Hardgrove grindability index (HGI)

PRESENTATION TOPIC
HARDGROVE GRINDABILITY INDEX
AND EFFECT OF COAL BLENDIN ON IT
PRESENTATED BY
KOMAL AROOSH
(2010-2012)
DEPARTMENT
CENTER FOR COAL TECHNOLOGY
PUNJAB UNIVERSITY LAHORE

GRINDABILITY OF COAL
The measure of the coal resistance
to crushing is said to be the grindability of a coal.
Generally, lignite and anthracites are more resistant to
grinding than bituminous coals. Two major factors, the
moisture and ash contents of a coal are affecting the
grindability. One commonly used method for assessing
grindability is the hard grove test.

COMMON TEST SERVINGS
TO DETERMINE THE GRINDABILITY
FACTOR
On industrial scale, WI work index by bond and
HGI tests serving are used to determine the
value of grindability. The Hard grove method is
mostly used to determine the grindability of
coal, but also in determining grindability of
other raw materials. Bond’s method is used to
evaluate the rock grindability.

HARDGROVE
GRINDABILITY INDEX (HGI)
• The Hardgrove Grindability index is designed to
determine the ease with which coal can be
pulverized.
• It measures the grindability of coal and is a
qualitative measure of coal.
• It provides information for determining grinding
power consumption and pulverizing capacities.
HGI of coal depends on the coal composition.

• It can be used by industry to determine power
consumption during the grinding process.
• Usually softer coals have higher HGI. Some
physical properties of coal such as
hardness, tenacity and fracture can also be
shown by it. Between grindability and rank of
the coal there exists a fixed relationship. Coal
easier to grind have low volatile matter and
lower moisture. The moisture content and the
humidity in which HGI has been conducted
affect the results.

HARDGROVE
GRINDABILITY
INDEX (HGI)

FEATURES OF HGI
MACHINE
NO OF BALLS = 8
BALLS MATERIAL = Iron
DIAMETER OF EACH BALL = 25.4±0.003 mm
NO OF REVOLUTIONS = 60
NO OF LOADED RINGS = 3
WT OF EACH LOADED EING =600gm

PROCEDURE TO
DETERMINE THE HGI VALUE
The experimental procedure for the determination of hard grove
grindability index HGI is as follow according to the ASTM
standard ;
• The 50gm sample of prescribed size -16+30 mesh is taken in
the ball mill of HGI machine along with 8 iron balls.
• Close the mouth of the ball mill , and set to rotate for exactly
60 revolutions. After the requisite rotation , the machine
stopped automatically.
• The sample left in the ball mill is then collected along with any
powdered substance sticking to the surface of the machine
with the help of a brush.
• This sample is then put in a sieve of 75μ size and is shaken for
about 10 minutes. The sample which passed through 75μ size
and retained on 300 μ size is weighed on the balance.

CALCULATIONS
The hard groove grind ability index of coal is than
calculated using the following formula.
HGI = 13 + 6.93 w
Where,
w = weight of the test sample passing
through 75μ sieve and retained on 300 μ
sieve after grinding in the HGI
machine.

FACTORS AFFECTING
THE GRINDABILITY VALUES
1. PETEROGRAPHIC COMPOSITION
The hard grove grind ability index value is
influenced by petrographic composition of
coal.

2. COMBUSTIBLE MATTER
Commonly, the increase of contents of volatile
combustible matter improves the grindability up
to the contents of volatile combustible matter of
approximately 30%, beyond which the
grindability deteriorates. Similarly the HGI
value increases with the growth of carbon
contents, the grindability then drops rapidly
with the contents of carbon exceeding
approximately 92%.

3. MASCERALS
a. LITHOTYPES
The lithotypes of black coal with similar
contents of volatile combustible matter include
differences in the HGI values.
b. DURITE
Durite (dull coal) is a lithotype characteristic by
low HGI and is generally the toughest.

c. FUSITE AS LITHOTYPE
Labeling fusite as lithotype with the highest HGI value is
disputable, because its extreme fragility is caused by
origination of significant amount of fine fractions rather
during screening that in the course of grinding in the
testing device (hower, 1988).
d. GLITTERING VITRINITE LITHOTYPES
Glittering vitrinite lithotypes in black coal have
significantly higher HGI values than durites in the same
coal group. The difference in grindability of individual
lithotypes allows for selective grinding.
It is generally valid that the occurrence of vitrinite in
coal increases the HGI value, whereas the micronite and
liptinite macerals decrease the grindability.

COAL PROPERTIES
COMPLEMENTARY TO HGI
Coal calorific value, moisture content and volatile
matter content/reactivity can be complementary
properties to HGI. If the volatile matter content or
reactivity is higher than normal, then coal particles
somewhat larger will burn out successfully in the
normal furnace volume. This coarser product
requirement allows the mill to provide more output.
Alternatively it allows the use of coal with a lower
HGI with effective particle burnout. Thus a
combination of coal properties is necessary to
ensure relatively complete combustion/reaction in
normal boiler furnaces

Coal with less volatile matter content or lower
reactivity will require more particles to be less
than 75 microns for fairly complete reaction.
This applies whether the reaction is:
• combustion in a boiler to generate power
• pulverized coal injection in a cement kiln to
make cement clinker
• pulverized coal injection in a blast furnace
to assist economic iron making
• entrained particle gasification to make syngas
for power generation
• entrained particle gasification to make syngas
for chemical production.

EFFECT OF BLENDING
ON HGI
Personally, I determined the HGI value for some of the coal
blends of Kotali and Afghan coal in the lab of CENTER
FOR COAL TECH PUNJAB UNIVERSITY LAHORE
under the guidance of Dr . Shafqat Nawaz and got the
following graph , while comparing the calculated and
measured values on the basis of wt blending ratio of Afghan
coal %.
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120
Hardgrovegrindabilityindex
wt blending ratio of Afghan coal %
HGI Measured
HGI Calculated

The results of this
study indicate :-
• Experimentally determined values of HGI of coal
blends differed by about 0.682 to 2.414 with
the weighted average computed values.
• These results are supported by the studies
Vuthaluru et al who studied the effect of
moisture and coal blending on Hard Groove
Grindability Index of Western Australian coal
where it has been observed that measured HGI
values of binary and tertiary blends
corresponded well with the weighted average
values of HGI within ±2.

• Rubeira et al found that the HGI value have linear
relationship when blending of two different hard
coals were considered. The difference between
weighted averages computed values and
experimental values are found to be between 1.88 to
3.522.
• Contrary to the above stated findings of
researchers, findings of Conroy and Bennett state
that even if coals of similar HGI were blended, the
measured HGI values of blends were lower than the
calculated values, in some cases lower than the
either blended components. Thus it was concluded
that there is no general method of predicting the
HGI of the coal blend, which must be determined
experimentally on case- to case basis.

• According to the above study, the blend no 3 shows
the optimum result for cement factories with
approximately HGI 45 and sulphur content
1.054, while blend no 5 is suitable for power sectors
with approximately HGI 66 and sulphur content
1.469.
• The value of the blends for HGI and moisture
content shows the inverse relation, as the moisture
content increases HGI content going to decreases.
• It is generally valid that the occurrence of vitrinite in
coal increases the HGI value, whereas the micronite
and liptinite macerals decrease the grindability.

REFFERENCES
[1]http://www.britannca.com/EBchecked/topic/286/grindability
[2] http://gse.vsb.cz/2008/LIV-2008-1-27-32.pdf
[3]http://wiki.answers.com/Q/What_is_Hardgrove_grindability_index
[4] http://gse.vsb.cz/2008/LIV-2008-1-27-32.pdf
[5] Hardgrove Grindability Index; www.acarp.com.au/Downloads/ACARP
Hardgrove Grindability Index.pdf.
[6] C. Ashley., B. Phill ., Evaluate combustion behavior of Australian export
and overseas low rank coal blends; < www.acarp.com.au/abstracts.aspx?
RepId=C3097 accessed on 20 Oct 2012.
[7] T. Wall., L. Elliott., D. Sanders., C. Ashley, Technology Assessment Report
14, Cooperative Research Centre For Black Coal Utilisation, NSW
Australia, 2001, p. 22.
[8] J. T. Riley., S. R. Gilleland., R. F. Forsythe., H. D. Graham., F. J.
Hayes, Non-additive analytical values for coal blends, Proceedings of the
7th international conference on coal testing, Charleston, WV, USA, 21-23
Mar. 1989. Ashland, KY, USA, Coal Testing Conference, Standards
Laboratories, Technical Services Division, pp. 32-38, 1989.

Hardgrove grindability index (HGI)

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