Sample Preparation Effects on Data Quality in Total Organic Carbon (TOC) Analysis of Bauxite Ore Samples0 pages

Application Note 37760312

Keywords
Aluminum
Bauxite
Bayer Process
Total Organic Carbon (TOC)
1030S Solids Module

Sample Preparation Effects on Data Quality in
Total Organic Carbon (TOC) Analysis of Bauxite
Ore Samples

Introduction
Aluminum is one of the most industrially important materials on the
earth, it has a multitude of uses, and it is relatively abundant. It is chiefly
obtained from its ore, bauxite, using the Bayer process. Caustic liquor is
used to extract Al from bauxite, and the presence of inorganic and organic
carbon in the extraction causes problems.(1, 2) First, carbonate ions rob
sodium ions from the caustic, reducing its effectiveness at extraction. The
presence of organic carbon compounds can interfere with the
precipitation of alumina trihydrate, (Al2O3)-3H20, after the extraction.
Small organics can also be converted to the sodium salts of oxalic acids,
which will coprecipitate with the alumina trihydrate, contaminating it
with sodium.
Bauxite ores vary in the proportion of inorganic and organic carbon
present, so assays of the ore are conducted to tailor the parameters of the
Bayer process for each load of ore. This determination is usually carried
out by combustion TOC measurements on representative samples of the
ore. Total carbon (TC) is measured by combusting a dry, weighed sample
of the ore, whereas total organic carbon (TOC) is measured by first
removing inorganic carbon from the sample by treatment with acid, then
combusting the sample. The amount of total inorganic carbon (TIC) is
determined arithmetically from those two values as shown in the
following relation.

TIC = TC – TOC
A number of factors can have an effect on the quality of data obtained
from combustion TOC of a solid sample of bauxite. Three such factors:
TIC removal method, sample size, and catalyst conditioning were
examined in this study.

Presented at the 2012 Pittsburgh
Conference on Analytical
Chemistry and Applied
Spectroscopy, Orlando, Florida,
March 11-15, 2012

Experimental
Three homogenized samples of bauxite ore, BXT-A, BXT-B, and BXT-C,
were obtained from a bauxite mine customer. The samples were dried in
an oven at 105 °C to eliminate water, and for TC measurements, no
further preparation was performed. For TOC measurements, two separate
TIC-removal methods were employed. In the in situ TIC removal method,
a roughly 80-mg sample of bauxite was added to a freshly-dried, closedbottom, approximately 1-cc quartz crucible, half filled with tufts of quartz
wool. Enough 2N HCl was added to the crucible to fill it, and the
crucibles were allowed to equilibrate for 20 minutes at room temperature,
then they were held at 105 °C for at least 3 hours until all liquid was
removed. The crucibles were massed after drying, after loading with