1. THEORY :

Ore grindability refers to the ease with which materials can be comminuted and data from

the grindability test is used to evaluate crushing and grinding efficiency. It is the one of the

basic properties of materials (such as cleavage, hardness, tenacity, elasticity and fracture). The

classification of materials is made through the properties “hard, friable, tough, soft, fibrous

and soapy”. The most widely used parameter to measure ore grindability is the Bond work

index Wi.

Comminution theory is concerned with the relationship between energy input and the

product particle size made from a given feed size. All the theories of comminution assume

that the material is brittle, so that no energy is absorbed in processes (such as elongation or

contraction which is not finally utilized in breakage.

The oldest theory is that of Rittinger, which states that the energy consumed in the size

reduction is proportional to the new surface are produced. Rittinger’s law equates to:

E = K (1/D2 – 1/D1)

E : Energy input,

D1: The initial particle size diameter,

D2: the final particle size diameter,

K : constant.

The second theory is that of Kick. He stated that the work required is proportional to the

reduction in volume of the particles concerned.

E = log R /log 2

E : Energy required,

R: Reduction ratio.

Bond developed an equation which is based on the theory that the work input is

proportional to the new crack tip length produced in particle breakage, and equals to the

work represented by the product minus that represented by the feed. For practical

calculations the size in microns which 80 % passes is selected as criterion of particle size.

The diameter in microns which 80 % of the product passes is designated as P, the size

which 80 % of the feed passes is designated as F, and the work input in kWh per short ton

is W :

W = 10 Wi (1 / √P – 1 / √F) (where Wi is the work index)

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Finally R.J. Charles stated that the energy input to the system as a function of the size

reduction. This equation can be calculated from simple experimental tests. The most

widely accepted and used test to measure the grindability is Bond’s test.

1.1 Grindability Tests:

1.1.a Bond Grindability Test: The Standard Bond test is a locked – cycle batch –

grinding test, which is done till the steady state condition. The used formula at Bond

Grindability Test is;

For Rod Mill:

Wi = 62 / [Pc0.23 * G0.625 * (1/√dp – 1/√df)]

For Ball Mill:

Wi = 44.5 / [Pc0.23 * G0.82 * (1/√dp – 1/√df)]

Where;

Wi: Bond work index (kWh/t)

Pc: Test sieve mesh size (µm)

G: Weight of the test sieve fresh undersize per mill revolution (g min-1)

dp: Diameter of the product (µm)

df: Diameter of the feed (µm)

1.1.b Hardgrove Grindability: R.M. Hardgrove proposed his grindability method

for rapid determination of coal grindabilities. The method is based on Rittinger’s law,

which states that the work done in pulverizing is proportional to the new surface area

produced. The used formula for finding the Hardgrove index is;

HI = 6.93 * W + 13

And also we can found work index from Hardgrove index;

Wi = 435 / HI0.91 (kWh/shton)

Where;

W: Weight of the ground product passing 200 mesh.

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2. OBJECTIVE:

The object of this experiment is to determine the grindability of lignite and

anthracite by using Hardgrove Grindability Test and to learn the importance of grindability

and also to be familiar with the equipments used in the experiment.

3. EXPERIMENTAL:

3.1 Material:

3.1.1 Lignite:

Lignite, often referred to as brown coal, is the lowest rank of coal and used almost

exclusively as fuel for steam-electric power generation. It is brownish-black and has a high

inherent moisture content, sometimes as high as 66 percent, and very high ash content

compared to bituminous coal. It is also a heterogeneous mixture of compounds for which

no single structural formula will suffice.

The heat content of lignite ranges from 9 to 17 million Btu per short ton (10 to 20

MJ/kg) on a moist, mineral-matter-free basis. The heat content of lignite consumed in the

United States averages 13 million Btu/ton (15 MJ/kg), on the as-received basis (i.e.,

containing both inherent moisture and mineral matter). When reacted with quaternary

amine, amine treated lignite (ATL) forms. ATL is used in oil well drilling fluids to reduce

fluid loss.

Because of its low energy density, brown coal is inefficient to transport and is not

traded extensively on the world market compared to higher coal grades. It is often burned

in power stations constructed very close to any mines, such as in Australia's Latrobe

Valley. Carbon dioxide emissions from brown coal fired plants are generally much higher

than for comparable black coal plants. The continued operation of brown coal plants,

particularly in the absence of emissions-avoiding technology like carbon sequestration, is

politically contentious

3.1.2 Anthracite:

Anthracite (Greek Ανθρακίτης, literally "a form of coal", from Anthrax [Άνθραξ],

coal) is a hard, compact variety of mineral coal that has a high luster. It has the highest carbon

count and contains the fewest impurities of all coals, despite its lower calorific content.

Anthracite coal is the highest of the metamorphic rank, in which the carbon content is

between 86% and 98%. The term is applied to those varieties of coal which do not give off

tarry or other hydrocarbon vapours when heated below their point of ignition. Anthracite

ignites with difficulty and burns with a short, blue, and smokeless flame.

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Other terms which refer to anthracite are blue coal, hard coal, stone coal (not to be

confused with the German Steinkohle), blind coal (in Scotland), Kilkenny coal (in Ireland),

and black diamond. The imperfect anthracite of north Devon and north Cornwall (around

Bude), used as a pigment, is known as culm. Culm is also the term used in geological

classification to distinguish the strata in which it is found and similar strata in the Rhenish hill

countries are known as the Culm Measures. In America, culm is used as an equivalent for

waste or slack in anthracite mining.

Anthracite is similar in appearance to the mineraloid jet and is sometimes used as a

jet imitation.

Physically, anthracite differs from ordinary bituminous coal by its greater hardness,

its higher relative density of 1.3-1.4, and lustre, which is often semi-metallic with a mildly

brown reflection. It contains a high percentage of fixed carbon and a low percentage of

volatile matter. It is also free from included soft or fibrous notches and does not soil the

fingers when rubbed. Anthracitization is the transformation of bituminous coal into anthracite

coal.

The moisture content of fresh-mined anthracite generally is less than 15 percent. The

heat content of anthracite ranges from 22 to 28 million Btu per short ton (26 to 33 MJ/kg) on a

moist, mineral-matter-free basis. The heat content of anthracite coal consumed in the United

States averages 25 million Btu/ton (29 MJ/kg), on the as-received basis (i.e., containing both

inherent moisture and mineral matter). Note: Since the 1980s, anthracite refuse or mine waste

has been used for steam electric power generation. This fuel typically has a heat content of 15

million Btu/ton (17 MJ/kg) or less.

Anthracite may be considered to be a transition stage between ordinary bituminous

coal and graphite, produced by the more or less complete elimination of the volatile

constituents of the former; and it is found most abundantly in areas that have been subjected

to considerable earth-movements, such as the flanks of great mountain ranges. Anthracite coal

is a product of metamorphism and is associated with metamorphic rocks, just as bituminous

coal is associated with sedimentary rocks. For example, the compressed layers of anthracite

that are deep mined in the folded (metamorphic) Appalachian Mountains of the Coal Region

of northeastern Pennsylvania are extensions of the layers of bituminous coal that are strip

mined on the (sedimentary) Allegheny Plateau of Kentucky and West Virginia, and Eastern

Pennsylvania. In the same way the anthracite region of South Wales is confined to the

contorted portion west of Swansea and Llanelli, the central and eastern portions producing

steam, coking and house coals.

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Structurally it shows some alteration by the development of secondary divisional

planes and fissures so that the original stratification lines are not always easily seen. The

thermal conductivity is also higher, a lump of anthracite feeling perceptibly colder when held

in the warm hand than a similar lump of bituminous coal at the same temperature. The

chemical composition of some typical anthracites is given in the article coal.

3.2 Equipment:

• 50 g of prepared -14 +28 mesh lignite and anthracite.

• Hardgrove mill : It consists of a stationary grinding bowl of polished steel

balls, each 1 inc in diameter. The balls are driven by an upper grinding ring which rotates at

20 rpm and exerts a total pressure of 29 kg.

• 200 mesh sieve and its pan.

3.3 Procedure:

A prepared sample (50 g lignite and anthracite) put into the Hardgrove mill which

contains steel balls. The grinding balls are spaced evenly in the lower grinding element and

the prepared lignite and anthracite sample is evenly distributed. The upper grinding

element is replaced on top of the grinding balls and the counter is set to zero and the

machine is switched on. The ground lignite and anthracite placed in the grinding chamber

is subjected to a grinding action by means of the steel balls under the required load of 29kg

for exactly 60 revolutions. Then the resultant powder is measured and put into the

vibrating screen for 15 minutes. After the 15 minutes the amount of lignite and anthracite

is measured and calculated in Hardgrove Grindability Index.

Figure 1: Hardgrove Mill

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4. RESULTS AND DISCUSSION:

4.1.a Grindability refers to the ease with which materials can be comminuted and data

from the grindability test is used to evaluate crushing and grinding efficiency.

4.1.b The Hardgrove Grindability Index Standard Reference Sample (HGI-SRS) is a

sample of coal used to calibrate instruments that are designed to determine the ease with

which coal can be pulverized. The HGI value provides information for determining

grinding power consumption and pulverizer capacities.

4.2.)

For lignite;

HI = 6.93 * W + 13 W = 4.42 gr.

So, HI = (6.93 * 4.42) + 13

HI = 43.63.

Wi = 435 / HI0.91

Wi = 435 / (43.63)0.91

Wi = 14 kW/shton

For anthracite;

HI = 6.93 * W + 13 W = 2.64 gr.

So, HI = (6.93 * 2.64) + 13

HI = 31.29.

Wi = 435 / HI0.91

Wi = 435 / (31.29)0.91

Wi = 18.95 kW/shton

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As we expected work index of anthracite is greater than the lignite. Therefore,

resistance of breakage of anthracite is greater than the lignite.

4.3 Energy Consumption for 60 tph:

For lignite;

W = 10 * Wi * (1 / √P – 1 / √F)

P = 0.149 mm = 149 µm & F = 6.35 mm = 6350 µm

W = 10 * 14 * [( 1 / √149) – (1 / √6350 )]

W = 9.7 kW/shton

W = 13 HP

E = W * C C: Capacity

E = 13 * 60 * 0.95

E = 741 HP

For anthracite;

W = 10 * Wi * (1 / √P – 1 / √F)

P = 0.149 mm = 149 µm & F = 6.35 mm = 6350 µm

W = 10 * 18.95 * [( 1 / √149) – (1 / √6350 )]

W = 13.1 kW/shton

W = 17.6 HP

E = W * C C: Capacity

E = 17.6 * 60 * 0.95

E = 1003 HP

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5. Conclusion:

From this experiment we learn the basis of grindability term. Grinding and crushing

steps are important for mineral processing. After doing this experiment we can know the

several grindability methods and especially Hardgrove Grindability Test in this experiment

we can use HGI for trona but after that we use it all of the materials easily by calculating

Wi.

Furthermore, we learn if the breakage characteristics of a material remain constant

over all size ranges, then the calculated Wi would be expected to remain since it expresses

the resistance of material to breakage. However, for most naturally occurring raw

materials, differences exist in the breakage characteristics depending on particle size,

which can result in variations in the Wi. For instance, when a mineral break easily at the

boundaries but individual grains is tough, then grindability increases with fineness of

grind. Consequently, Wi values are generally obtained for some specified grind size which

typifies the comminution operation being evaluated.

6.) References:

Mineral Processing Technology, Pergamon Press, 1985, pp. 137-141

Mineral Processing Lab manual

http://en.wikipedia.org/wiki/Lignite

http://en.wikipedia.org/wiki/Anthracite

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