Compressor Instability with Integral Methods Episode 1 Part 3 ppsx

Chapter 2

Abrasive Materials

2.1 Classification and Properties of Abrasive Materials

A large number of different types of abrasive materials is available for blast clean￾ing applications. Most frequently applied abrasive materials are listed in Table 2.1.

Table 2.2 lists numerous physical, chemical and technical properties of commercial

abrasive materials. Basically, there can be distinguished between metallic abrasive

materials and non-metallic abrasive materials.

The evaluation of an abrasive material for blast cleaning applications includes

the following important parameters:

material structure; material hardness; material density; mechanical behaviour; particle shape; particle size distribution; average grain size.

2.2 Abrasive Material Structure and Hardness

2.2.1 Structural Aspects of Abrasive Materials

Structural aspects of abrasive materials include the following features:

lattice parameters; crystallographical group and symmetry; chemical composition; crystallochemical formula; cleavage; inclusions (water–gas inclusion and mineral inclusion).

A. Momber, Blast Cleaning Technology 7

C Springer 2008

8 2 Abrasive Materials

Table 2.1 Annual abrasive consumption in the USA for blast cleaning processes (Hansink, 2000)

Abrasive type Consumption in Mio. of tonnes

Coal boiler slag 0.65

Copper slag 0.1–0.12

Garnet 0.06

Hematite 0.03

Iron slag 0.005

Nickel slag 0.05

Olivine 0.03

Silica sand 1.6

Staurolite/zirconium 0.08–0.09

Steel grit and steel shot 0.35

Table 2.3 lists typical values for some abrasive materials. Table 2.4 displays

a commercial technical data and physical characteristics sheet for a typical blast

cleaning abrasive material.

Abrasive particles contain structural defects, such as microcracks, interfaces,

inclusions or voids. Very often, these defects are the result of the manufacturing

process. Strength and fracture parameters of materials can be characterised through

certain distribution types. A widely applied distribution is the Weibull distribution,

and it was shown by Huang et al. (1995) that this distribution type can be applied to

abrasive materials. The authors derived the following relationship between fracture

probability, particle strength and particle volume:

F(σF) = 1 − exp

−VP ·

σF

σ∗

mW



(2.1)

The strength parameter σ* is a constant, which is related to the defects distri￾bution. The power exponent mW is the so-called Weibull modulus; it can be read

from a graphical representation of (2.1). Low values for m indicate a large intrinsic

variability in particle strength. A Weibull plot for aluminium oxide abrasive par￾ticles, based on the results of compressive crushing tests, is displayed in Fig. 2.1.

Values for the Weibull modulus estimated for different abrasive materials are listed

in Table 2.5. There is a notable trend in the values that both fracture strength and

Weibull modulus drop with increasing particle size. Therefore, scatter in strength

of abrasive particles can be assumed to be wider for larger particles. The relation￾ship between abrasive particle size and fracture strength of the particles is shown in

Fig. 2.2. This phenomenon can be explained through the higher absolute number of

defects in larger particles. The probability that a defect with a critical dimension (for

example, a critical crack length in a fracture mechanics approach) exists increases

with an increasing number of defects.

This effect was also observed by Larssen-Basse (1993). This author found also

that the Weibull modulus of abrasive particles depended on the atmospheric humid￾ity. Larssen-Basse (1993) performed crushing tests with SiC-particles, and he found

that, if humidity increased, the Weibull modulus and the number of fragments both

2.2 Abrasive Material Structure and Hardness 9 Table 2.2 Selected abrasive properties (References: manufacturer data) Brand name Bulk densitya in t/m3 Apparent density in t/m3 Hardnessb–e Melting point in

◦C

Grain size

min–max in mm

Major composite

in %

Technical name

Abrablast – 4.3 9b 1,900 – Al2O3 (71.9) Zirconium corundum

Abramax 1.0–2.0 3.95 2,200c 2,000 – Al2O3(99.6) Corundum

Abrasit 1.1–2.3 3.96 2,100c 2,000 – Al2O3 (96.4) Corundum

Afesikos 1.4 2.6 8b – 0.04–1.4 SiO2 (53) Aluminium silica

Afesikos HS 2.83 4.1 8b – 0.04–1.4 SiO2 (36) Garnet

Afesikos SK 1.8 3.96 9b – 0.06–2.8 Al2O3 (99.3) Corundum

Asilikos 1.3 2.5–2.6 7–8b – 0.06–2.8 SiO2 (51) Aluminium silica

Cast steel – – 60d – 0.12–3.36 – –

Garnet – 3.9–4.1 8–9b 1,315 – SiO2 (41.3) Garnet

Glass beads 1.5 2.45 6b – 0.07–0.4 SiO2 (73) –

GSR 3.7–4.3 7.4 44–58d – 0.1–2.24 – Cast steel

Cast iron 2.7–4.3 7.4 56–64d – up to 3.15 – –

Ceramic spheres 2.3 3.8 60–65d – 0.07–0.25 ZrO2 (67) Ceramics

MKE 1.75 3.92 1,800–2,200e – 0.001–2.8 Al2O3 (99.6) Corundum

Olivine 1.7–1.9 5.3 6.5–7b 1,760 0.09–1.0 MgO (50) –

Scorex 1.35 – – – 0.5–2.8 SiO2 (40) Refinery slag

Steel grit – 7.5 48–66d – 0.2–1.7 – –

Steel shot – 7.3 46–51d – 0.2–2.0 – –

Testra 1.2–1.4 2.5–2.7 7b – 0.09–2.0 SiO2 (54) Melting chamber slag

aDepends on grain size

Hardness parameter: bMohs; cVickers; dRockwell; eKnoop

10 2 Abrasive Materials

Table 2.3 Structural properties of abrasive materials (Vasek et al., 1993)

Material Damaged grains (%) Lattice constant (A) Cell volume ( ˚ A˚ 3)

Almandine 5–60 11.522 (0.006) 1,529.62

Spessartine – 11.613 (0.005) 1,566.15

Pyrope – 11.457 (0.005) 1,503.88

Grossular 30 11.867 (0.005) 1,671.18

Andradite 80–90 12.091 (0.009) 1,767.61

increased. This feature can be attributed to moisture-assisted sharpening of the tips

of surface defects present in the particles.

The presence of defects, such as cracks and voids, affects the cleaning and

degradation performance of abrasive materials. Number and size of defects are,

Table 2.4 Data sheet for a garnet blast cleaning abrasive material (Reference: GMA Garnet)

Parameter Value

Average chemical composition

SiOa

2 36%

Al2O3 20%

FeO 30%

Fe2O3 2%

TiO2 1%

MnO 1%

CaO 2%

MgO 6%

Physical characteristics

Bulk density 2,300 kg/m3

Specific gravity 4.1

Hardness (Mohs) 7.5–8

Melting point 1,250◦C

Grain shape Sub-angular

Other characteristics

Conductivity 10–15 mS/m

Moisture absorption Non-hydroscopic

Total chlorides 10–15 ppm

Ferrite (free iron) <0.01%

Lead <0.002%

Copper <0.005%

Other heavy metals <0.01%

Sulphur <0.01%

Mineral composition

Garnet (Almandine) 97–98%

Ilmenite 1–2%

Zircon 0.2%

Quartz (free silica) <0.5%

Others 0.25%

aRefers to SiO2 bound within the lattice of the homo￾geneous garnet crystal (no free silica)