In mineral processing, the reduction of minerals by crushing and grinding may be regarded as having one or other of two main objectives: the attainment of a size appropriate for the direct industrial application of the mineral, e.g. barytes, sand, aggregate; or the release of metallic or ore inclusions from an unwanted matrix with a view to maximum separation. In both cases, quarrying, as a rule by explosives, followed by coarse crushing of the quarried lumps and then by intermediate or secondary crushing of the product, is the normal course of reduction and, with a few exceptions, is irrespective of the ultimate objective. After this, the methods of fine crushing and grinding and the accompanying ancillary processes are chosen in accordance with the objective in view and with certain physical properties of the mineral. In the later stages of reduction, power consumption increases rapidly with fineness of product, and it follows therefore that grinding beyond the desired size or optimum range is to be avoided as far as possible. In practice it is more often the case that power is unnecessarily expended due to inadequacy of the ancillary equipment, its inherent inefficiency or unsuitability.
The basic principle upon which a crusher works is the application of the necessary force in a suitable way to overcome bonding forces by which a lump of mineral is held together. In machines where the opposing crushing members are held mechanically apart this force is applied either as direct pressure or squeezing until fracture occurs; or by impact, where the rock may either be freely suspended, e.g. as in hammer mills, or stationary as in stamp mills.
In such machines the feed size of mineral may range from 1/4″ cube—as in the case of smooth-faced rolls—up to large run-of-mine or quarried lump of 30-in. cube.
The determining factor in the choice of the primary crusher is often the tonnage to be handled and the size of the largest lumps, for where both are large the gyratory type has many advantages, foremost of which are lower power consumption, first cost and choke feeding—in fact the gyratory may be ‘buried’, and truck loading is common practice. The jaw crusher on the other hand needs a feed controller, which in the case of the very large units involves the provision of a massive apron feeder the cost of which may be high. The jaw crusher, however, is capable of receiving a larger lump for any rated capacity and in certain cases is applied as a primary sledging breaker, although, unless there is an abnormal quantity of massive lumps present in the mine or quarry, it would seem preferable to break such boulders by the use of explosives.
The following comparison between the respective feed and discharge areas of a 48 in. gyratory and a 48 in. x 60 in. jaw crusher serves to illustrate some of the foregoing factors:
Jaw crusher…..one opening of 20 sq. ft
Gyratory…..two openings each of 43-4 sq. ft
Jaw crusher…..3*1 sq. ft
Gyratory…..10-9 sq. ft
An important advantage of the jaw crusher over the gyratory crusher is that of being able to deal with materials having a high clay content, although this advantage is less where discharge openings are large.
In mineral processing, it is assumed, for the present purpose that intermediate crushing is not necessary and that the run-of-mine or quarried mineral has, in one pass, been reduced in size so that all is below say 6-in. ring size. From this stage forward the utilization of the product assumes primary importance. For example, if the economic mineral is wolfram or scheelite, necessitating separation from the matrix by hydro-gravity separation, the further size reduction must be effected with the aim of minimizing the production of ‘fines’, whereas if flotation separation is to be used no such consideration applies. Similarly, in the production of road-surfacing aggregate the shape of the secondary crushed product is important and here particles approaching cubic shape are preferable.
Prior to secondary crushing it is important and desirable to remove the fines already below the set of the crusher. Run-of-mine and quarry product when accepted into the plant comprises rock of varying sizes some of which is below the primary crusher open setting, but its removal from the crusher feed at this stage is not so important as in secondary crushing where the feed is of a shorter range and hence packing by fines more serious. Moreover the mechanical and siting problems involved in removing, say, minus 6 in. ring size from quarried rock of 30 in. cube would outweigh any increased efficiency of the crushing operation.
It is desirable to remove undersize material from the crushing unit for a number of reasons: power has been expended in effecting its size reduction; its presence in the crushing unit and the packing of the voids between the uncrushed oversize not only reduces throughput but results in increased wear and higher power costs. If, in addition, the fines are of an argillaceous character the presence of such in the crusher will prove to be an intolerable nuisance.
The secondary crushers to be considered are the following: cone-type gyratory, rolls, hammer mills, gravity stamps. This range of four secondary crushing machines includes two in which size reduction is effected by pressure and two by impact. Of the four to be discussed the hammer mill has its own particular field of use from which other types of crushers are excluded; rolls are extensively used in the crushing of minerals preparatory to gravity separation and whilst much of their former use has been taken over by the cone gyratory, the spring roll makes an efficient crusher to sizes from 3/8″ to 1/16″, taking over where the cone crusher leaves off. Despite occasional claims to the contrary it is unwise to effect size reduction much below 1″ by a cone crusher.
The cone-type gyratory, of which the “Symons’ is perhaps best known, is pre-eminent as a secondary crusher and is capable of effecting a size reduction ratio of the order of 6-8:1. This type of machine is best employed in close-circuit with a screen but is unsuitable for minerals of an argillaceous character. Protection against the inclusion of steel in the feed is imperative and all units of this type are more satisfactory handling dry feed. In cases where the feed is damp to wet it is advisable to limit the closed setting to 1/2″ and unless extra water can be added to ensure the non-build-up of fine material in the bowl regular inspection is advisable.
The use of hammer mills is in the field of softer minerals, such as gypsum, barytes and limestone, and particularly where the presence of clay would most definitely exclude the use of crushing machines in which fracture of the mineral is effected by pressure. In this latter field in particular, the hammer mill is also used as a primary crusher. The hammer mill is an impact breaker and is capable of effecting large reduction ratios. Where the mineral is soft and would easily clog, this type of crusher is extensively and successfully employed, modifications being made to the cage to facilitate screening and retention of material in the grinding zone.
The gravity stamp, which crushes by impact and is a wet crusher, is being superseded by the rod and ball mill in fields where formerly it was extensively used. In particular, the stamp was used extensively in crushing gold-bearing quartz and cassiterite lode material as in the Cornish mines. The stamp gives a very big reduction ratio feed of from 1.5″ to 2 in. is reduced to 30 mesh but its inefficiency from the viewpoint of power expended must be largely attributed to the ‘hit and miss’ method of removing the pulp from the stamp box.
The use of smooth-faced rolls as a secondary crusher preparatory to ball milling in a lead-zinc differential flotation is exemplified by practice at the Zinc Corporation Ltd mill, Broken Hill, New South Wales. Here run-of-mine ore is reduced to minus 0.5″ in two stages of primary crushing and subsequently by slow-speed rolls to 0.25″, the latter being in closed circuit with screens. The use of rolls in this case was influenced by the desire to feed to the ball mills a product minus 0.25″, to be able to operate the circuit wet, and to use bucket elevators to raise the roll discharge to the close-circuiting screen. Further advantages in this particular installation were the elimination of the dust problem and the ability to change the size of the ball mill feed to a finer product if desired.