Use of macadamia nut shells in the tin production and their effects on the slag
MK Wa Kalenga
School of Mining, Metallurgy and Chemical Engineering Faculty of Engineering and the Built Environment,
University of Johannesburg
P.O.BOX 17011, Doornfontein 2028, South Africa
Corresponding author: [email protected]
Abstract. Conventional reductants have been and are still being used for most of the high temperature reduction processes. However, new reductants are being sought for.
Macadamia nut shells were used as reductant in this investigation and their effect on the slag. Cassiterite containing Nb and Ta have been used in this investigation. The basicity of the feed was kept at 1 through the addition of CaO-bearing material as a flux. The raw macadamia nut shells were characterized using proximate and ultimate analysis, XRF, XRD, SEM-EDS and FTIR. The ore and flux were analyzed using XRF, XRD and SEM-EDS. Experiments were conducted using an alumina tube furnace. The temperature was set at 1450 oC, a graphite crucible placed into a silica crucible was used for the experiments. Reduced products were characterized and compared to the products currently obtained using conventional reductants. Products obtained when raw macadamia nut shells were used and when using coke were compared. Results show that macadamia nut shells present great potential to use as reductants in tin industry on condition of further investigations on optimization. The slag produced presented a more needle-like structure as opposed to the conventional process that presented a glassy slag.
Keywords: Metallurgy, Tin, production, macadamia use 1 Introduction
Tin industry has become very attractive due to High Technology progress in telecommunication and computer industry that has increased the demand in tin and its products. A comparison study between the oxidation of coconut charcoal mix with CO-CO2 and the reduction of SO2 with coconut charcoal was made and the rate of reduction established [1]. Carbothermic reduction of cassiterite in ionic melt of NaCO3-NaNO3 salt was conducted a temperature range of 600-9500C. The process resulted a 95%
of tin in the crude alloy [2]. A study was conducted on the Abu Dabbab concentrate using char coal in a muffle furnace at 1200oC after preheating. It was found that using charcoal as a reducing agent of cassiterite concentrate in the melt of Na2CO3- NaNO3 salt system provided a high yield of above 95%
tin metal using carbon stoichiometry of 0.75X [3]. The replacement of coal with biomass has been tried in pig iron production and its assessment on the environment conducted [4, 5]. Although it is always
said that Carbothermic reduction smelting using charcoal, coke is a very effective way to reduce oxides [6], it remains open to try and assess other materials that may lead to if not the same but better results and reducing environmental threats. Tin has been recovered from obsolete computers using hydrometallurgical process. The feed was leached using using different leaching agents [7]. The use of biomass under argon has been tested on the manganese ore and have shown good prospects [8]. This investigation constitutes a preliminary study in the reduction of cassiterite and investigates the possibility of replacement traditional coal, charcoal use with biomass.
2 Experimental
The tin ore used in this study has been provided by Congo Premier from the Republic Demoacratic of Congo. The macadamia nutshells were sourced from South Africa. The ore was crushed and milled to - 75 microns. The ore was analyzed using XRD for the mineralogy of the sample, XRF for chemical composition and SEM/EDS. Proximate and ultimate nalysis have been used to analyse the biomass.
XRD, XRF and FTIR were also used as analytical technique. Two reduction experiments were conducted in this study. The first was conducted using raw macadamia nutshells only under argon gas and the second using raw macadamia nutshells under argon gas and a comparison was conducted thereof.
The manganese ore was mixed with the required amount of macadamia nutshells and milled together with the aim to homogenize the feed. A graphite crucible was used for the experiments. The crucible was weighed before and after the experiment to assess whether there was any reaction between the crucible and the manganese ore. The graphite crucible was placed into a silica crucible to prevent any possible spillages from any probable defection of the graphite crucible. The feed was then placed into an alumina tube furnace that was set to reach a temperature of 1450oC under either argon or carbon monoxide depending on what experiment as described above. After reaching the desired temperature, the sample was kept for one hour and the furnace was switched off keeping the gas flowing until room temperature. The product was then removed from the furnace, weighed and prepared for characterization.
Experimental set up
2 Results and discussion
2.1 Materials
2.1.1 Tin Ore. The chemical composition of the tin ore used in this investigation is provided in Table 1 below. From the Table 1, it can be seen that the ore is acidic. However the tin content is high, thus the ore is of good quality and is expected to produce quality products that might be of great utilization.
Table 1. Chemical composition of the tin ore Comp
ounds
MgO Al2O3 SiO2 P2O5 K2O TiO2 MnO Fe2O3 ZrO2
%mass 0.13 3.20 0.40 1.30 0.30 1.30 0.30 7.39 1.60 Comp
ounds Nb2O5 SnO2 CeO2 Ta2O5 La2O5 WO3 PbO ThO2
%mass 0.59 75.6 0.79 0.79 0.30 0.20 0.02 0.04
0 20 40 60 80 100
0 100 200 300 400 500 600 700 800
csc k c
q
c pc
k k
k s
p
p
p c
c f qf
Intencity (cps)
2 theta (degree)
q:quartz k: cassiterite s: sillimanite t:tapiolite f:ferrotapiolite c:calcium iron oxide p:pyrochlore
f
p
s k
Figure 1. XRD analysis of the tin ore.
From Figure 1 it is observed that cassiterite is the major pick amongst all. The presence of tapiolite, ferrotapiolite, calcium iron oxide and pyrochlore is confirmed.
Table 2. Proximate and ultimate analysis of the macadamia nutshells Parameter
Proximate analysis Ultimate analysis
Macada mia nutshells
Fixed C Volatiles Ash Moisture H2 N2 C CV(MJ/kg) 19.2 72.6 0.7 7.5 5.9 0.2 48.7 18.5
*AR: As Received, CV: Calorific value, Fixed carbon by difference [100 - (Volatile matter + Moisture + Ash content)]
From Table 2, the amount of fixed carbon is 19,15%. This could lead to the use of a considerable amount of macadamia to provide the required amount of carbon for reduction.
Table 3. Chemical composition of the raw macadamia nutshells
% MgO Al2O3 SiO2 CaO K2O SO3 Na2O LOI%
Nutshells 0.01 0.07 0.20 0.08 0.09 0.04 0.01 99.50
From Table 3 it is observed that silica, alumina as well as MgO are in low amounts. This shows that the basicity of the feed will not be impacted. Therefore we consider the basicity of the ore as the basicity of the feed.
Figure 2. SEM-EDS analysis of raw macadamia
The spectra of the chosen points in Figure 2 have their chemical compositions provided in Table IV below. The observation is that carbon is the major component compared to other elements. However, the volatile materials combined with moisture is in considerable amount.
Table 4: Analysis of the spectra
Sample Spectrum C K Si Ca Mg
1 70.67 - - - -
2 71.27 - - 0.07 -
3 77.57 0.42 0.06 0.96 -
4 74.77 0.71 - 0.06 0.32
It is important to mention that the same macadamia nutshells have been used in all papers presented at COMAT 2018.
Figure 2. SEM of the product after reduction with raw macadamia
From Figure 2, the grey areas are tin whereas the dark areas are the typical slag that has formed.
However, the right Figure shows a white spot which is tin and the dark areas representing the slag. It is seen that the slag is flaky in shape. It is believed that time to allow good settlement of metal would allow better separation between the slag and the metal although it is noticed that there is a clear demarcation between the metal and the slag. However, it was noticed a great decrease in mass after reduction. This was attributed to high amount of volatile matters that are present in the macadamia nutshells. Also, the mass of the graphite crucible used decreased by only 0.08 g, this led to conclude that there was no interaction between the crucible and the oxide tin ore. The reduction that took place was mainly led by the presence of carbon fixed in the macadamia nutshells.
Table VI.Chemical composition of the metal Element Sn Fe
% mass 95.7 4.30 3 Conclusion
1) The preliminary results on the use of raw macadamia nutshells in tin production show a very good prospect. Due to low carbon fixed, the required mass of macadamia is relatively high, hence leading to a high volume of the feed. However, there is still room for improvement on the quality of the metal and the slag. The metal quality needs to be improved as well as the quality of slag essentially with regard to Nb and Ta.
2) The immiscibility between the metal and the slag can be improved during optimization of the process. The increase of settling time as parameter may be of great contribution in the metal- slag separation. The miscibility of slag and metal might be attributed to the sticky macadamia during fusion. Also, the high amount of macadamia needed due to low carbon fixed content led to high volume of slag. The two above mentioned slag behavior might have played a big role in the miscibility of the slag and metal. An appropriate flux and optimum conditions need to be investigated.
Acknowledgements
The authors would like to express their gratitude to Congo Premier SPRL for providing the tin sample that made the present investigation possible
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