Sample characterization
Results of XRF analysis showed that SnO2 constituted 0.199% of the sample. The major oxides were SiO2, Al2O3, MgO, Fe2O3, and CaO. The oxides contents assayed 51.94%, 11.87%, 10.97%, 8.73%, and 5.68%, respectively, Table 2. They were related to different minerals phases, e.g. albite, quartz, tremolite, clinochlore hornblende, biotite, and talc, Fig. 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, Fig. 2. Rare earth oxides e.g. ZrO2, Rb2O, SrO, and Nb2O5 assayed 0.021%, 0.014%, 0.014%, and 0.002% were identified, Table 1. Amounts of nickel, copper, and zinc oxides were 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 SnO2 reaching 0.39% − 0.25% in 0.59 − 0.074 mm fractions, Table 2. Fewer cassiterite concentrations assayed 0.17–0.11% SnO2 were noted in coarser fractions 6.0- 0.83 mm, and in the finer fraction − 0.074 mm, Table 3. On the other hand Table 3 illustrates the size/chemical analyses of the attrition product. It was noted that low content of fines below 0.074 mm reaching about 15% by wt., were produced after the attrition process, Table 4. The constitution of these fines besides cassiterite may include the soft minerals e.g. talc and clinochlore.
Results of sink/float tests using bromoform (sp.g. 2.89 g/cc) showed that 24.40% and 21.40% by weight of the attrition − 0.51 + 0.211 mm, and − 0.211 + 0.074 mm samples were separated as heavy products with increase in MgO from 10.97–13.38%, TiO2 from 0.75–1.02%, Cr2O3 from 0.17–0.47%, and Fe2O3 from 8.73–14.38%, Table 5. On the other hand, the silica and alumina contents assayed 46.16% SiO2, and 10.04% Al2O3, respectively, Table 5. The increase in content of these oxides may be attributed to the presence of tremolite, hornblende, and biotite granites in the sink fraction. Meanwhile, SnO2 content increased from 0.199% in the original sample to 0.76% in the sink product, Table 5. 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.
Different samples cuts that were picked from the 6.0-2.28 mm original sample according to grain color were illustrated in Fig. 3. The petrography investigation of polished sections of these stones 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 Fig. 4. as follows:
-
Thin veins and grains of cassiterite (from 20 to 120 um) found in the fractured quartz Fig. 4. A.
-
Patches of cassiterite (from 30 to 5 um in diameter) coating the surface of biotite, Fig. 4.B.
-
Finely disseminating elongate crystals of cassiterite (from 10 to 5 um diameter) found in groundmass of Hornblende and Biotite, Fig. 4.C.
-
Large crystal of cassiterite "Hexagonal" (150 um diameter), Fig. 4.D.
-
Very large crystal of cassiterite (from 200–250 um diameter), Fig. 4.E.
-
Large cassiterite particle (120 um diameter) and large vein of cassiterite found between biotite and hornblende (light brown in color), Fig. 4.F.
-
Grains of cassiterite (from 40–120 um in diameters) found in the open space between quartz and microcline, Fig. 4.G.
-
Grains of cassiterite (from 40–120 um in diameters) found in the open space between quartz and microcline (yellow brown in color), Fig. 4.H and I.
-
Large grains of elongated cassiterite and topaz minerals were detected together in the 400 microns particle size, Fig. 4.J.
Table 2
XRF elemental analysis of the original sample
Oxide
|
Wt%
|
Oxide
|
Wt%
|
Na2O
|
0.615
|
NiO
|
0.083
|
MgO
|
10.967
|
CuO
|
0.011
|
Al2O3
|
11.869
|
ZnO
|
0.008
|
SiO2
|
51.939
|
As2O3
|
0.000
|
P2O3
|
o.292
|
Rb2O
|
0.014
|
SO3
|
0.031
|
SrO
|
0.014
|
K2O
|
1.490
|
ZrO2
|
0.021
|
CaO
|
5.683
|
Nb2O5
|
0.002
|
TiO2
|
0.754
|
SnO2
|
0.199
|
Cr2O3
|
0.175
|
PbO
|
0.007
|
MnO
|
0.402
|
Cl
|
0.004
|
Fe2O3
|
8.730
|
|
|
Table 3
Size/chemical analysis of the original sample (as received)
Fraction, mm
|
Wt.%
|
Cum.Wt.%
|
SiO2%
|
Al2O3%
|
Fe2O3%
|
SnO2%
|
-6.0 + 1.168
|
14.60
|
14.60
|
47.66
|
5.25
|
7.66
|
0.17
|
-1.168 + 0.833
|
11.00
|
25.76
|
47.66
|
5.25
|
7.66
|
0.17
|
-0.83 + 0.598
|
10.10
|
36.66
|
45.52
|
5.65
|
7.39
|
0.38
|
-0.59 + 0.417
|
10.80
|
47.61
|
49.83
|
5.45
|
7.40
|
0.39
|
0.295
|
9.10
|
58.51
|
45.84
|
4.82
|
7.53
|
0.34
|
0.208
|
7.40
|
67.14
|
51.12
|
5.15
|
7.87
|
0.31
|
0.106
|
4.40
|
80.04
|
51.65
|
7.07
|
7.36
|
0.29
|
0.074
|
10.00
|
83.82
|
53.84
|
4.49
|
6.35
|
0.25
|
-0.074
|
22.60
|
100.00
|
46.18
|
5.37
|
5.91
|
0.11
|
Total
|
100.0
|
|
47.52
|
|
7.16
|
|
Original
|
100.0
|
|
51.94
|
11.67
|
8.73
|
0.199
|
Table 4
Size/chemical analysis of the attrition sample
Fraction, mm
|
Wt.%
|
SiO2%
|
Al2O3%
|
SnO2%
|
Dist., %
|
-0.5 + 0.211
|
50.75
|
52.91
|
14.09
|
0.179
|
35.71
|
-0.211 + 0.074
|
11.56
|
55.52
|
6.91
|
0.239
|
10.85
|
Total
|
62.31
|
|
|
0.190
|
59.49
|
-0.074
|
37.59
|
51.62
|
8.77
|
0.21
|
40.51
|
Calculated
|
100.00
|
|
|
0.199
|
100
|
Original sample
|
100.00
|
51.94
|
11.87
|
0.199
|
100
|
Table 5
XRF analysis of sink fraction of the attrition product − 0.51 + 0.074 mm
Oxide
|
Wt%
|
Oxide
|
Wt%
|
Na2O
|
0.51
|
CuO
|
0.004
|
MgO
|
13.38
|
ZnO
|
0.012
|
Al2O3
|
10.04
|
As2O3
|
0.003
|
SiO2
|
46.19
|
Rb2O
|
0.010
|
P2O5
|
0.17
|
SrO
|
0.010
|
SO3
|
0.02
|
Y2O3
|
0.004
|
K2O
|
0.66
|
ZrO2
|
0.009
|
CaO
|
6.76
|
Nb2O5
|
0/006
|
TiO2
|
1.02
|
SnO2
|
0.759
|
Cr2O3
|
0.47
|
PbO
|
0/012
|
MnO
|
0.65
|
Cl
|
0.001
|
Fe2O3
|
14.38
|
Br
|
0.012
|
NiO
|
0.104
|
|
|
Cassiterite Enrichment Using Shaking Table
After applying some exploratory experiments on the shaking table separation of the 0.51 + 0.21 mm sample, it was noted that at table inclination 3o, and at constant wash water flow rate of 20 l/min., the gradual increase in stroke length from 2 cm, 2.5 cm, and to 3 cm was not accompanied by notable change in mineral recovery, yet an improvement in recovery reaching 82.5% at stroke length 2.5 cm was recorded, Fig. 5.
However, at table inclination of 4o, and the wash water flow rate of 20 l/min., no change in recovery (all recovery values were 78–80%. In addition, the results showed again some improvement to reach 85% at 2.5 cm stroke length, Fig. 5. At table inclination 5o, and water wash 20 l/min., again the same behavior as previously noted, no change in recovery (75% − 77%), and again the recovery showed improvement to 88% at 2.5 cm stroke length, Fig. 5. At constant table tilt 5o and stroke length 2.5 cm, the wash water flow rate seemed very effective on the mineral recovery. Gradual decreasing of water rate from 25 l/min. to 20 l/min., and 15 l/min., the recovery decreased from 89–84%, and then 76%, Table 6. At constant stroke length 2.5 cm and at low wash water flow rate at 15 l/min., the recovery gradually decreased from 81–76% by the gradual increase on the table tilt from 3o to 5o, Table 6.
At constant stroke length 2.5 cm and wash water flow rate 20 l/min., it was noted that the change in the table tilt showed no effect on the level of cassiterite recovery. Whereas by increasing the tilt from 4o to 5o, the recovery percentage remained un-changeable at the level of 85%, Table 6. At constant stroke length 2.5 cm, and wash water flow rate reaching 15 l/min, the recovery percentage decreased from 81–76% by decreasing the inclination from 4o to 5o. Meanwhile, at wash water flow rate reached 20 l/min., there was no change on the recovery% by changing the table tilt from 4o to 5o, Fig. 6. This information proved again that the wash water flow rate was a detrimental parameter on the table separation efficiency. As the wash water flow rate increased, the transport of gangue minerals to the tailings fraction increased which in turn improved the grade of the concentrate fraction. Moreover, higher grade of the concentrate fraction was obtained at higher level of deck tilt angle and feed flow-rate. With a lower level of deck tilt angle, an increase in the feed rate decreased the quality of the concentrate fraction.
However, the black to deep gray strip of heavy concentrate fraction was discharged over the far end of the deck, Fig. 7. In addition, great amount of middling mass composed of two distinguished colors (light gray and beige) were formed on the table surface. The gray mass was shown to contain most of the heavy minerals that constituted the original sample, i.e. hornblende, biotitie, and tremolite, Fig. 7. Additionally, the middling beige in color mass was composed of light minerals which contained cassiterite inclusions i.e. quartz, and feldspars (albite, and microcline), Fig. 7. On the other hand, fragments of these light minerals that were nearly free from cassiterite inclusions were carried by the flowing water towards the tailings launder, Fig. 7. However, the variation in color between these cuts helped a lot in their identification.
It could be concluded that the coarse fraction − 0.5 + 0.21 mm was stratified in a good manner at table inclination 5o, 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 (Burt and Mills 1984). The Schematic Diagram showing the shaking tabling separation process was depicted in Fig. 7 (N.B. in all figures the cassiterite grade% were multiplied by the factor 50 to facilitate the illustration). The evaluation of the final end-products with respect to cassiterite recovery and grade after shaking table was illustrated in Fig. 8 and Table 6.
Table 6
Evaluation of shaking table products
Product
|
-0.51 + 0.211 mm
|
-0.211 + 0.074 mm
|
-0.51 + 0.074 mm
|
Wt.%
|
SnO2%
|
SnO2% Dist.opt.
|
Wt.%
|
SnO2%
|
SnO2% Dist.opt.
|
Wt.% SnO2% Dist., %
|
Heavy
|
2.66
|
1.45
|
20.30
|
2.10
|
1.98
|
18.08
|
|
Middling 1
|
23.45
|
0.31
|
39.26
|
2.88
|
1.32
|
16.53
|
|
Middling 2
|
36.34
|
0.20
|
38.25
|
69.22
|
0.19
|
65.39
|
|
Total
|
62.45
|
0.29
|
97.81
|
74.20
|
0.28
|
91.61
|
40.27 0.29 96.94
|
Light tailing
|
37.55
|
0.01
|
1.98
|
27.15
|
---
|
---
|
|
Calc. opt.
|
100.00
|
0.190
|
100.00
|
100.0
|
0.23
|
100
|
|
Feed
|
50.75
|
0.190
|
47.75
|
11.56
|
0.23
|
11.45
|
62.31 0.19 60.49
|
Statistical Optimization Of Rer Magnetic Separation Process
The design summary and the factorial design results of the RER magnetic separation in terms of cassiterite recovery% as a response are shown in Tables 7 and 8. It can be seen that the cassiterite recovery% reached an optimum of 97% (runs 2, 5, 7, 8, 10, and 11) at splitter angle 72.50o, and belt speed 150.00 rpm. A regression Eq. (1) was obtained by multiple regression analysis of the experimental data as follows:
Recovery% = -24.76 + 66.22*A + 0.68*B − 0.44*A2 -3.00E- 004*B2 -8.00E-003*A*B (1)
where, A is the splitter inclination angle, and B is the separator belt speed.
The optimization of Eq. (1) 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 9. The values of R2 and the standard deviation suggested that there was a good agreement between the experimental and predicted values obtained from the model, Fig. 9.
Results showed that with increasing the splitter inclination angle and the separator belt speed, cassiterite recovery value increased, and reached its maximum value of 97% at splitter angle 72.50o and at belt speed 150 rpm. However, it was evident that in these selected ranges of separator angle (70o-75o) and belt speed (100–200 rpm), there was a dominant effect of the separator splitter inclination angle on cassiterite recovery % compared to the effect of belt speed, Fig. 10.
On the other hand, by increasing both splitter inclination and belt speed more than 72.50o and 150 rpm respectively, a slight decrease in the recovery value reaching 96% was remarked. Meanwhile, the decrease in the values of both variables to 70o and 100 rpm, respectively, a pronounced decrease in the cassiterite recovery value reaching 86% was remarked, Fig. 10. In addition, the interaction of both variables showed a reversible effect on the mineral recovery, Fig. 11. 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 86–92%, and then to 96%. However, at belt speed of 150 rpm, the change in splitter angle showed promising recovery reaching 90% at 70o, and then the recovery increased to reach 97% at splitter angles 75o. Additionally, the same trend was shown by increasing the belt speed to reach 200 rpm, Fig. 12. Response surface for cassiterite recovery% as function of RER magnetic separation variables, i.e. splitter angle, and belt speed was illustrated in Fig. 13.
An interesting study was carried out about the effect of feed size of both magnetic and nonmagnetic particles on the efficiency of the RER magnetic separator (Gehauf 2004). It was found that, when nonmagnetic particles travel over a roll and are allowed to drop unhindered, they are roughly classified by their particle size. Large particles will travel 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 will usually be 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 (Gehauf 2004). 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 is too great. The problem can normally be overcome by screening the feed before magnetic separation and thus produce a better product. The practicality of screening is ultimately based on the difference in the magnetic responses of the particles to be separated and the value of the product(s). That was the way the present study started, i.e., feed fractionation into − 0.51 + 0.21 mm and − 0.21 + 0.074 mm (Ibrahim et al. 2017).
However, it could be concluded that the optimum separation conditions were reached by keeping the splitter angle fixed at 72.50o and by applying the belt speed at 150 rpm for coarse fraction and 200 rpm for finer fraction. At these conditions, the optimum operational concentration results of the − 0.50 + 0.21 mm fraction was 2.10% wt., 12.96% SnO2, and 97.62% recovery from a feed containing 0.29% SnO2. On the other hand, the optimum operational concentration results of the − 0.21 + 0.074 mm fraction was 3.72% wt., 6.21% SnO2, and 90.00% recovery from a feed containing 0.28% SnO2. From these results, it could be concluded that an overall final cassiterite concentrate contained 11.25% SnO2, 2.29% wt, and 94.08% recovery from a feed contained 0.19% SnO2, was produced, Fig. 14 (N.B. in all figures the cassiterite grade% were multiplied by the factor 50 to facilitate the illustration).
Important to mention that a non magnetic product containing topaz mineral was separated from the coarse fraction − 0.51 + 0.21 mm only at splitter angle of 65o and belt speed 150 rpm. This may be attributed to the presence of topaz that was impeded as inclusions within the hard quartzite matrix that resisted over-grinding and remained in the coarser size 0.50 + 0.21 mm. SEM/EDX and XRD analyses of topaz mineral, cassiterite/topaz, and cassiterite mineral are illustrated in Figs. 15–21. However, the schematic diagram for the suggested processing flow-sheet to recover cassiterite and topaz minerals from some mines scraps of the Eastern Desert of Egypt is illustrated in Fig. 22.
Table 7
Stydy type
|
Response surface
|
Experiments
|
13
|
Initial design
|
Central composite
|
Blocks
|
No blocks
|
Design model
|
Quadratic
|
Responce
|
Name
|
Units
|
Obs
|
Minimum
|
Maximum
|
trans
|
Model
|
Y1
|
recovery
|
%
|
13
|
86.00
|
97.00
|
None
|
Quadratic
|
Factor
|
Name
|
Units
|
Type
|
Low actual
|
High actual
|
Low coded
|
High coded
|
A
|
Splitter angle
|
Degree
|
Numeric
|
70.00
|
75.00
|
-1.000
|
1.000
|
B
|
Belt speed
|
rpm
|
Numeric
|
100.00
|
200.00
|
-1.000
|
1.000
|
Table 8
Results of full factorial design
Std
|
Run
|
Block
|
Variable 1
A: Splitter angle, degree
|
Variable 2
B: belt speed, rpm
|
Response 1
Recovery %
|
3
|
1
|
Block 1
|
70.00
|
200.00
|
90.00
|
8
|
2
|
Block 1
|
72.50
|
220.71
|
97.00
|
2
|
3
|
Block 1
|
75.00
|
100.00
|
96.00
|
1
|
4
|
Block 1
|
70.00
|
100.00
|
86.00
|
9
|
5
|
Block 1
|
72.50
|
150.00
|
97.00
|
4
|
6
|
Block 1
|
75.00
|
200.00
|
96.00
|
12
|
7
|
Block 1
|
72.50
|
150.00
|
97.00
|
10
|
8
|
Block 1
|
72.50
|
150.00
|
97.00
|
6
|
9
|
Block 1
|
76.04
|
150.00
|
96.00
|
11
|
10
|
Block 1
|
72.50
|
150.00
|
97.00
|
13
|
11
|
Block 1
|
72.50
|
150.00
|
97.00
|
7
|
12
|
Block 1
|
72.50
|
79.29
|
86.00
|
5
|
13
|
Block 1
|
68.96
|
150.00
|
90.00
|
Table 9
ANOVA for response surface quadratic model in terms of recovery% as a responce
Source
|
Sum of squares
|
DF
|
Mean square
|
F Value
|
Prob > F
|
Model
|
134.63
|
5
|
26.93
|
6.97
|
0.0121
|
A
|
74.94
|
I
|
74.94
|
19.39
|
0.0031
|
B
|
2.00
|
I
|
2.00
|
0.52
|
0.4593
|
A2
|
52.61
|
I
|
52.61
|
13.61
|
0.0078
|
B2
|
3.91
|
I
|
3.91
|
1.01
|
0.3479
|
AB
|
4.00
|
I
|
4.00
|
1.03
|
0.3429
|
Residual
|
27.06
|
7
|
3.87
|
|
|
Lack of Fit
|
27.06
|
3
|
9.02
|
|
|
Pure Erro
|
0.000
|
4
|
0.000
|
|
|
Cor Total
|
161.69
|
12
|
|
|
|