grinder has usually been used.

Hereby it is of course possible to grind the ore or slag, but it is not possible to guarantee that the components may be separated from each other in the separation step following the ball mill, when the fragile or light par- tides easily stick again to metal-containing particles, and thus the efficiency of the prosess significantly dec¬ reases.

The object of the present invention is to remove the abo- ve-mentioned disadvantages by using a counterjet grinder such that the recirculation is repeated, until a desired portion of the initial weight of the ore, ore-concentrate

or the slag batch has left the centrifugal classif er as a fine fraction, whereby the metallically rich coarse fraction left in the equipment is led via a product out¬ let opening to a next refining step.

By means of this method, it is possible to effectively separate the particles of the fragile or light consti¬ tuent from poorly ground metal-containing particles. In the grinding occurring in the counterjet grinder, which grinding is practically based on collision, the different components of the initial material, i.e. the ore or the slag, are ground in a completely different way due to their grinding properties, and furthermore the different components of the initial material shutter in the col- lision effectively apart from each other and remain sepa¬ rate from each other in the gas-solids mixture occurred as it enters immediately into the centrifugal classifier, in which a very accurate classification occurs.

By means of the method, it is possible to very effective¬ ly concentrate metal e.g. from such a slag, which has previously been regarded as a very problematic waste. Such a slag is e.g. a slag produced in the melting of aluminium, whose aluminium concentration is low to such an extent that it cannot be utilized by conventional ways, but it is yet so high that it causes serious envi- romental problems e.g. due to the occurrence of hydrogen.

The remaining characteristics of the invention appear from the enclosed claims 1 to 10.

The invention is next described in more detail with refe¬ rence to the accompanying drawing, in which

Fig. 1 shows the equipment to be used in the method, seen from one side, and

Fig. 2 shows the equipment of Fig. 1 seen from the right.

The inventive method is performed step-wise. The ore, the ore concentrate or the slag batch is fed via a feeding funnel 1 and a screw conveyor 2 to another feeding funnel 3 of a counterjet grinder and therefrom further via a double-valve feeder 4 to a pressurized distributing cham¬ ber 5. The quantity of the batch fed to the equipment is observed by means of a scale weighing the weight of the equipment. The ore or slag batch drops from the feeding funnel 3 to an intermediate container 4b of the double- valve feeder 4 when a top valve 4a is open and a bottom valve 4c is closed. When the intermediate container 4b is filled to a predetermined level, the top valve 4a is clo- sed and the intermediate chamber 4b is pressurized by means of gas, fed via a gas feeding line 6. As the pres¬ sure has risen to a desired level, a valve 6a in the gas feeding line is closed and the bottom valve 4c is opened, whereby the ore or slag batch in the intermediate con- tainer 4b drops to the distributing chamber 5 located below the double-valve feeder. The bottom valve 4c is then closed and a valve 7a located in a pressure reducti¬ on pipe 7 extending from the upper end of the interme¬ diate container 4b is opened and maintained open, until the pressure of the intermediate chamber has decreased to normal air pressure. The valve 7a is then closed and the top valve 4a is opened for a new batch. These feeding functions are repeated, until the total quantity of a dry concentration step is fed to the equipment.

The material batch in the pressurized distributing cham¬ ber 5 is fed at an even rate by means of the screw con¬ veyor 5a to a fluidization chamber 8, in which the ore or slag material is fluidized by means of a high-pressure working gas flow fed via a working gas pipe 9. The flui¬ dized gas-solids mixture is led via a bipartition device 10, in which it is divided into two equivalent partial

flows. Each partial flow is accelerated in response to the overpressure prevailing in the fluidization chamber 8 via its own acceleration pipe 11 to a grinding chamber 12 of the counterjet grinder, where gas-solids materials jets surging from mainly oppositely directed acceleration nozzles collide with each other. The fragile mineral or slag fraction then decomposes ja loosens from poorly grinding metal-containing particles. The pulverized gas- solids mixture produced in the grinding is immediately led by means of a afterpressure prevailing in the grin¬ ding chamber 12 of the counterjet grinder to a centrifu¬ gal classifier 13, which is either a static or dynamic centrifugal grinder, depending of the required classifi¬ cation accuracy. The metallically poor fine fraction is removed from an axial outlet opening 13a of the classi¬ fier and led either to an intermediate or waste storage. The metallically rich coarse fraction is removed to a pocket 13b located on the shell surface of the classi¬ fier, from which it is fed batchwise, by means of a com- partment feeder 13c or a double valve device to a two- directional screw conveyor 14, which, when rotating in one direction, is arranged to return the coarse fraction to the second feeding funnel 3 of the jet grinder for recirculation, which recirculation is repeated, until a desired portion of the initial weight of the ore, ore concentrate or slag batch has been removed as a fine fraction from the centrifugal classifier 13. The screw conveyor 14 is then caused to rotate in the other direc¬ tion, whereby the metallically rich coarse fraction that has remained in the equipment is led via a product outlet opening 15 to a next refining step.

The inventive method is especially suitable for dry con¬ centration of aluminium in the slag produced in the mel- ting of aluminium. The aluminium content of such a slag is generally in the range 15-25% by weight. If the dry concentration method is performed such that the slag

batch is circulated in the equipment, until ca. 50% by weight of the initial weight of the slag batch has been removed from the axial outlet opening 13a of the centri¬ fugal classifier 13, an end product is obtained, whose aluminium content is ca. 35-50% by weight, which is suf¬ ficiently high such that the aluminium of the end product may be used in a conventional melting process. The alu¬ minium content of the fine fraction that has left the dry concentration process is then ca. 5-15% by weight, which considerably reduces the environmental problems caused by the slag.

In a test run performed, in which the slag produced in the melting of aluminium, the aluminium content of which slag was ca. 20% by weight, was treated in the equipment shown in the drawing, in which as a working gas was used oil-free pressure air of ca. 3 bar, the aluminium content of the end product or the coarse fraction became ca. 40- 45% by weight and the aluminium content of the fine frac- tion became ca. 7-11% by weight, when the treatment con¬ tinued, until ca. 50% by weight of the initial weight of the slag batch had been removed from the equipment as a fine fraction. The quantity of the fine fraction that has left the equipment may be observed either by means of a scale measuring the total weight of the equipment or by means of a scale measuring the weight of the fine fracti¬ on that has left the classifier.

The invention may also be utilized when it is necessary to remove from a blast furnace slag the iron contained therein. The iron content of the blast furnace slag is less than 1% by weight, and in the inventive treatment, it accumulates in the coarse fraction.

The inventive method may be used for concentrating a me¬ tal from such an ore or an ore concentrate, which is com¬ prised of a fragile and/or light component as well of a

tough and/or heavy metal-containing component. Such com¬ ponents include e.g. copper, tin, nickel and lead ores and ore concentrates as well as noble-metal ores.

According to the invention, the noble metal may be dry concentrated also from a noble-metal ore or some other waste.

As a working gas is generally used oil-free pressurized air, but in case the metal to be dry concentrated is e.g. easily oxidable, also nitrogen, argon or some other inert gas may be used.