Abstract
This project aims at investigating the reduction behavior of a South African manganese ore,
Nchwaning, reduced in hydrogen presence and biochar generated from biomass palm nutshells
as reducing agents. The aim was achieved by conducting four sets of experiments whereby the
ore was reduced using biochar and hydrogen as reducing agents in the range of 1200- 1500 °C.
The first set of experiment was conducted using biochar only in a temperature range of 1200-
1400 °C, using Argon as a carrier gas. The second set was conducted at 1500 °C, using biochar
in 10,20 and 30% excess, to study the impact of biochar on the manganese reduction behavior.
The third set of experiments was performed in a between 1200-1400 °C, using hydrogen gas at
different hydrogen ratios (H2/Ar: 25/75, 50/50, and 75/25), in the absence solid carbon
(biochar). Finally, the fourth set of experiments was conducted between 1200-1400 °C. The
reduction experiments were conducted using biochar and hydrogen.
The reduction behavior and extent were calculated based on the weight loss recorded after
reduction. The chemical composition of the reduced samples and the phase analysis was
examined through XRF and SEM/EDS and Scanning probe microscopy (EPMA) analysis
coupled with WDS analysis respectively.
The biochar palm nutshells were produced trough pyrolysis of raw biomass palm nutshells.
The pyrolysis parameters were temperature, residence time, and biomass particle size. A
correlation between the pyrolysis parameters and biochar quality was established. The biochar
quality was assessed trough proximate analysis, sulfur content measurements, porosity, and
calorific value. The ash chemical composition was determined by XRF analysis. The results
revealed that increasing the pyrolysis residence time and temperature and decreasing the
biomass palm nutshell particle size correlated with an increase in the calorific value, fixed
carbon content, and porosity as opposed to sulfur content. The biochar Ash content increased
as the temperature and residence time were increased The optimum conditions were found to
be 900 °C, 60 minutes and biomass particle size of 5 mm. The volatile matter, fixed carbon
content, moisture, and ash content of the resulting palm nutshell biochar were 9.23%, 88.57%,
1.12%, and 1.09 respectively. The calorific value, and sulfur content measured were 32.47
kJ/kg 0.0410 % respectively.
A comparison of the reduction conducted using biochar and the reduction conducted in
Hydrogen gas presence had shown the following:
v
• The recorded weight loss was the highest in the reduction using biochar in hydrogen
presence. At 1200 °C , within 120 minutes of reduction a weight loss of 8.45, 9.12, and
9.57 g respectively while the weight loss recorded at 1200 °C in the reductions using
biochar only was 7.94 g. At 1400 ° C, in the reduction using biochar in hydrogen
presence, the weight loss obtained were 10.21, 10.67, and 11.1 g respectively. The
weight loss obtained at 1500 °C in the reduction with excess of biochar were 10.67,
10.26 and 9.31 g for 10,20 and 30 % biochar excess respectively. Addition up to 30%
biochar excess was found to give significantly lower reduction extent.
• The manganese reduction rate of MnO was affected not only by the reducing agents but
by the biochar content and reduction temperature as well. The sample reduced in
absence of hydrogen and excess biochar content showed the slowest reduction rate
while the reduction in hydrogen presence and biochar was the fastest. The addition of
hydrogen in the reductions using biochar improved the rate of reduction and increasing
the temperature also showed a positive impact.
• The SEM micrographs confirmed the formation a metallic phases for the reductions
with biochar. The Mn/Fe in varied from 4.51, 9,55 to 12,86 for the reductions using
biochar at 1400 °C, for 30, 60 and 120 minutes of reduction. For the samples reduced
in hydrogen presence without the presence of biochar, the results revealed that the
metallic phase was Fe-rich with negligeable amount of manganese, indicating that no
formation of metallic manganese had taken place. For the reduction using biochar in
hydrogen presence, the Mn/Fe varied from 8.28, 8.58, to 13.09 from the reduction in
25/75 gas atmosphere. In the 50/50 and 75/25 the Mn/Fe ratio varied from 10.53, 9,65
and 14.18, 10.99, 12.40, and 12.38 respectively. The phase evolution in the reductions
with 10,20 and 30 % biochar were examined by electron probe microscopy coupled
with WDS. The Mn/Fe ratio and metal/slag separation was significantly affected by the
biochar content.
The optimum reductions conditions were then established as follows: 1400 °C was the
favorable temperature with higher weight loss, metallization degree and reduction rate.
Although temperature had a great effect on the reduction, it had shown little impact on the
reduction in the absence of biochar. The hydrogen ratio showed a minimal effect at 0 and 25%,
the highest extent was obtained from 50%. Going up to 75 % improved the reduction to a
greater extent. The biochar content did not affect the process beyond 20% excess.