CA2276473A1 - Cryogenic crushing of ore - Google Patents

Description

This invention relates to a method of treating ores and more particularly to a cryogenic method of crushing ores.
Crushing and pulverizing are processes in ore dressing in which large chunks of ore are reduced to a size at which the valueless gangue can be separated from valuable minerals by such means as froth flotation, magnetic separation and the like.
In general, particles of ore are extremely hard and un-yielding and must be crushed in stages. Large chunks of ore from a mine are crushed in at least three stages each char-acterized by the size of feed material and the size of the output. Output from each stage is usually screened to ensure uniformity of feed material to the next stage. Material which passes through the screen passes to the next stage while material which does not is returned to the preceding stage for re-crushing.
Crushing at the primary stage is usually carried out by so-called Blake-type jaw crushers or Gates-type gyratory crushers. Such crushers can also be used at the secondary stage as can hammer crushers. At the tertiary stage, pul-verizers or milling machines are generally used to crush the relatively fine particles of ore.
Jaw crushers and gyratory crushers are heavy, expensive machines which consume large amounts of energy. The cost of such machines and the cost of operating them are a significant part of the overall cost of a mining operation.
It has been found that the hardness of ores and their resistance to crushing can be significantly diminished if their temperature is reduced to about -90 degrees Celsius or lower. If ores are crushed at this temperature, significantly less energy is required to do so than if they are crushed at higher temperatures. In addition, crushing may be accomplished in fewer stages at significantly lower cost.
The hardness of ore is substantially greater at temp-eratures above about -90 degrees C. than at lower temper-atures. As a result, the energy required to crush ore at a temperature above -90 degrees is significantly higher. For example at ambient temperature it was found that about seven time more energy was required to crush samples of ore and at -35 degrees C., it was found that about twelve times more energy was required. At about -188 degrees approximately the same amount of energy was required to crush the ore than at -90 degrees.
The preferred range of temperature for crushing of ore is about -90 to about -95 degrees C. At higher temperatures the hardness of the ore is greater as indicated above and at lower temperatures its hardness does not appear to change signific-antly from its hardness at temperatures within the preferred

2 range. Since it becomes increasingly costly and difficult to cool ore below -95 degrees C., there appears to be no ad-vantage to doing so.
The samples of ore referred to above and elsewhere below were fine-grained quartz diorites and coarse-grained ferro-gabbros. The diorites contained phrrhotite and chalcopyrite and the ferrogabbros contained magnetite, ilmenite, phrrhotite and pentlandite.
The effect which temperature had on the energy required to crush the ore samples is illustrated in Figure 1. In that Figure, it will be noted that the energy needed to crush the ore samples fell off sharply as the temperature of the samples at their core fell below -35 degrees C. and levelled off at -95 degrees. As the temperature decreased further the energy required to crush the ore samples remained about the same.
Not only were the energy requirements for crushing less but the time required to crush the ore was less. It was found for example that crushing of the ore samples could be carried out about five times more rapidly at temperatures within the range of -90 to -95 degrees C. than at ambient temperature and nine times more rapidly than at -35 degrees C.
Equipment which may be used to crush ore at temperatures within this range is illustrated schematically in the drawings in which:

3 Figure 2 is a side view of the equipment, shown schematically, Figure 3 is a plan view of the equipment; and Figure 4 is a perspective view of the equipment.
Like reference characters refer to like parts throughout the description of the drawings.
With reference to Figures 2 to 4, chunks of ore from a mine are deposited onto a conveyor belt 10 which carries them to and drops them onto an annular tray 12. The tray is mounted on bearings 14, 16 and is rotated by means of a motor 18.
A semi-cylindrical covering 20 is mounted above the tray Only a portion of the covering is shown in Figure 3 but the covering in fact extends around substantially the entire circumference of the tray and defines the upper wall of a tunnel through which the particles of ore on the tray travel.
Nozzles 22 are spaced along the covering to introduce liquified nitrogen into the tunnel. When the nitrogen enters the tunnel it vaporizes and causes the temperature of the outer surface of the chunks of ore travelling on the tray to drop. Depending on ambient conditions, the temperature drop is generally about 20 to 30 degrees C.
The direction of rotation of the tray is counterclockwise in Figure 3. The rate of rotation can be increased or de-creased to ensure that the temperature of the ore is reduced

4 to the required value. The ore is thus pre-cooled in the tunnel.
A number of tubes 24 extend radially inward and upward from the tray and terminate at a central opening 26. Flaps 28 are mounted within the tunnel and each flap is adjacent to the opening of a tube. The flaps are downstream, relative to the direction of travel of the tray, of the openings and the flaps serve to direct chunks of ore into the tube. Means ( not illus-trated) is provided for adjusting the angle of each flap so that the quantity of ore directed by the flap into each tube can be adjusted.
Mounted for rotation within each tube is an auger 30 for conveying the ore upwardly from the tray to the opening 26. A
nozzle 32 is mounted to the tube adjacent to the tray for introducing coolant into the tube and a drain 34 adjacent to the discharge end of the tube draws off coolant and re-circulates it to nozzle 32 or to the refrigerating system described below. Each tube is filled with the coolant which serves to carry out the final cooling of the particles of ore.
The coolant is preferably liquid alcohol and is cooled to the preferred temperature of -100 degrees C. The alcohol can be cooled to this temperature by means of a conventional cascade refrigerating system ( not illustrated ) in which two or more refrigerating systems are interconnected and operate simultaneously. A cascade system can be provided for each auger or a single system can supply coolant to all the augers .
Alcohol is a desirable coolant for a number of reasons:
first, it readily penetrates into the interstices of the ore and causes more rapid cooling than a coolant such as liquid nitrogen which tends to coat or blanket the surface of the ore but does not penetrate the ore. Secondly alcohol lubricates the ore and makes it slippery. Being slippery, it travels more smoothly on the conveyor belts and augers with less wear on the apparatus. In addition lubricated ore can be crushed with less wear on the jaws of the crusher than dry ore.
A third reason why alcohol is desirable is that alcohol wets the surface of the ore and causes the ore fines to adhere to the larger particles of ore and not to become airborne as dust. Finally, alcohol tends to act both as an insulator and a refrigerant. As a result, the ore remains longer at the de-sired temperature range for longer when coated with alcohol than when uncoated. This is because the alcohol, when it coats the ore, insulates it and, when it evaporates, it cools the ore.
The alcohol must have a freezing point of below about -100 degrees C. to avoid freezing in the auger. N-butyl alcohol and a denatured corn alcohol sold under the trade mark "Van-Col 729", both products of Van Waters & Rogers Ltd., a subsidiary of Univar of Weston, Ontario, Canada, are suitable for this purpose.
Most alcohols are volatile and highly combustible, some explosively so. Their vapours are likewise highly combustible.
If the liquid and gaseous alcohol are not insulated from oxy-gen, it may be ignited by a spark and cause serious damage to the equipment containing the ore. It may also cause injury or even death to an operator in the vicinity of the equipment.
Nitrogen in the liquid and as a gaseous blanket sur-rounding the ore and the equipment is effective to prevent the alcohol and its vapour from igniting.
During final cooling, the chunks of ore should be cooled until the temperature at their cores is preferably within the range of -90 to -95 degrees C. The temperature at their sur-faces may be lower than this without significant impairment in the effectiveness of the method of this invention. Excess nitrogen in the auger can be collected and injected by nozzle 22 into the tunnel to pre-cool the incoming chunks of ore.
The cooled ore chunks fall through opening 26 and into a conventional jaw crusher 40. The crusher serves to break up the chunks to particles of about 6.5 mm in size or smaller.
The crushed particles discharge onto a vibrating screen 42 having openings of about 9.5 mm to 6.5 mm in size. The angle of the screen is adjusted such that larger particles which do not pass through the screen travel to a secondary crusher 44.
This crusher can be a jaw crusher or a roller crusher.
Particles which pass through screen 42 fall onto a conveyor 48 and to a lower conveyor 50 where they travel to a hopper 52 and from there to a final refining stage ( not illus-trated) for separation of the gangue from the minerals.
Material which discharges from the secondary crusher 44 falls to conveyor 50. Alternatively the material discharged from the crusher may fall onto a screen (not illustrated) where particles of about 3 mm are separated from coarser particles. The former particles fall to conveyor 50 while the latter coarse particles are returned to crusher 44 for re-crushing.
In general, ore can be reduced to the size at which the minerals can be separated from the gangue in the two stages described above. Under certain circumstances however a third stage of crushing or milling may be required.
Crushing has been found to be the most effective method for reducing the size of the cooled ore. Impacting the part-icles with a blunt force or tumbling the particles have been found to be not as effective. Crushing has also been found to have little effect on the surface morphology of the cooled particles. It also has little effect on the size of the part-icles following crushing. In other words, ore crushed by the method of this invention is little, if any, different from ore crushed by conventional means. As a result the crushed ore of this invention may be treated by conventional methods to sep-crate the minerals which it contains from gangue.
It will be understood, of course, that modifications can be made in the steps in the preferred method illustrated and described herein without departing from the scope and purview of the invention as described in this application.

Claims