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All rocks a nd s oils

cons ist o f minerals that

have a distinctive

sc ratc h hardness . To

define this hardness, the Moh’s

hardness scale is the standard

reference used . The s ca le is

divided into 10 increments ,

ranging from talc (with a hardness

of 1) as the softest, up to

diamond (hardness 10) as the

ha rdest. The sc ale is natura lly

linear from a hardness of 1 to 9,

with each mineral being able to

sc ratch the one b elow it in the

scale.

Among the mos t common

minerals, mica and ca lcite are

very s oft (hardness 2.5 and 3,

respectively), while feldspar,

pyroxene and amphibole ma y be

cha racterised a s medium hard

(hardness 6). Quartz and garnet

are very hard (hardness 7 and 7-

7.5, respectively), and to a great

extent, determine the degree of

TB M cutter we ar.

Cutter life can be estimated from the

rela tive pe rcentag e o f minera ls of different

Moh’s hardness classes (>7, 6-7, 4-5 and

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Thes e method s no rmally g ive a fa irly reliable

estimation of the abras iveness . The greates t

challenge in most cases is to collect

represe ntative samp les.

Vickers hardness defines the micro-

indentation hardness of a mineral, a nd

provides a Vickers hardness number (VHN).

The ha rdness number is defined a s the ratio

of the load ap plied to the indentor (gram or

kilogram force) divided by the conta ct a rea

of the impression (mm2). The Vickers

indentor is a square based diamond pyramid

with a 130˚ included ang le betw een oppo site

faces, so that a perfect indentation is seen

as a sq uare with equa l diag ona ls. A virtually

linear relation has been found b etwee n

Moh’s hardness and VHN (in log-scale), as

shown in Figure 1.

As with Moh’s hardness, the use of VHN is

primarily for the purpose of preliminary

estimates of abrasivity and the expected

cutter wea r.

The Cerchar tes t is performed b y

sc ratching a freshly broken rock surfac e w ith

a sharp pin of heat-treated alloy steel (figure2). The C ercha r Abra sivity Index (CAI) is the n

calculated as the average diameter of the

ab raded tip of the s teel pin in tenths of mm

after 10mm of travel ac ros s the rock surface.

The ad vanta ge o f this tes t is tha t it ca n be

performed o n irregular roc k sa mples. The

CAI value is related directly to cutter life in

the field. C AI values va ry betwee n less tha n

0.5 for soft rocks (such a s s hale and

limestone) to more than 5.0 for hard rocks

(such as quartzite).

The LCP C Abrasimeter Tes t involves the

taking of s amples o f rock, s oil or

synthetically created material a nd tes tingusing the 4mm-6.3mm frac tion. C oa rse

grained material has to be crushed and

sieved and fine gra ined ma teria l (

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of a pprox. 2kg is us ed for preparation of

abras ion po wder.

• Crushing is d one g ently in several crusher

steps to avoid excessive production of

fines . The initial crushing is pe rformed in a

jaw crusher with the outlet opening

adjusted to 10mm. Further crushing is

performed using a sma ller lab oratory

crushe r in minimum 2 s teps . The outlet

opening is a djusted to approx 3mm-4mm

prior to the first crusher step.

• The c rushed ma terial is sieved o n a 1mm

qua dratic mes h. The fraction < 1mm is

transferred to a suitable pan and the

frac tion > 1mm is c rushed ag ain after

ad justment of the o utlet ope ning to

app rox. 1mm. This proce ss is repea ted

until the g rain size d istribution is 99%