Recovery of Cassiterite and Topaz Minerals from an Old Metallurgical Dump, Eastern Desert of Egypt


 Huge amounts of tailing dumps as a result of mines’ blasting operations were impacting both economical and environmental problems. This issue was in serious need to be treated with suitable solutions. Evaluation of one of these tailing dumps in the Eastern Desert of Egypt showed the presence of reasonable amount of cassiterite mineral reaching 0.199%. The mineral was found as finely disseminated particulates within varieties of quartz-feldspar-hornblende-biotite granitic formations. In the present study, the processing regime relied upon the synergy between reaching liberation size, and mineral over grinding due to its extreme brittleness. However, delicate grinding via attrition scrubbing was adopted to produce − 0.51 + 0.074 mm attrition product with fine fractions, reaching 62.31% and 37.59%, respectively. The recovery of cassiterite from the − 0.50 + 0.074 mm size fraction was accomplished by the physical difference between mother granitic formations that shielded the mineral grains. Under these conditions, joint shaking table/dry high intensity magnetic separation techniques were conducted to recover cassiterite mineral. The CCD statistical system was used as a mathematical approach to optimize the effect of the main working parameters of the magnetic separator, i.e., splitter inclination angle, and belt speed, and their interactions on the cassiterite recovery of the final concentrate. The suggested flow sheet succeeded to recover cassiterite mineral with a grade reaching 11.25% SnO2 with 94.08% operational recovery from a feed contained 0.19% SnO2. These results are highly imperative to achieve applicable processing flow-sheet of such kind of minerals’ secondary resources.


Introduction
Cassiterite is considered to be the only economically significant tin mineral in the earth's crust. The mineral is hard and heavy, but also extremely brittle. Cassiterite has the chemical formula SnO 2 (78.8% Sn, and 21.2% O 2 ), density of 7.02 g/cm 3 , and Mohs hardness 6 -7. Cassiterite is of black, brown-black, yellowish brown, reddish brown coloration. It is nonmagnetic and conductive for electricity [1] [2] [3] [4].
Cassiterite mineral is almost enriched in the "tin granites", which contain approximately 15 -50 g Sn/ton. It is found also as "mine tin" in domes of granite intrusions, in feldspar-topaz granites, and titano-magnetite deposits. Because it is very resistant to all weathering influences, it is a typical placer mineral. There are no specifications for commercial concentrates of cassiterite because it is always smelted directly, depending on the concentration, without further treatment [5] [6].
However, its high hardness property is accompanied by the unfortunate quality of extreme brittleness. This factor must be considered during the size reduction operations prior to concentration. Therefore, the general policy is that cassiterite grains should, where possible, be recovered at the earliest possible stage and at their largest size to avoid softness that is difficult to handle [7] [8].
It is obvious from the physical characteristics of cassiterite that its high relative specific gravity in relation to those of accompanied minerals constituents, in most cases, makes it an ideal mineral for the application of gravity-separation techniques. However, development of cassiterite beneficiation techniques has been carried out using flotation, electro and bio-flotation [9]- [23]. Sometimes, cassiterite was also could be separated using different magnetic separation techniques [24]- [33]. Microwave pre-treatment and hydrometallurgical methods were adapted also to recover cassiterite mineral and extract the tin metal [34]- [41].
However, all mines generate waste which are being stored at or near the mine site itself. Mine site rehabilitation can be expensive, and often the burden falls on the taxpayer rather than the mining company. This burden could be minimized if mining companies change their perception and start to view these disused materials not as waste, but as potential resources. Among these mines, are those locating in the Egyptian Eastern Desert that contains very large quantities of mine blasting remnants which are accumulated after different mining operations ( Figure 1) [1]. Many scientific studies have succeeded to a large extent in developing research plans to concentrate many important minerals from these wastes such as gold, silver, copper, lead and zinc, and other important ores.
However, very few studies have been devoted to recovering the mineral cassiterite. Therefore, this study aims to assess the extent to which a sample of these mining tails of cassiterite ores is capable of upgrading by simple physical techniques to obtain from them economically acceptable products for complementary industries.

Methodology
A technological sample from the tailing dumps of the Eastern Desert of Egypt was supplied for evaluation and treatment. The sample was about −6 mm in size.
It was subjected to elemental evaluation using XRF unit model PANalytical-Axios, and Perkin-Elmer Analyst 200 atomic absorption units. Phase analysis was applied using X-ray diffract-meter model "pw 1010" with CuKα radiation under target voltage 40 kV and current 30 mA in a scanning rate of 5˚ 2θ/min. Scanning electron microscope (SEM) type JEM-1230, JEOL equipped with energy dispersive X-ray (EDX) spectrometers was used to evaluate the final products.
A representative sample at the size fraction −6.0 + 2.28 mm from the original sample (14% by wt.) was divided into different cuts according to color. Polished sections of these samples were prepared for reflected light microscope investigation. Size reduction of the sample was conducted via stirring ball milling technique using wet batch laboratory mill Union Process "Attritor" model 1S, with working capacity 4.2 liters. The attrition time was conducted for 15 min as three steps, in closed circuit with 0.50 mm vibrating screen. The stirring speed was 1500 rpm, and the solid/water/balls ratio was 1/1/1 by wt. (each 800 g). The grinding zirconium ball diameter was 2 mm. The wet ground product was further screened on 0.074 mm sieve. Both the over and under screens products were collected, dried, and directed to evaluation. The under screen −0.074 mm fines S. S. Ibrahim et al.
were kept aside for a separate study. The over screen product was further divided after dryness into −0.51 + 0.21 mm and -0.211 + 0.074 mm fractions for gravity and magnetic separation treatment.
The size fractions −0.51 + 0.211 mm and −0.211 + 0.074 mm were subjected separately to a "Wilfley" shaking table. Throughout the experiments, the samples were kept in suspension at 20% -25% solids by weight with continuous mechanical stirring. However, the slurry was fed onto the inclined table surface at feeding rate of 1.30 kg/min. The wash water flow rate was kept at 20 l/min with table speed 300 rpm. Stroke length and deck inclination were optimized throughout the separation study. The middling and light products were further subjected to tabling separation at relatively lesser wash water flow rate, lesser tilt, and a shorter stroke length. An increase in the wash water flow rate showed separation improvement. As wash water flow rate increased, the transport of light minerals to the tailings fraction increased which in turn improved the separation. Improving in the heavy fraction was noticed at higher level of both deck tilt angle and feed flow rate [23] [35] [36] [37] [44].
At the end of shaking table process, various products were collected, dried, and directed to evaluation. Heavy and the two middling products were fed separately onto laboratory "Eriez" rare earth roll (RER) dry high intensity magnetic separator at permanent field strength of 21,000 gauss. Feeding rate was adjusted at 2 kg/ hr. Box-Behnken Design (BBD) [45] was constructed to optimize the separation process. The working variables were the separator splitter inclination angle and the roll belt speed. On the other hands, cassiterite recovery% was the sole response of the design ( Table 1). The products after magnetic separation were directed to evaluation.
Software package, Design-Expert 6.0.5, Stat-Ease, Inc., Minneapolis, USA, was used for regression analysis of experimental data and to plot the response surface. Analysis of variance (ANOVA) was used to estimate the statistical parameters. The extent of fitting the experimental results to the polynomial model equation was expressed by the determination coefficient, R 2 . F-test was used to estimate the significance of all terms in the polynomial equation within 95% confidence interval.   Figure 2). According to XRD pattern, semi quantitative analysis, these phases represented about 27.30%, 22.8%, 12.30%, 21.90%, 11.70%, and 4.00%, respectively, of the original sample ( Figure 2). The sample could be grouped according to density into light minerals and heavy minerals. The lights included albite, quartz, clinochlore, and talc with average density 2.65 g/cc. On the other side, heavy minerals included hornblende, tremolite, and biotite with average density 3.50 g/cc. Rare earth oxides e.g. ZrO 2 , Rb 2 O, SrO, and Nb 2 O 5 assayed 0.021%, 0.014%, 0.014%, and 0.002% were identified ( Table 1). Amounts of nickel, copper, and zinc oxides were also detected ( Table 2). Size/chemical analyses of the original sample (as received) are shown in Table 3. It is clear that cassiterite showed concentration in SnO 2 reaching 0.39% -0.25% in 0.59 -0.074 mm fractions (Table 2). Fewer cassiterite concentrations assayed 0.17% to 0.11% SnO 2 were noted in coarser fractions 6.0 -0.83 mm, and in the finer fraction −0.074 mm ( Table 3). The evaluation of the −6 + 2 mm fraction as major and minor components according to weight% was shown in Figure 3(A) and Figure 3(B), respectively. The major components represented about 58%, 18%, 9%, 5%, and 3%, by wt. of the fraction sample, respectively ( Figure 3(A)). On the other hands, the minor components 6, 7, and 8 represented about 4% by wt. of the overall fraction sample −6.0 + 2.1 mm, where few particles of components 9 (mainly metallic particles), and component 10 (mainly mica) were found (Figure 3(B)). The semiquantitative XRD analyses of these components were shown in Table 4.

Results
On the other hands, Table 5 illustrates the size/chemical analyses of the ground product after the attritor. It was noted that low content of fines below 0.074 mm reaching about 15% by wt., were produced after the attritor milling process ( Table 5). The constitution of these fines besides cassiterite may include the soft minerals e.g. talc and clinochlore.      (Table 6). On the other hand, the silica and alumina contents assayed 46.16% SiO 2 , and 10.04% Al 2 O 3 , respectively ( Table 6). The increase in content of these oxides may be attributed to the presence of tremolite, hornblende, and biotite granites in the sink fraction. Meanwhile, SnO 2 content increased from 0.199% in the original sample to 0.76% in the sink product (Table 6). This showed that almost all the cassiterite content in the original sample was concentrated as inclusions of different concentrations within the matrices of these heavy minerals.
Petrography investigation of polished sections of these samples' cuts showed that cassiterite inclusions were found almost throughout all the minerals matrices granitic formations in different concentrations and with various diameter sizes between 120 microns to 5 microns as shown in Figure 4 as follows:  Thin veins and grains of cassiterite (from 20 to 120 um) found in the fractured quartz (Figure 4(A)).  Patches of cassiterite (from 30 to 5 um in diameter) coating the surface of biotite (Figure 4(B)).  Finely disseminating elongate crystals of cassiterite (from 10 to 5 um diameter) found in groundmass of Hornblende and Biotite (Figure 4(C)).

Cassiterite Enrichment Using Shaking Table
Several exploratory experiments were conducted to separate the −0.51 + 0.21 mm fraction sample using "Wilfley" shaking table. It was remarked that the feeding rate 1.30 kg/min with 25% solid and wash water flow rate 20 l/min were suitable working conditions to proceed reasonable separation on the table surface. By applying these conditions at table inclination 3˚, there was no notable qualitative improvement on the segregation process (which was easily to follow up due to the pronounced differences in color) at various stroke lengths 2.0, 2.5, 3.0 cm. By increasing the table inclination to 4˚ and 5˚, no remarkable changes were noticed ( Figure 5). However, at table inclination 5˚ and stroke length 2.5 cm, the wash water flow rate seemed very effective on the separation process. It was noticed that by increasing the water flow rate to 25 l/min, an improving in the separation process was occurred ( Figure 6). This information proved that the wash water flow rate was a detrimental parameter on the table separation efficiency in this case study. As the wash water flow rate increased, the transport of light minerals to the tailings fraction increased which in turn improved the grade of the heavy concentrate portion. however, a remarkable narrow black strip fraction was observed at the far end of the deck towards the wash water source was produced at table inclination 5˚, stroke length 2.5 cm, and table speed 280 -300 rpm.
In case of applying the finer fractions −0.21 + 0.074 mm, the suitable stroke length was reduced to 2 cm with increased table speed up to about 320 -330 rpm. Usually the increase in the length of the stroke required a decrease in the number of strokes per minute and vice-versa to achieve efficient separation. High amplitude was necessary when treating relatively coarse particles in order to create complete dilation along with lower acceleration [

Statistical Optimization of RER Separation of Cassiterite Concentrates
The statistical design summary and results of the rare earth roll (RER) separator process in terms of cassiterite recovery% as a response are shown in Table 8 and where, A is the splitter inclination angle, and B is the separator belt speed. The optimization of the equation was performed using State-Ease program, by an iteration method. Statistical testing of the model has been carried out by F-test to produce ANOVA-the analysis of variance (Table 10). The values of R 2 and the standard deviation suggested that was in a good agreement between the experimental and predicted values obtained from the model (Figure 9). On the other hand, the effect of both variables and their interaction effect on the mineral recovery were shown in Figure 10 and Figure 11 respectively. Response surface for cassiterite recovery% as function of the separator variables splitter angle, and belt speed was illustrated in Figure 12.
Results showed that with increasing both splitter angle and belt speed to 72.50˚ and 150 rpm, cassiterite recovery increased to reach its maximum at 97% ( Figure 13). It was noticed that at low belt speed of 100 rpm, the increase in splitter angle was accompanied by gradual increase in cassiterite recovery from 86% to 92%, and then to 96%. However, at belt speed of 150 rpm, the change in splitter angle showed promising recovery reaching 90% at 70˚, and then the recovery increased to reach 97% at splitter angles 75˚. Additionally, the same trend was shown by increasing the belt speed to reach 200 rpm ( Figure 13).
The effect of the feeding material particle size on the efficiency of the rare earth roll separation showed that when nonmagnetic particles travel over a roll and allowed to drop unhindered, they were classified by their particle size, i.e.
large particles travelled further from the centerline of the roll than smaller particles. Therefore, large particles are typically processed using lower surface speeds      than small particles. When a magnetic roll is used, strongly attached magnetic particles are being usually pinned to the roll surface until they are released from the magnetic field. Weakly attached magnetic particles may only be deflected by the magnetic field, altering them from their normal path. When this occurs, there will be an overlap in the large weakly attached magnetic particles and the small nonmagnetic particles. If the splitter is set to eliminate the large weakly attached magnetic particles, many of the small nonmagnetic particles will report to the magnetic product. On the other hand, if the splitter is set to recover the small nonmagnetic particles, the nonmagnetic product will contain many of the large weakly attached magnetic particles. This is an indication that the particle size range of the feeding material is too great. The problem can normally be overcome by screening the feed before magnetic separation and thus produce a bet-

Conclusions
A technological sample from the mine's scraps of the Eastern Desert of Egypt was subjected to chemical and mineralogical investigation. The sample was below 6 mm in size. It was shown to contain 0.199% cassiterite mineral (SnO 2 ).
Cassiterite mineral was found in different sizes particulates from 250 microns to 5 microns imbedded in granitic ground mass containing quartz-feldspar-hornblende-clinochlore-tremolite-biotite-and talc minerals of different contents in the rock.
Stirring ball milling was adapted to mill the sample into 100% −0.51 mm with minimum fines below 0.074 mm (reached 37.69 wt.%, with 39.51% cassiterite distribution. This fine product was kept aside for a separate study. The heavies and the two middling products after shaking table were directed after dryness to "Eriez" Rare Earth Roll separator, where the light products were rejected. At magnetic field strength approaching 21,000 gauss and feeding rate 2 kg/hr., mathematically designed experiments were applied to optimize the separation process.
At optimum conditions 72.50˚ splitter angle and belt speed 150 rpm, 200 rpm for the two size fractions, respectively, a cassiterite concentrate of 2.29% overall weight, 11.25% SnO 2 with an operational recovery 94.08%, was obtained. In addition, a high-grade topaz mineral product was separated from the coarse fraction −0.50 + 0.211 mm, as a non-magnetic fraction at splitter angle 65˚.

Declarations
• No funds, grants, or other support was received.
• The authors have no conflicts of interest to declare that are relevant to the content of this article. • All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
• The authors have no financial or proprietary interests in any material discussed in this article.