CMD 124 Class 5 Comminution- Crushing Circuits

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CMD 124 Introduction to Mineral ProcessingClass 5 – Comminution: Crushing Circuits;

Grinding Mills

Outline

1. Open Circuit Crushing2. Closed Circuit Crushing3. Grinding Mills

Introduction to Mineral Processing

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Comminution: Open Circuit Crushing


Definition of Open Circuit Crushing • This is crushing in which the undersize material from a

scalping screen is combined with the secondary crusher product and then conveyed to the next unit operation.

• Application:– Intermediate crushing stages– To prepare feed to a rod mill

Open Circuit Crushing

Introduction to Mineral Processing 3

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Comminution: Closed Circuit Crushing


Definition of Closed Circuit Crushing • This is when the crusher product is screened and the undersize

material is the final product while oversize material is re-circulated back to the crusher.

• Application– To prepare feed for the ball mill

Closed Circuit Crushing


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Comminution: Closed Circuit Crushing -2


• What are the benefits of closed circuit crushing?– crusher can be operated at wider closed setting to prevent

packing when crushing wet ores;– correct product size cut is made on the screens, and– wider closed setting increases crusher throughput, thus

compensating for the increased circulating load.

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Comminution: Closed Circuit Crushing-2


Why Scalping screens are used ahead of crushers?• To remove undersize material;• Scalping ensures that undersize material bypasses the

crusher thus improving the capacity of the crushing circuit, and

• Scalping removes undersize which tends to pack in the voids between large rock pieces in the crushing chamber causing choking which can damage the crusher.

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Comminution: Grinding-1


• Grinding is the last stage of comminution. • What is a grinding mill?

– Grinding mills are cylindrical steel vessels and are also known as tumbling mills.

• Typical feed size is 5mm – 250mm and this is ground to a product with a size range of 10 microns – 300 microns

Mill

5-250mm10-300 microns

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Definition of terms• Mill Charge – A mixture of medium, ore and water• Medium – steel balls, steel rods or pebbles or the ore itself

that is used to grind ore. • Optimal mesh of grind - that size at which the valuable

mineral particles are liberated from the gangue.• Recall that the optimal mesh of grind depends on:

– Ore texture (how the ore is dispersed in the gangue) – Separation process to be used

• The medium must be large, hard and heavy relative to ore particles

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Why Grinding is the most important stage in comminution• Grinding is done up to the optimal mesh of grind;• If the ore is ground to a size larger than the mesh of grind, the

product is too coarse, which will lead to less than optimal recoveries;

• If the ore is ground to a very fine size less than the mesh of grind, this reduces the size of both the gangue and the valuable minerals, thus wasting energy, and

• Grinding consumes the most energy in comminution circuit therefore want to avoid over grinding e.g. energy costs of grinding to an 80% passing for one size smaller than the optimal size on the √2 mesh series is ≈20%. e.g. grinding to 240 mesh (63 microns instead of 200 mesh (74 microns)

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Exception to grinding up to mesh of grind:• If a subsequent separation process requires a much larger

surface area of the particle to be exposed then we can over grind the ore e.g. gold ore treatment through cyanide leaching.

• Over grinding results in large surface area relative to the particle mass. Leaching in this case results in improved gold recovery, which offsets the costs of additional energy used in grinding.

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Determinants of Grinding in a Tumbling mill – Size, quantity and motion of the medium– Spaces between the medium particles

• Two important facts about grinding– The extent of grinding of a particle therefore depends on

the probability of it being caught in the right spaces at the right time.

– A very small fraction (<5%) of the energy input to a mill goes to grinding with the rest being lost as heat, noise, etc

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Mechanics of grinding• In most cases, grinding is a wet process. In a tumbling mill,

the ore particles are subjected to breakage by impact or compression, chipping or abrasion (see Fig 7.1).

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Grinding Mills / Tumbling Mills-1


The motion of the charge in a Tumbling mill• The rotation and friction of the mill shell liners lifts the medium

up to a point where part of the medium cascades over the rising parts of the charge while the other cataracts over a dead zone down to the toe of the charge.

At low mill speeds, the medium cascades or rolls down to the toe:• Get grinding through abrasion• Release of more fines (i.e.

finer grinding)• High mill liner wear

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At high mill speedMedium is thrown clear of the charge to descend in a parabolic path towards the toe of the charge. This is called cataracting.• Get impact grinding at the toe• Coarse grind is achieved• Low mill liner wear

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Critical speed, Nc = 42.3/(D-d)0.5 rev/min, Where D= mill diameter in m; d= rod or ball diameter in m.

The speed at which medium cataracts down to the toe instead of following a circular path. At speeds above critical speed, the mill behaves like a centrifuge.In practice a mill is run at 50-90% of the critical speed.

Grinding Mills / Tumbling Mills -4

There are three types of Grinding / Tumbling Mills

1. Ball mill –steel ball as medium

2. Rod mill –steel rods as medium

3. Autogenous mill –ore as medium can add up to 10% steel balls. This becomes a semi-autogenous mill.

• Key variables for a mill are:• Diameter –determines the pressure that the medium exerts on

ore particles• Length –for a given diameter, the length determines the

capacity of the mill • In general the larger the feed size the bigger the diameter of

the mill.


Grinding Mills / Tumbling Mills -5

How do grinding mills operate?• A tumbling mill is supported by trunnions at both ends• It rotates on its axis• Feed is introduced at one end and the product leaves the

mill at the other end.• Mills operate continuously and therefore are fed from

storage bins.• Can be wet or dry grinding


Ball Mills

Ball Mills - Grinding media is steel balls• Ratio of Length/Diameter, L/D ≤ 1.5• if L/D 3-5 , these are called Tube Mills• Can have compartments along the length of the tube, each

with a different charge e.g. rods, steel balls or pebbles• Used dry to grind cement, clinker, gypsum and phosphate• A tube mill with only one compartment and using screened

hard ore particles as media is called a pebble mill• As pebbles are less dense than steel balls the power input

for pebble mills is less than for ball mills


Ball Mills -2

Mechanics of Grinding in a Ball Mill• Random impact crushing of ore particles• Balls impact on ore particles at the toe• Because grinding is random can have a wide size

distribution of the product• Can have problems of over grinding• Therefore closed circuit grinding with short residence time

for the ore particles is done to control over grinding


Ball Mills - 3

Factors that affect grinding efficiency in a ball mill • Need a high pulp density (65%-85% solids). This ensures that

the ore coats the balls to reduce metal to metal contact (metal to metal contact increases the wear of the steel balls);

• As viscosity of the pulp increases with fineness of the ore can lower pulp density in fine grinding circuits

• Efficiency depends on the surface area of the medium - Need small balls for fine grinding as they have more surface area. Therefore a mature charge will contain new balls(large)and small balls. Typically primary grinding uses 5-10 cm balls and secondary grinding uses 2-5cm balls.


Ball Mills -4

Factors that affect grinding efficiency in a ball mill • Charge volume - This is maintained 40 -50%of the mill volume• Mill speed – A ball mill should be run at 70 – 80% of the

critical speed to achieve cataracting of the medium.• Medium consumption may account to 40% of milling costs


Rod Mills -1

Rod Mills • Ratio of L/D is 1.5-2.5X• Limited to about 6M length as at this lengths rods will bend• Mechanics of Grinding in a Rod Mill• A rod mill is run at 50 -65% of critical speed to achieve

cascading only


Rod Mills -2

Mechanics of Grinding in a Rod Mill• At the feed end the coarse feed spreads the rods so that there is

preferential grinding of the coarse ore;• This preferential grinding results in the minimum production of

fine material and • A product with a relatively narrow size range


Rod Mills -3

Rod Mills - Application• Used as fine crushers or coarse grinding machines- can achieve

reduction ratios of 15- 20:1 e.g. feed size of 50mm to produce a product size of 300 microns

• Preferred in clayey or damp ores that tend to choke crushers• Used in the preparation of feed to gravity concentrators• Used where there are flotation problems associated with slime

in the ore• Preparing feed for Ball mills


Rod Mills -4

Factors that affect the efficiency of a Rod Mill• Need a high pulp density (65%-85% solids) to reduce metal to

metal contact;• Medium-charge with rods of different diameters (25-150mm) to

maximize the grinding surface;• A charge with more smaller rods increases the surface area for

grinding;• Use high carbon steel rods;• Optimum grind with new rods at 35% of the volume;• This lowers to 20-30% with wear and is kept at this level, and• The rods charge is therefore about 45% of the volume.


Autogenous / Semi Autogenous Mills

Autogenous Mills (AG)• Can be semi autogenous (SAG) or fully autogenous (AG);• Ore particles are tumbled in a mill to achieve comminution;• Ore on ore Vs balls and rods on ore;• In SAG, steel ball charge ≈ 6 -10 % by volume(medium is steel

ball + ore);• Wet or dry process: dry milling is not suitable for clayey ores.• Dry milling is more difficult to control than wet milling.

– Advantageous in grinding materials that would be degraded if wet, e.g. asbestos, talc and mica where closed circuit SAG produces a finished product in one stage.


AG / SAG Mills -2

Mechanics of AG and SAG• Ore breaks by impact and abrasion or attrition –get breaking at

the boundary between the mineral and gangue Vs impact breaking in a ball mill

• Minerals are liberated without much over grinding• Smoother particle surface leads to better separation by flotation


AG / SAG Mills -2

Factors that affect the efficiency of AG milling• In tough ores i.e. grains strongly bonded together, AG milling

fails to liberate the individual mineral particles but instead breaks these down by abrasion, to fine sizes not suitable for separation by subsequent processes;

• If the ore lumps have structural weakness, the ore will breakup too early affecting the mill charge(pebbles too small for efficient grinding);

• Need pilot testing to test the ore for suitability to AG milling


Grinding Mills – Mill Liners -1

Mill Liners• Primarily used to prevent the wearing of the shell of a tumbling

mill;• Constructed of wear resistant material such as white cast iron,

nickel alloyed steel, or rubber;;• Liners can be smooth, for fine grinding through abrasion (high

wear rates);• Many shapes are available to provide lift and impact and

abrasion to the mill charge;


Grinding Mills – Mill Liners- 2

Mill Liners• Mill liners are a major cost item, but usually prefer a good liner

over a cheaper one considering the labor costs of replacing liners;

• While rubber liners may also be used, this depends on the operation. If high forces and speed are required in the mill then rubber liners show excessive wear rates.


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CMD 124 Class 5 Comminution- Crushing Circuits