Chemical differentiation of products of crushing  by the grain-size fractions: example of Kovdor magnetite ore

Zaitsev V.A., Rosсhina I.A.

Vernadsky Institute of Geochemistry and Analytical Chemistry

 

Sample selection and sample preparation are the most critical stages in the analysis of a rock - this sentence has become a truism of geochemistry. The process of quartering, reducing the weight of a sample without reducing representativeness of this sample was many times described in textbooks for students. However it sometimes happens, that after the crushing of a sample only the fine fraction is used for the chemical analysis, and the coarse one is used for the separation of pure mineral fractions.

The danger of this approach was emphasized in many publications but it was never done in terms of quanttive analyses. In this connection it is interesting to compare the composition of different grain-size fractions and to estimate the possible error resulting from the irregular sample preparation.

For example, we carried out the experiment with a magnetite ore from the Kovdor deposit. The ore consists of magnetite and calcite (the main minerals), phlogopite and apatite (minor) and baddeleyite (accessory).

The sample was crushed in an iron mortar and divided into the grain-size fractions: +1 mm, -0.5+1 mm and  -0.5 mm. Each fraction was powdered in an agate mortar. The results of the X-ray fluorescent analyses are given in Table 1.

 

Table 1. The chemical compositions of grain-size fractions after the crushing of one sample of a magnetite ore.

Фракция:

"+1"

"+0.5-1"

"-0.5"

max/min

SiO2

4.35

6.63

7.28

1.7

TiO2

0.24

0.16

0.15

1.7

Al2O3

2.13

1.86

1.49

1.4

Fe2O3*

73.71

57.81

43.45

1.7

MnO

0.30

0.28

0.25

1.2

MgO

9.14

11.92

14.30

1.6

CaO

5.03

11.33

17.13

3.4

Na2O

0.23

0.24

0.21

1.2

K2O

0.07

0.11

0.09

1.4

P2O5

0.14

0.05

0.07

2.6

SO3*

0.11

0.16

0.13

1.5

CO2 (as LOI)

4.40

9.78

15.21

3.5

V

0.031

 

0.029

1.1

Co

0.0131

0.0115

0.0083

1.6

Sr

0.037

0.073

0.1052

2.8

Zr

0.0195

0.0383

0.0773

4.0

Nb

0.005

 

 

 

Cu

0.0105

0.0175

0.007

2.5

Zn

0.0304

0.0233

0.0157

1.9

Ni

0.0107

0.0103

0.0079

1.4

Y

0

0.0008

0.0003

 

Ba

0.024

0.0313

0.077

3.2

As

0.0009

0.0008

0.0007

1.3

Pb

0.0019

0.0018

0.0011

1.7

Cr

 

 

0.0078

 

 

100.03

100.54

100.08

 

* All iron as Fe2O3

 

If one looks at this table he can decide that it contains the analyses of three different samples. The coarsest fraction is enriched in iron, alumina, titanium and zinc (mainly concentrated in magnetite) and also in phosphorous (mainly concentrated in apatite). The finest fraction is enriched in magnesia, calcium, strontium and CO2, mainly concentrated in calcite and in zirconium, which forms its own mineral baddeleyite. The medium-size fraction is enriched in potassium, sulphur and copper the elements, concentrated in phlogopite and sulphides.

The last column of the table characterizes the degree of elements distribution as the ratio of maximum concentration/ minimum concentration.

The most contrast distribution show the elements, which concentrate in the finest fraction: Zr concentrates four times as much, Ca, Ba, CO2 more than triple. For other elements the concentration ratio is rarely less than 50 %.

After all, we can conclude, that the difference of composition of the grain-size fractions can result in bigger and more significant errors in estimation of a rock composition than any analytical errors of chemical analyses. Our experiment shows that grain-size separation after the crushing can be effective mechanism for mineral concentration, at least for carbonatite-related ores, and can be regarded as effective way of ore-dressing.

 

This study was funded by grants of President of Russian Federation MK-860.2008.5, grant RFBR 09-05-90436 and 08-05-00054.


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