Milling
INDEX
Chapter 1 Process description
Chapter 2 Ball loading system
Chapter 3 Ball mill
Chapter 4 Cyclone
Chapter 5 Linear screens
Chapter 6 Maintenance
Chapter 1
Process Description
Introduction
Milling or Grinding is the last stage in the process of comminution. Communition means simply to break ore into smaller particles. In this section the particles are reduced in size by a combination of impact and abrasion breakage
The objective of milling and classification sections of a concentrator is to further reduce the size of the ore that has been crushed so that the valuable mineral can be liberated or unlocked from the gangue or waste rock. The size to which you grind the ore is a very important parameter which is worked out based on financial calculations, based on the mineralogy of the ores.
In order to produce good, clean concentrates, you need to grind fine enough to liberate all the valuable material, but if you grind it too fine, this could increase the energy costs and can lead to the production of very fine, untreatable “slime” particles which may be lost to tailings.
Grinding is done in cylindrical steel vessels known as tumbling mills. These contain a charge of loose crushing bodies known as the grinding medium which is free to move around inside the mill, thus breaking the ore particles smaller. Grinding medium can be steel rods or balls, hard rock or the ore itself.
There are various types of milling circuits, and each circuit is designed, so that the best milling efficiency can be obtained. Run of mine ball mills or SAG mills can handle ore feed sizes up to 200mm whereas rod mills or ball mills usually need a finer crushed feed? The type of milling circuit used depends on the properties of the particular ore body.
Milling circuits vary from mine to mine. A concentrator can either have a single stage milling circuit or two stages of milling where a primary and secondary mill is used. Primary and secondary mills can either be consecutive stages or the primary stage can be followed by an initial stage of flotation, and the tailings are then sent to the secondary mill. This is called an MF2 circuit – Mill Float Mill Float.
The circuits can also be either being open or closed circuits. An open circuit mill is where the mill product is sent directly to the next milling stage or the next process in the concentrator. An open circuit is usually used where the mill product size is not crucial to the next stage, or where over grinding of material is likely. In a closed circuit mill, the mill product is first classified into fine and coarse material and only the fine material is sent to the next stage. The coarse material is returned to the mill to be ground again, until it becomes fine enough to move on to the next circuit. A closed circuit is used when it is crucial that the size of the material is within a certain range.
Classification can either be done by cyclones or vibrating screens. The objective of classification in a milling circuit is to separate a stream of material into coarse and fine products. Therefore, where the size at which you need to separate the material is large enough, a vibrating screen can be used, but when the size of separation is finer, a cyclone can be used.
Fig.1.Typical process flow sheet
Upstream and downstream problems
Upstream
Delays and breakdowns in the milling and classification section as a whole will have the following impact on the upstream (crushing) operations:
o The crushing plant will have to be stopped when the milling ore storage facilities are full. This would lead to a serious loss of production.
o The delay in crushing plant could lead to the delays in mining operations and the entire mine’s production targets may not be met.
· Downstream
Delays and breakdowns in the crushing and screening section as a whole will have the following impact on the downstream (milling) operations:
o In the event of a coarse grind in the primary mills, the secondary mills may become overloaded.
o If the primary mills are feeding the rougher flotation section then less mineral will be recovered in the rougher flotation section as less minerals will be liberated.
o In the event of a coarse grind, the minerals which are not yet will not be recovered during flotation and they will report to the tailings section, decreasing the overall plant recovery.
o If the grind size distribution is not consistent it will result in ineffective control of the flotation circuits.
o If the grind is too fine, it will cause a lot of waste material to be mechanically entrained in the froth and it will report to the concentrate which will decrease the overall grade of the process.
· Basic overview of Specific Components/ Machinery/ Equipment
The following pieces of equipment are found in the milling section
o Ore Storage Silos / Stockpiles and Bins
o Conveyor belts
o Feeders
o Suspended ball loading system
o Run of Mine Mill
o Ball Mill
o Vibrating screen
o Mill Discharge sump
o Centrifugal pump
o Cyclone
o Linear Screen
· Suspended loading system
The suspended ball loading system is a system which loads steel balls used as grinding media into the mill to maintain optimum grind. It consists of a magnet with which to pick up the steel balls, a chute into which to drop the balls and a sputnik or other mobile container with which to transport the balls to the loading point. The loading point is usually on the mill feed conveyor belt. The balls are deposited onto the moving convey and enter the mill with the feed. The balls should be deposited via a chute.
Fig.2.Suspended loading system
· Run-Of-Mine Mill
A run-of mine mill is a mill which can accept a feed size consistent of that which is supplied by the mine. It uses steel balls as grinding media and in some cases, hard rock. The charge of grinding media is about 20-30% of the mill volume. It consists of a drive unit with oil lubricating system, feed chute, girth gear, cylindrical shell with liners and lifter bars, and a discharge end with trunion bearing.
Fig.3.Run-of-mine mill
· Ball Mill
The final stages of comminution are performed in tumbling mills using steel balls as the grinding medium and they are so called ball mills. Since balls have a large surface area per unit weight, they are better suited for fine finishing. The term ball mill is restricted to those mills having a diameter to length ratio of 1.5:1 and less. Ball mills are also classified by the nature of their discharge ends. They can be trunion overflow mills or grate discharge mills.
Fig.4.Ball mill
· Vibrating screen
The purpose of the vibrating screen is to separate the mill product into an oversize product and an undersize product. The vibrating screen can be used for two different functions. The first is as a trash screen. In this case the oversize product is the material which been discharged from the mill, but which is too big to be pumped by the mill discharge pump and could cause damage to the pump. The undersize product goes directly into the mill discharge sump from which it is pumped to the next section. The second function of a vibrating screen in a milling circuit could be as a classification screen which will form part of a closed circuit milling process. In this case the coarse fraction (oversize) will be returned to the mill for further grinding and the fine fraction (undersize) will continue to the next process or the next stage of milling.
Fig.5.Vibrating screen
· Mill discharge sump
The mill discharge sump is the vessel into which the mill discharge flows, before being pumped. The sump should be designed to accept maximum the flow rate of slurry discharging from the mill. The sump is also a vessel into which any dilution water required for the next process be it a classification screen or cyclone, or a flotation process, can be added. The sump level should be carefully monitored to ensure that the pump pumping the slurry from the sump does not run without any feed, or that the sump does not overflow and create unnecessary spillage.
Fig.6.Mill discharge sump
· Cyclone
The function of a cyclone is to separate a stream of material into a coarse and fine fraction. It is a static piece of equipment which uses centrifugal forces and gravity, to separate heavy and light material. The centrifugal force is created by the design of the inlet and the pressure at which the feed is introduced into the cyclone. The heavy material is flung to the outside of the cyclone and then directed to the underflow and discharged through the spigot. A vortex or upward suction is created by a vortex finder which sucks the water and the fine material to the overflow. The cyclone separates material based on size and density.
Fig.7.Cyclone
· Linear screen
A linear screen is a mechanical screen which removes unwanted coarse material from slurries. The material is directed onto a screen cloth, which has apertures or holes as small as 500 microns. Its application can be to remove tramp material from the slurry, but it was originally designed to remove woodchips from slurries. The woodchips come from underground packs, which are used to support the ore body underground after blasting the supporting rock out. The woodchips which cannot be removed from the conveyor belts end up being milled and they can cause blockages of valves, chutes and flow meters later in the process.
· Instrumentation
Instruments are the tools we use to measure parameters around the milling circuit. The commonly measured parameters are:
o Mill feed rate – weightometer
o Belt speed – tachometer
o Mill inlet dilution water flow rate – flow meter
o Mill mass – load cells or mill bearing pressure
o Mill discharge sump level – level sensor
o Mill discharge dilution flow rate – flow meter
o Cyclone feed flow rate – flow meter
o Cyclone feed density – densitometer
o Cyclone feed pressure – pressure transducer or pressure gauge
o Mill product size – PSI/PSE monitor.
· Detailed Information on Specific Equipment
Detailed information concerning the location, components, purpose, function, and technical specifications trouble shooting and safety information of the following equipment will be discussed in this section:
o Steel ball loading system
o Run of Mine mill
o Ball mill
o Cyclone
o Linear Screen