Crushing/Grinding
Crushing/grinding is typically the machining technique used in the first instance with freshly mined/quarried rocks, ores and minerals. Tolerant of large sized ‘raw’ input materials, crushing can be thought of as a cruder technique than milling for example, as it produces comparatively larger sized output products; such as aggregates for construction and infrastructure building, small to medium sized rocks for landscaping, and materials that can be further processed using other techniques.
African Pegmatite offers the following equipment for crushing treatments:
Cone Crusher
A cone crusher operates by dropping hardcore materials down a funnel onto a steel cone-shaped cylinder spinning in an off-centre pattern. The cone is set to a predefined crushing measurement, usually of a 4:1 to 6:1 reduction ratio. The materials work their way down past the cylinder which crushes them to the desired size before releasing them into the filters below. Rocks can pass through up to four different sized crushers in order to achieve the desired size. When a rock passes through a crusher its size is further reduced each time, this is referred to as the ‘ratio of reduction’.
Unprepared rocks and freshly mined ores, alongside other aggregates, are the most common applications that cone crushers are used for, producing rocks for landscaping, erosion control, road building, and ore mining.
Roller Crusher
There are two main types of roller crushers. The first is where the rolls of each two parallel rollers are rotating in the opposite direction and where one roll is spring loaded. In this instance, the mineral particles are trapped and crushed as they pass between each of the two rolls.
The second type is known as ‘high pressure grinding roll’ (HPGR), in which each of the minerals or rocks are reduced in size by using compressive and interparticle pressure which is mathematically configured beforehand.
Like the cone crusher, some of the most common applications that use roll crushers are ores, aggregates and unprepared rocks.
Jaw Crusher
A jaw crusher is the main type of primary crusher utilised in a mine or ore crushing facility. The size of the feeder at the top of the crusher usually denotes the size of the jaw crushers and the size of unprepared aggregate or ore the machinery can handle. As the name suggests, the jaw crushers are aligned into a V shape and act with a jaw movement using hydraulic or pneumatic compression to crush materials to the desired size.
Essentially, the outward motion of the jaws allows gravity to draw the rocks (or material) down further into the V-shaped jaws where they become trapped. Then the inward motion of the jaws crushes the rocks. This process continues until each rock has reached the base of the machine and is finally compressed to the right size. The rock will remain in the machine until it has been sufficiently crushed to allow it to pass through the gap at the base which only allows a specific gauge on size.
As a general rule, primary jaw crushers are usually square in design and secondary jaw crushers are usually a rectangle, but there are exceptions to this.
Milling
In contrast to crushing, milling is a technique that produces a much finer physical product. Oftentimes, minerals are required for use in a ground or powdered form, making them much easier to add to the process that needs them. Be it a refractory material for furnace lining production, ground glass for filtration applications, or milled anthracite for solvent extraction, milling is the key to getting the right product.
With in-house fully customised equipment to effectively mill all across the mineral spectrum, easily down to grind sizes as low as 45 μm, African Pegmatite offers the broadest tolerance of materials and reliable results at any scale. The following milling technologies are offered:
Ball Mills
The ball mill could be thought of as a type of grinder. The principle of which has been used for hundreds of years, but has been refined in the modern era to be more tolerant of a wider profile of minerals, operating for longer time periods, offering better performance. The machine is partially filled with balls and serves to offer repeated and constant impact using the movement of the balls to crush and grind the materials.
The types of material that the balls are made of is dependent on the type of mineral to be treated. Steel is the most widely used material, but stainless steel, rubber or ceramic could all be used depending on the material that needs to be ground and the desired product outcome.
The balls are placed inside a long hollow cylinder which is about as long as it is wide, the tube is often lined with abrasive wall linings or in some cases an incredibly hard material like manganese. The cylinder is rotated around a central axis, the minerals inside are slowly spun along with the ball contents and are subjected to constant impact and attrition from the balls to ultimately crush and mill the contents to a specific size or powder ratio.
Ball milling is a widely used and reliable technique, often employed for the machining of pigments, ceramics and fine refractory materials.
Hammer Mills
Water powered hammer mills were used as far back as 488 CE. Essentially, a hammer mill is a steel drum housing a horizontal rotating shaft mounted vertically or horizontally on which numerous hammers are mounted. They can be used as a secondary or tertiary means of crushing, and the primary means on occasion.
The hammers are spun on their axis at high speed and as the material is fed in through the hopper it shatters on impact, all the way down the mill, becoming smaller as it goes. This can be a small or larger scale. Fragments exit through the bottom of the mill, and can then be sieved or filtered depending on the size requirements. Medium to large sized aggregates are most suited to this technique.
Raymond Mills
The Raymond mill is a superior choice for small scale powder making projects and is used widely in the mining industry. The Raymond mill will easily satisfy the demand for 80 to 600 mesh (180 to 23 μm) powder tasks.
A large upright cylinder houses a central shaft, which spins at high speed. The shaft has attached to it a number of heavy high-speed rollers that act with a grinding motion as large material particles are fed into the side of the machine. The result is almost instant powder residue resulting from the grinding motion which is then fed out using specialist filtering tubes.
Common applications that use Raymond mills include barite, calcite, potassium feldspar, limestone, talcum, dolomite, gypsum, marble, clay and glass. Such wide material scope is testament to the Raymond mill’s extensive utility.
Attritor Mills
Attritors can be found in many sizes and a variety of places, from small scale laboratories or pilot plants to factory production floors. Attritors, also referred to as stirred ball mills, work by use of ‘shear’ and ‘impact’ methods on a given material by another stronger media.
The attritor mill is similar to the ball mill and uses much the same principles, but with slight variations. Essentially, the material is put into a fixed tank which contains a grinding media, this media could be carbon steel, ceramic, stainless steel, tungsten carbide, ceramic or, most commonly, chrome steel balls. These balls are then violently agitated, propelled by a fast-moving shaft with arms which causes them to impact upon and break up mineral material. After a period of ‘attrition’ the material is pulverised, often into a fine powder.
Common applications that use attritors include: ceramics, pigments, paper coatings, glazes, pharmaceuticals, fine chemicals and coal products.
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