Mineral Processing- Lecture 7: Grinders

Grinding

• is the last stage in the comminution process

• particles are reduced in size by a combination of impact and abrasion, either dry, or more commonly, in suspension in water

• performed in cylindrical steel vessels that contain a grinding medium

• the most energy intensive operation in mineral processing

• economic degree of liberation is the principal purpose

• increase mineral surface area

• usually performed wet or dry

Grinding Medium

• a charge of loose crushing bodies

• is free to move inside the mill, thus comminuting the ore particles

• due to the rotation and friction is lifted along the rising side of the mill until a position of dynamic equilibrium is reached (the shoulder), when the bodies cascade and cataract down the free surface of the other bodies, about a dead zone where little movement occurs, down to the toe of the mill charge

Tumbling Mills

• the mill shell is rotated and motion is imparted to the charge via the mill shell

• the grinding medium may be steel rods, balls, or rock itself

• use of loose crushing bodies, which are large, hard, and heavy in relation to the ore particles, but small in relation to the volume of the mill, and which occupy (including voids) slightly less than half the volume of the mill

• are rated by power rather than capacity, since the capacity is determined by many factors, such as the grindability, determined by laboratory testing, and the reduction in size required.

• use both impact and shear (abrasion/attrition) energy in roughly equal measure

Stirred Mills

• the mill shell is stationary mounted either horizontally or vertically

• motion is imparted to the charge by the movement of an internal stirrer

• found applications in multiple fine to ultrafine grinding applications from pharmaceuticals to industrial minerals

• has become more prevalent in milling applications, corresponding to the increase in processing of more complex

  fine-grained ores demanding liberation grinds of 10 μm and

  less

• use predominately shear energy

• effective for fine grinding

• more energy efficient

Breakage (of ore)

result of repeated, random impact and abrasion, events which break liberated as well as unliberated particles.

Dry Grinding

is used in applications such as cement production or when a material is simultaneously ground and dried

Mill Charge

• the mixture of medium, ore, and water

• is intimately mixed, the medium comminuting the particles by

  any of the above methods

Rod, Ball, and Autogenous/Semi-Autogenous

Three Types of Tumbling Mills

Cascading

• leads to finer grinding and increased liner wear

• At higher speeds the medium is projected clear of the charge to describe a series of parabolas before landing on the toe of the charge

Cataracting

leads to comminution by impact and a coarser end product with reduced liner wear

Centrifuging

• occurs at the critical speed of the mill

• the medium is carried around in an essentially fixed position against the shell

Lifting Section

• near to the shell liners

• circular

Drop Back

• to the toe of the mill charge

• parabolic

Gross Power Draw

by a rotating mill is the sum of the no-load power (to account for frictional and mechanical losses in the drive power) and net power drawn by the charge

Net Power

is calculated by considering the energy to lift a single ball, summing over the total number of balls and multiplying by the number of times the balls are lifted per second, given by the speed (N, revs per second) of the mill

Rod Mills

• they are often preferred to fine crushing machines when the ore is “clayey” or damp, thus tending to choke crushers

• They are still to be found in older plants but are often replaced upon modernization and are now relatively rare

• the length of the cylindrical shell is between 1.5 and 2.5 times its diameter

• are charged initially with a selection of rods of assorted diameters, the proportion of each size being calculated to approximate to a seasoned or equilibrium charge

• are normally run at between 50% and 65% of the critical speed, so that the rods cascade rather than cataract

• many operating mills have been sped up to close to 80% of critical speed without any reports of excessive wear

• are therefore suitable for preparation of feed to gravity concentrators, certain flotation processes with slime problems, magnetic cobbing, and ball mills

Center Peripheral Discharge Mills

• are fed at both ends through trunnions and discharge the ground product through circumferential ports at the center of the shell

• the short path and steep gradient give a coarse grind with a

  minimum of fines, but the reduction ratio is limited

End Peripheral Discharge Mills

• are fed at one end through the trunnion, discharging the ground product from the other end of the mill by means of several peripheral apertures into a close-fitting circumferential

  chute

• This type of mill is used mainly for dry and damp grinding, where moderately coarse products are involved

Trunnion Overflow

• the most widely used type of rod mill in the mining industry

• the feed is introduced through one trunnion and discharges through the other

• is used only for wet grinding and its principal function is to convert crushing-plant product into ball mill feed

Seasoned Charge

contain rods of varying diameters, ranging from fresh replacements to those that have worn down to such a size as to warrant removal

Overcharging

results in inefficient grinding and increased liner and rod consumption

Mill Feed, Mill Speed, Rod Length, and Product Size

Rod consumption varies widely with the characteristics of

Ball Mills

• the final stages of primary comminution

• using steel balls as the grinding medium

• they are better suited for fine grinding

• in which the length to diameter ratio is between 3 and 5 are designated tube mills

• are also classified by the nature of the discharge

• they may be simple trunnion overflow mills, operated in open or closed circuit, or grate discharge (low- level discharge) mills

• operated at higher speeds than rod mills

Pebble Mills

• tube mills having only one compartment and a charge of hard, screened ore particles as the grinding medium

• They have the advantage over ball mills when iron contamination needs to be avoided

• the power draw and capacity are correspondingly lower

Grate Discharge Mills

• usually take a coarser feed than overflow mills

• not required to grind so finely

• with many small balls forming the charge, the grate open area plugs very quickly

Trunnion Overflow Mill

• is the simplest to operate

• is used for most ball mill applications, especially for fine grinding and regrinding

Efficiency of Grinding

depends on the surface area of the grinding medium

Hardinage Mill

• cylindroconical ball mill, made in three sections--a flattish cone at feed end followed by a cylindrical drum, and finishing with a steep cone leading to the discharge trunnion

• due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, that is, the largest diameter and greatest centrifugal force, and the smallest are at the discharge.

Doering Cylpebs

• are slightly tapered cylindrical grinding media with length equaling diameter, and all the edges being rounded

• have greater surface area than balls of the same mass

Grinding Balls

are usually made of forged or rolled high-carbon or alloy steel, cast alloy steel and some of white iron

Finer Grinding

may lead to improved metallurgical efficiency, but at the expense of higher grinding energy and media consumption

Abrasion, Corrosion, and Impact

Three Wear Mechanism

Abrasion

the direct removal of metal from the grinding media surface

Corrosion

• electrochemical reactions with oxygen, sometimes enhanced by galvanic effects with sulfide minerals

• continual removal of oxidation products by abrasion means the fresh surface is always being exposed to such reactions

Spalling, Breaking, or Flaking

Three Impact Wears

Low-Speed Stirred Mills

gravity plays a role

High-Speed Stirred Mills

which fluidize the pulp

Stress Intensity

• this is a measure of the pressure acting on the particles and is fixed during the comminution process

• the conditions—grinding media size and density, stirrer speed and slurry density—are normally constant during operation.

Orientation of the Mill, Shape of the Impeller, Mill Shell Surface, and Speed of the Impeller

Four Properties of Stirred Milling Technologies

TowerMills / Vertimills

• steel balls or pebbles are placed in a vertical grinding chamber in which an internal screw flight provides medium agitation

• the feed enters at the top, with mill water, and is reduced in size by attrition and abrasion as it falls, the finely ground particles being carried upward by pumped liquid and overflowing to a classifier

Stirred Media Detritor (SMD)

• normally, natural silica or ceramic is used as the grinding media

• Grinding media are added through a pneumatic feed port or the manual feed chute located on top of the mill

• Feed slurry enters through a port in the top of the unit

IsaMills

• being used for ultrafine grinding applications in various chemical industries

• to make this horizontal mill suitable for the mining industry, the engineering challenge was to expand the volume by a factor of 6

• inside the horizontal shell is a series of rotating discs mounted on a shaft that is coupled to a motor and gearbox

• the high-speed discs fluidize the media and the slurry that is continuously fed into the feed port

VXP Mill / HIGMills

the technology has been utilized for more than 30 years in the calcium carbonate industry, but until recently, was not available for mineral processing

Vibratory Mills

• These are available for continuous or batch grinding materials to a very fine end product

• Two vibrating cylinders charged with ball or rod media are placed either directly above the other or inclined at 30 to

  perpendicular

Centrifugal Mills

it has so far not gained full-scale industrial application

Roller Mill

• these mills are often used for the dry grinding of medium soft materials of up to 45 mohs hardness

• above this hardness, excessive wear offsets the advantage of lower energy consumption compared with conventional mills

Table and Roller Mills

• These units are used for dry grinding of medium-hard materials such as coal, limestone, phosphate rock, and

  gypsum

• Two or three rollers, operating against coiled springs, grind material which is fed onto the center of a rotating grinding table

• Ground material spilling over the edge of the table is air-swept into a classifier mounted on the mill casing, coarse particles being returned for further grinding

Pendulum Roller Mills

• Used for dry fine grinding non-metallic minerals such as barytes and limestone, material is reduced by the centrifugal action of suspended rollers running against a stationary grinding ring

• The rollers are pivoted on a spider support fitted to a gear-driven shaft

• Feed material falls onto the mill floor, to be scooped up by ploughs into the “angle of nip” between the rolls and the grinding ring