The present invention relates to a method and to apparatus for dry-grinding particulate material in an upstanding mill with agitated grinding medium.

When grinding with an agitated grinding medium, there is used a mill that has an upstanding stationary cylindrical grinding chamber. This grinding chamber is provided with inlet and outlet means for material to be ground and for ground material respectively and containing a filling of grinding bodies that are held in motion by means of a rotor having generally radial rods and mounted in the centre of the mill. The grinding effect is thus achieved by agitating grinding bodies and the material being ground by means of the rapidly rotating rotor, e.g. at a speed of about 80 r.p.m., therewith creating pressure forces and shear forces between grinding bodies and material. The grinding bodies are typically from 10-20 times larger than the maximum particle size of the material to be ground. This grinding technique enables large quantities of grinding energy to be supplied per unit of mill volume and therewith per unit weight of material being ground, and is particularly suited for fine-grinding, e.g. extreme fine-grinding, of material that already exists in a relatively finely-divided state, e.g. material finely-ground in a ball mill .

Grinding with agitated grinding medium has earlier been described in, e.g., EP Patent 0 267 170, where grinding is effected on a slurry of water and mineral particles. However, it is sometimes desirable to grind with an agitating grinding medium in a dry environment, e.g. in an environment in which the moisture content of the particles does not exceed some few percent by weight .

SE Patent 466 485 describes apparatus for grinding material in an essentially dry state. According to this patent, there is maintained in the mill a predetermined residence time and therewith a predetermined grinding energy supply per unit weight of material, partly by maintaining a predetermined, generally constant outfeed of ground material from the mill bottom, and partly by adjusting the material feed to the upper part of the mill in relation to the outfeed of material from the mill such as to increase the volume of material in the mill. The infeed of material is interrupted in response to a signal delivered by a level monitor in the upper part of the mill when a predetermined highest level of material in the mill has been reached, and is restarted after the lapse of a given time period or in response to the receipt of a signal from a second, lower level monitor. When practicing this technique, the material being ground is transported vertically through the mill solely by the effect of gravimet- rical forces, i.e. the material is self-running. This mode of transportation restricts the transport capacity to those flow conditions that apply to the passage of the fine-grain material concerned through the bed of this material and the grinding bodies, and through the ground material outlet, which has the form of a bottom plate perforated with conical holes adapted to separate the grinding bodies but to allow the ground material to pass through.

Because the energy supply is transferred via grinding bodies to a dry material in a closed space with a large amount of energy per unit of mill volume, the temperature of the material being ground will rapidly increase. In the case of the known dry-grinding method, only a small part of the heat- converted grinding energy can be removed by cooling via a cooling jacket, which is effective when wet-grinding, and the temperature of the material increases by about 4C per kWh/tonne. When grinding heat-sensitive material, e.g. cement, with the risk of dewatering included gypsum at

temperatures above about 150C, it is necessary in the absence of a suitable cooling technique to restrict the degree of grinding to that corresponding to an energy supply of at most 30 Kwh/tonne for instance.

The object of the present invention is to provide a novel and advantageous dry-grinding method and novel and advantageous apparatus for carrying out the method while avoiding the aforesaid drawbacks to a large extent.

To this end, it is proposed in accordance with the invention that in a method of the aforedefined kind the dry material to be ground is pressed into the lower part of the mill and discharged in a ground state from the upper part of the mill, air or some other gas, e.g. an inert gas, being blown into the mill for fluidizing and optionally also for cooling purposes and to facilitate transportation of ground material up through the mill and out thereof. There is achieved in this way an accurately controllable supply of material to be ground with the possibility of increasing the throughput of material in comparison with conventional dry-grinding techniques, which utilize solely gravimetrical forces for transporting material through the mill. Further advantages are afforded by the invention by virtue of obtaining several degrees of freedom with regard to the residence time of the material in the mill, and by better conditions for cooling the material and for simplifying the grinding process control system.

Advantageous embodiments of the inventive method are defined in Claims 2-6.

As before mentioned, the invention also relates to improved dry-grinding apparatus, more particularly apparatus of the kind defined in the preamble of Claim 7.

The inventive apparatus is mainly characterized in that the infeed device is constructed to press dry material to be ground into the mill mechanically, said device particularly having the form of a dry material screw feeder which dis¬ charges into the mill, means further being provided for introducing into the mill and/or the screw feeder means for fluidizing and optionally also cooling the material being ground. This arrangement provides the advantages aforementioned with respect to the inventive method.

Advantageous further developments of the inventive arrange¬ ment are set forth in Claims 8 to 10.

The invention will now be described with reference to the accompanying drawing which illustrates very schematically an exemplifying embodiment of apparatus for carrying out the inventive me hod.

The reference numeral 1 identifies a hopper having an upper material inlet and a lower material outlet which is connected to a feed device 2. The supply of material to be ground to the inlet adjacent the bottom of the mill 3 is controlled in response to the level of material in the hopper 1 and by the drive of the feeder 2. The main purpose of the feeder 2 is to meter material from the hopper 1, and there is provided between the feeder 2 and the mill 3 a screw feeder 4 which functions to mechanically press material to be ground into the mill 3 through its material inlet. As indicated in the drawing, the hopper 1 will preferably have a height which equals twice the heighth of the grinding charge in the mill 3, so as to facilitate transportation through the mill. When grinding very fine particulate material which runs relatively freely, the pressure generated on the particles in the hopper 1 simplifies transportation of the material and imparts unitary and even motion to the particles up through the mill and uniform distribution over the mill cross-section. The

thus controlled flow of material contributes towards achiev¬ ing a dynamic classification function, by virtue of the fact that the smallest particles pass more easily through the bed of relatively small grinding bodies 3 ' , normally between 2 and 15 mm in diameter, wherewith the grinding effect is particularly directed to the larger of the particles being ground, which contributes towards making grinding so effective as to render unnecessary subsequent classification in many cases.

As illustrated in the drawing, the feeder 2 is conveniently a screw feeder, although it will be understood that it may have some other design provided that the feeder can be controlled to a desired capacity for metering material to the feeder 4 whose function is to feed material into the mill 3 at the rate determined by the feeder 2.

The material is ground in the mill 3 with the aid of an agitated grinding medium, wherewith the material being ground is agitated by means of a rotating stirrer or rotor 3" having radial high-speed stirring or agitating rods, e.g. moving at a speed of about 80 r.p.m. The grinding effect is thus obtained by pressure forces and shear forces acting between the grinding bodies 3' and the particulate material being ground, wherein the power output per unit of mill volume is high, normally in the order of at least ten times the power consumption per unit of mill volume in a conventional ball mill.

With the intention of further facilitating transportation of the material being ground up through the mill, the mill is provided with means for introducing air or some other gas into the mill chamber, for instance a gas which is inert in the present context. The illustrated apparatus includes to this end supply nozzles 5 which open into the mill adjacent its bottom, or in the feeder 4 adjacent the feeder outlet and

through which gas can be delivered for fluidizing and optionally cooling purposes. A chemical substance can also be introduced through the nozzles 5, or in some other way, as a grinding auxiliary for improving dispersion and avoiding agglomeration of the particles influenced by the grinding process.

The gas used for fluidizing purposes will, of course, cool the product being ground to some extent. However, if the cooling effect thus achieved is insufficient, water or some other liquid which will vaporize upon contact with the material is sprayed into the mill 3 for cooling and optional¬ ly fluidizing purposes.

The ground material leaves the mill 3 through its upper outlet, which is suitably provided with a grating, sieve, or like device for preventing departure of grinding bodies from the mill. In the illustrated embodiment, the ground material leaves the mill by self-running into a product hopper 6.

The grinding energy supplied to the mill may be in the region of 1-100, suitably 5-50, Kwh/tonne supplied particulate material, and the material to be ground may be delivered in a particle size distribution such that 80 percent by weight of the particles will pass through a sieve having a largest sieve opening of 2 mm. The material may conveniently be ground to a particle size distribution such that 80 percent by weight of the ground particles will pass through a sieve having a sieve opening of at most 1 mm, suitably at most 0.1 mm, for instance at most 0.01 mm and particularly at most 0.001 mm.

It will be understood that the invention is not restricted to the aforedescribed and illustrated embodiment and that it can be practiced in any way that lies within the scope of the inventive concept as defined in the following Claims.