Abrasives

©2009 Dr B C Paul

Acknowledgement is given to the following sources SME Industrial Minerals, mcrocks.com, geology.com, webmineral.com, mindat.org, USGS, cs.cmu.edu,

yuprocks.com, gc.maricopa.edu

Good Abrasives

• They are cheap

• They last a long time

• They cut or wear surfaces that you want cut or worn quickly

• They don’t cut things you don’t want them to

• Hardness is an obvious important factor

• Size and shape also are important

Diamonds

• Hardness of the diamond is unmatched and for many applications they are irreplaceable for cutting– Industrial diamonds lack the color and

perfection of jewlery

• Major uses– Diamond bits for rock and concrete– Diamond dies for wire drawing– Diamond tipped tools and wheels

(from Rockhoundblog.com)

Diamond Locations

• Diamonds are typically found in Kimberlite and Lamproite intrusions

• The intrusions can be as recent as the Cenozoic– Diamonds themselves are usually older

• Most are 3,200,000,000 to 990,000,000 years old• A few have been found back to 4,200,000,000

• All seem to be associated with Archean basement (maybe a few younger)

Roots of Continents Deeper than Crust

At these depths stable form of carbon switches from graphite to Diamond

Melts Coming Up from Depth Drag Diamonds to the Surface

Kimberlite does not make diamondsIt drags them

Where is Archean Crust Known to be Found

Red and Dark Green Mark the Spot

The Missing Crust Problem

• Stabilization of the Mantle Composition relative to earth’s crust appears to happen around 2,700,000,000 years ago– We should have 70 to 80% of the continental

crust by then

Closer View of North American Old Crust Areas

Industrial DiamondsC S.G. 3.5 - 3.53 Hardness 10Color Colourless, yellowish to yellow, brown, black, blue, green or red, pink, champagne-tan, cognac-brown, lilac (very rare)

How Diamonds are Processed

• Crushing and Grinding– (if the deposit is weathered or a placer this may not

be needed)

• Gravity Concentration– Diamonds are heavier than surrounding minerals– some unique designs produce a concentrate of

heavy minerals

Rotary washing panMake mud and theHeavies settle

Heavy MediaThis diagram shows how cones (left) and cyclones (right) use heavy-media separation. Diamond-bearing concentrate is mixed with a fluid near the density of diamond. Separation occurs in cones and cyclones by swirling the mixture at low and high velocities respectively. In the cone, rotational mixing permits lighter minerals to float to the top and run out as overflow, while diamonds and dense minerals sink to the bottom and are sucked out with a compressed air siphon. In the cyclone, fast rotation of the suspension drives heavy minerals to the conical wall, where they sink to the bottom and are extracted, while float waste minerals are sucked from the center of the vortex. Cyclones are about 99.999% efficient at concentrating diamonds and similarly dense minerals from the original ore. Adapted from Bruton (1978)

Other Minerals Besides Diamonds are Heavy

• The grease method– Freshly exposed

diamonds grab onto axle grease

– Use greased shaking tables to pull out diamond

An Alternative to Hand Sorting of Concentrates

Using Fluorescence

Reserves and Production

Industrial Diamond Reserves

0

10

20

30

40

50

60

70

80

90

Production Reserves/7

Mil

lio

ns

of

Ca

rra

ts

Other

South Africa

Russia

Congo

China

Botswana

Australia

Catch – 88% of industrial diamonds are now synthetic

What are Diamonds Worth

Ability to make synthetic diamonds has brought down the price but they areStill over $1,000,000 per ton (of course they are sold by carrots)

Corundum/ Emery

• Lots of gems are corundum

• Much of abrasive market has been taken by synthetic minerals– Get more uniform material without random

natural weaknesses– Still used for super heavy duty concrete and

durable anti-skid material

Corundum

Al2O3

S.G. 3.98 - 4.1 Hardness 9Color Colourless, blue, red, pink, yellow, grey, golden-brown

Gem forms include Rubies and Sapphires, Emery

Where Corundum is Found

Garnet

• Use as a grinding and polishing agent– Garnet papers still common abrasive– Also used as a “sand blasting agent”

• It does not cause silicosis

– Filtering material– A lot of natural sharpening rocks in fact get

their properties from well distributed small garnet crystals

Almandite

Fe3Al2(SiO4)3 S.G. 4.09 - 4.31, Average = 4.19 Hardness 7-8 Color Brown, Brownish red, Red, Black, Black red. Fluorescence None

Grossularite

Ca3Al2(SiO4)3

S.G. 3.42 - 3.72, Average = 3.57 Hardness 6.5-7.5 Color Brown, Colorless, Green, Gray, Yellow. Fluorescence None

Pyrope

Mg3Al2[SiO4]3

S.G. 3.5 - 3.6 Hardness 7.5Color Blood red, orange red, purple red, pink, or black red Fluorescence not reported

Andradite

Ca3Fe2(SiO4)3

S.G. 3.8 - 3.9 Hardness 6½ - 7½ Color Green, yellow, orange, reddish-brown, brown, black Fluorescence not reported

Staurolite

• Industrial application is as a “Sand Blasting Agent”

• Also used as a gemstone

Staurolite

(Fe,Mg)2Al9(Si,Al)4O20(O,OH)4 S.G. 3.65 - 3.77, Average = 3.71 Hardness 7-7.5 Color Brownish yellow, Brownish black, Yellow brown, Dark brown, Reddish brown. Fluorescence not reported

Silica

• Sand is still dominant material for “Sand Blasting”

• Carefully graded and sized sand is used for “flint” sandpapers– Dominant market is home use

• Fine pure sands are an important raw material for glass, ceramics, and synthetic abrasives

• Naturally fine almost amorphous material that breaks up easily is called Tripoli

Sand

SiO2

S.G. 2.6 - 2.65, Average = 2.62 Hardness 7Color Brown, Colorless, Violet, Gray, Yellow. Fluorescence not reported

Tripoli

SiO2S.G. 2.7Hardness 7ColorWhite, buffFluorescenceNoneMagnetic- No

Soft Scrubbing Powders

• Diatomite– Used in silver polishes– More common use is filtering media

• Pumice– Main ingrediant in polishing powders and

household cleaners• Including tooth paste

– Used in hard rubbers– Used for washing stones for blue jeans

Pumice

Pumice is about 60-70% silicaIt has no crystal structure – it is a frothy glassS.G. < 1

Other Soft Abrasives

• Ground Feldspar in window cleaners

• Chalk for fine soft metal polishes

• Kaolin

Synthetic Abrasives

• Silicon Carbide and related (Tungsten Carbide)