Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.1, pp.13-25, 2011
jmmce.org Printed in the USA. All rights reserved
13
Recovery of Chromite Values from Plant Tailings by
Gravity Concentration
Sunil Kumar Tripathy*, Y. Ramamurthy and Veerendra Singh
Research & Development Division, Tata Steel,
Jamshedpur, INDIA – 831001
* Corresponding Author: [e-mail]
ABSTRACT
Large tonnages of chromite tailing were discarded during processing of chromite ore in the
conventional circuit. A typical chromite plant tailing was treated in wilfley table for the recovery
of chromite values. Optimisation study was carried out for the process parameters of wilfley
table using empirical models, developed from the experimental data. It was found that grade and
recovery (% Cr2O3) in the concentrate fraction majorly depended on the variation of deck tilt
angle. To achieve high grade (>45%) with acceptable recovery (>40%), set of optimisation
condition of parameters have derived which resulted large quantity of wash water (>5lpm of
flow rate) is necessary. Validation of the empirical models were done with set of tests which
resulted good agreement with the predict values (R2 is 0.96 and 0.99 for the grade and recovery
respectively).
Key Words: Chromite Plant Tailing, Beneficiation, Wilfley Table, Modeling, Process
optimisation.
1. INTRODUCTION
Most of heavy minerals including chromite are treated by gravity concentration methods at
different stages of upgradation [1&2] and produces huge quantity of tailings which composes
unrecovered valuable minerals. The popularity of gravity concentration is due to their simplicity,
low operating cost and easy to operate. Wilfley table is one of key unit operations which can help
in diagnostic or amenability of the gravity concentration process for different minerals/ore. The
detailed principle of wilfley table has been discussed at different places [3-5].

14 Sunil Kumar Tripathy, Y. Ramamurthy and Veerendra Singh Vol.10, No.1
The wilfley table is one among the gravity concentration unit operation that separate minerals or
other bulk material of different specific gravity by their relative movement in response to gravity
and other forces. Usually Wilfley tables are used for the concentration of different heavy
minerals such as chromite, iron and beach sand including different plant tailings. The separation
performance of Wilfley table depends on both feed characteristics and operating parameters
associated with the equipment. Feed characteristics include particle size, density distribution,
volumetric flow rate and solid concentration. Similarly the operating parameters include wash
water flow rate, deck tilt angle, shake amplitude, shake frequency, splitter position and side tilt
angle [3, 5, 6]. Wilfley table has been used to recover celestite ore [7] and also for the
beneficiation of low grade manganese ore [8].
Significant research effort has been focused on recovery of chromite values from the plant
tailings which need to be focused on mineral conservation, utilisation and environment
protection point of view [9]. The tailing generated from the Turkish chromite beneficiation plants
were treated in the multy gravity separator for producing the desirable grade concentrate [10-13].
Low grade chromite sample from Karaburhan was treated with combination of wet shaking table
and multi-gravity separator for obtaining marketable grade [14]. A combination of multi gravity
separator and column flotation has been studied for the upgradation of the plant tailing in Turkey
[9].
In the present research, beneficiation of chromite plant tailing by Wilfley table was studied.
Optimisation of Wilfley table performance was determined for the grade and recovery of Cr2O3
in the concentrate fraction using empirical models which were derived from the experimental
data. The effect of different process variables and their interactions are analysed using 3D
surface plots. Also validations of the obtained empirical models were done by set of tests.
2. EXPERIMENTAL.
2.1. Material
The chromite sample used in the present study was collected from the tailing fraction of a typical
chromite beneficiation plant of Sukinda region, India. As received tailing sample contains
24.26% Cr2O3, 23.51% total iron, 13.61% alumina, 17.58% silica, 5.35% MgO and 7.6% loss
of ignition (LOI) having 0.7 Cr/Fe ratio. So the sample is of ferruginous in nature and having
specific gravity of 3.3.
2.1.1. Particle size distribution
Particle-size measurement was carried out using the Vibratory Laboratory Sieve Shaker (Fritsch,
Germany). Micro precision sieves were used for separation of -75 micron particles. Graphical

Vol.10, No.1 Recovery of Chromite Values 15
representation of the size analysis data of the tailing sample is presented in Figure 1. It may be
elucidated from the size measurement that the slime is extremely fine in nature and substantial
amount of the tailing i s below 25 micron (33.45% by weight).
Figure 1: Size distribution of the chromite plant tailing sample.
2.1.2. Sizewise chem ical analysis
The chemical analysis of the different size fractions were carried out by ICP analyser and the
result of the size wise chemical analysis is given in Table 1. This table shows that the Cr2O3
content varies from 18.2% to 29.26% and most of the Cr2O3 (about 51.18% by wt) is distributed
in the size range -250 and +25 microns. But huge quantity (30.56% by wt) of Cr2O3 value is
distributed at finer sizes i.e. below 25 micron. The total iron content varies from 13.23% to
23.61% and the maximum quantity of total iron (38.74% by weight) is distributed at finer sizes
i.e. below 25 micron. Similarly, maximum amount of alumina, silica and MgO distributed at
finer size (below 25 micron) are 29.07%, 28.32% and 35.43% respectively.
2.1.3. XRD study
X-Ray Diffraction study was carried out to identify mineral phases in the chromite tailing
sample. The diffractogram of the XRD study is shown in Figure 2. From this figure, it can be
seen that hematite and goethite are the major iron-bearing mineral phases along with chromite
whereas gibbsite, kaolinite and quartz occur as minor gangue mineral phases.

16 Sunil Kumar Tripathy, Y. Ramamurthy and Veerendra Singh Vol.10, No.1
Table 1: Size wise chemical analysis of the chromite tailing sample.
Size Wt% Assay value (%)
(µm) Retained Cr2O3 Fe(T) Al2O3 SiO2 MgO
+600 2.47 23.76 19.2 11.8 11.21 5.8
-600+500 6.74 23.18 22.2 14.35 13.1 4.5
-500+250 11.26 18.2 21.86 16.14 18 3.7
-250+150 11.26 21.2 21.87 16.4 18.77 4.34
-150+106 8.35 27.5 18.9 14.5 17.8 6.1
-106+75 5.80 26.3 18.9 15 21.1 5.7
-75+45 8.49 24 13.23 15.93 24.36 5.45
-45+37 5.97 28.52 14.32 13.66 21.4 6.2
-37+25 6.22 29.26 15.19 12.12 18.14 6.26
-25 33.45 21 23.61 12.16 14.74 5.6
Figure 2: X-Ray Diffraction (XRD) pattern of the tailing sample with identified phase s ( :
chromite, :hematite, : kaolinite, : gibbsite, : quartz, α: goethite).
2.2. Methods
The experimental set up consists of feed slurry tank, a peristaltic pump and Wilfley table. The
feed slurry tank (100-litres capacity) was attached with a stirrer to keep the solids in uniform
suspension throughout the test programme. The peristaltic pump was used to feed the desired
Position [°2Theta]
20 30 40 50 607080
0
2500
10000 CO B Pl ant Taily (Head)_R
▲
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20 3060 70 80 40 50
Position (Degree 2 ǿ)
α
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Position [°2Theta]
20 30 40 50 607080
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10000 CO B Pl ant Taily (Head)_R
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20 3060 70 80 40 50
Position (Degree 2 ǿ)
α
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Counts

Vol.10, No.1 Recovery of Chromite Values 17
quantity of slurry to the separation unit. The laboratory Wilfley table used in the study is a
typical one, commonly used to concentrate minerals (laboratory model No. 15 S, supplied by
M/s The Deister Concentrator Company Inc., USA). It has one deck of rectangular form with
350 x1000 mm, with linoleum as surface material. It is partially riffled, with riffles parallel to the
deck motion. The riffles have 5 mm height on the feed end with decreasing height from the feed
edge to the concentrate edge. The gap between riffles is 12 mm. The process variables wash
water flow rate, deck tilt angle and feed slurry flow rate were varied by keeping the other
variables such as solid concentration at 20% solids by weight, 15 mm of shake amplitude,
200cycles/min. of shake frequency and splitter position at 25 cm from the concentrate end were
kept constant.
2.3. Experimental Program
A statistically designed test program was performed to obtain the necessary data needed to
develop empirical models that accurately describe the effect of three key operating parameter
values and their interactive relationships when treating chromite plant tailings using wilfley
table. The models were further used to optimize the parameters which could maximize the grade
and recovery of the Cr2O3 in the concentrate fraction. The range of operating values for each
parameter tested is shown in Table 2. Number of tests as per the Box–Behnken experimental
design was conducted and the product samples were dried, weighed and analysed in terms of
grade and recovery. After the model development, a set of optimum operating conditions were
identified by considering the limit of grade and recovery of Cr2O3. Subsequent table tests were
conducted for validation purposes.
Table 2: Process variable ranges used in the 3-level Box-Behnken design.
Process variables Parameter Values
-1 Level 0 Level +1 Level
X1. Wash water flow rate (l/min)2.5 5 7.5
X2. Deck tilt Angle (degree) 2 4 6
X3. Slurry feed Rate (l/hr) 100 130 160
3. RESULTS AND DISCUSSION
The experimental programme provided a broad range of grade and recovery values which is
shown in Table 3. It is observed from the table that the concentrate fraction is enriched up to
with 60.88% Cr2O3 with recovery of 13.66% where as a maximum of 61.37% Cr2O3 recovery is
reported with Cr2O3 content of 49.58%. The test results were analysed in further section.

18 Sunil Kumar Tripathy, Y. Ramamurthy and Veerendra Singh Vol.10, No.1
Table 3: Observed results at different levels of variables.
Test Observed Conditions Observed Results
No. X1 X
2 X 3 % Cr2O3 % Rec.(Cr2O3)
1 -1 -1 0 42.76 57.52
2 +1 -1 0 48.95 37.33
3 -1 +1 0 54.86 12.64
4 +1 +1 0 52.41 13.74
5 -1 0 -1 40.33 33.25
6 +1 0 -1 44.9 19.11
7 -1 0 +1 45.93 16.4
8 +1 0 +1 37.04 11.94
9 0 -1 -1 49.58 61.37
10 0 +1 -1 32.2 7.77
11 0 -1 +1 28.2 34.82
12 0 +1 +1 60.88 13.36
13 0 0 0 52.16 37.79
14 0 0 0 52.16 37.79
15 0 0 0 52.26 37.79
3.1. Model Evaluation
By using the test results empirical models have developed for the chromite grade and recovery of
concentrate fraction. Table 4 is the ANOVA (Analysis of Variance) for developed models for the
concentrate fraction of Cr2O3 grade and recovery. All major statistics indicate that the models
can be used for effectively describing the operating parameter effects on the response variables.
Both the models have higher value of R2 (0.98 and 0.99 for grade and recovery respectively)
which indicates the models are well agreement with the experimental data. The models are
significant as the F value is high, the Prob>F value is less than 0.05 and the standard deviation is
very small (1.67 for the grade and 0.97 for the recovery).
The coefficient estimate and the significance level of the parameter and parameter interactions of
the empirical models are provided in Table 5. The data clearly shows that the Cr2O3 grade of the
concentrate fraction has affected majorly by deck tilt angle (X2) and the interaction between the
deck tilt angle and slurry feed rate (X2X3) has major influence on grade of the concentrate
fraction. Similarly for the recovery of % Cr2O3 in the concentrate fraction is influenced by deck
tilt angle (X2) and interaction between wash water flow rate and deck tilt angle (X1X2). So, it is
evident that deck tilt angle has

Vol.10, No.1 Recovery of Chromite Values 19
major influence on the separation of the chromite bearing minerals to the concentrate fraction.
The high specific gravity minerals (chromite) are forced to spread out in thin, wide band which
decides the particle traveling path (allows much sharper cuts between the concentrate, middling
and tailing). In case of the grade model terms, the wash water flow rate (X1), slurry flow rate
(X3), double interaction of wash water flow rate (X12) and deck tilt angle (X22) and for the
recovery model terms, the double interaction of the deck tilt angle (X22) are not significant as the
Prob>F are higher than 0.05.
Table 4: ANOVA table derived for the grade and recovery models.
Statistics
Source
Cr2O3(%) grade model Cr2O3(%) recovery model
Sum of square 1147.44 4092.09
Degree of freedom 9 9
Mean square 127.49 454.68
F-Value 45.64 480.12
Prob>F <0.0001 <0.0001
Standard deviation 1.67 0.97
R2 0.98 0.99
Table 5: Estimated coefficient values for the parameter and paramet er interaction effects.
Factor
(%) Cr2O3 Grade (%)Cr2O3 Recovery
Coefficient F-value Prob>F Coefficient F-value Prob>F
estimate estimate
X1 -0.072 0.015 0.9058 -4.71 187.51 < 0.0001
X2 3.86 42.62 0.0003 -17.94 2719.24 < 0.0001
X3 0.63 1.14 0.3217 -5.62 267.05 < 0.0001
X12 -1.54 3.57 0.1006 -8.32 307.68 < 0.0001
X22 -0.88 1.15 0.3183 0.84 3.11 0.1212
X32 -8.57 170.71 < 0.0001-9.3 384.24 < 0.0001
X1X2 -2.16 6.68 0.0362 5.32 119.66 < 0.0001
X1X3 -3.36 16.21 0.005 2.42 24.74 0.0016
X2X3 12.52 224.29 < 0.00018.03 272.7 < 0.0001

20 Sunil Kumar Tripathy, Y. Ramamurthy and Veerendra Singh Vol.10, No.1
For better understanding of the results, the predicted models are described in terms of three
dimensional (3D) response surface plots which show the effect of process variables on grade and
recovery of Cr2O3 in concentrate fraction. Figure 3 explains the effect of the process parameters
of wilfley table on grade of concentrate fraction. Figure 3(a) shows the effect of wash water flow
rate (X1) and deck tilt angle (X2) on grade of the concentrate fraction at center level of slurry
feed rate. It is observed that higher grade is obtained at lower level of wash water flow rate and
higher level of deck tilt angle, which is caused due to decrease in the residence time of the
gangue minerals a nd that result wash away of the low density minerals to the tailing fraction.
Figure 3(b) shows the effect of wash water flow rate (X1) and slurry feed rate (X3) on grade of
the concentrate fraction of the wilfley table at center level of deck tilt angle. The grade of the
concentrate fraction is maximum at intermediate of slurry feed rate and lower level of the wash
water flow rate. It is also observed that as the wash water flow rate increases, there is an increase
in the grade of the concentrate fraction at lower level of feed flow rate and at higher level of feed
flow rate and vice versa. As the wash water flow rate increases, the transport of the gangue
minerals to the tailing fraction increases which in turn improves the grade of the concentrate
fraction.
Figure 3(c) shows the effect of deck tilt angle (X2) and feed flow rate (X3) on grade of the
concentrate fraction of the wilfley table at center level of wash water flow rate. The higher grade
of the concentrate fraction is obtained at higher level of both deck tilt angle and feed flow rate. It
is also noted that at lower level of deck tilt angle, as the feed rate increases there is decrease in
the quality of the concentrate fraction but at higher deck tilt angle and vice versa.
Figure 3. Response surface plots showing the effects on grade (%) of Cr2O3 in the concentrate
fraction: (a) Between wash water flow rate (x1) and deck tilt angle (x2) , (b) Between wash water
flow rate (x1) and feed flow rate (x3), (c) Between deck tilt angle (x2) and feed flow rate (x3).
-1
-0.5
0
0.5
1
-1
0
1
40
45
50
55
60
Wash Water Flow Rate(x1)
Deck Tilt Angle(x2)
Grade(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
35
40
45
50
55
Wash Water Flow rate(x1)
Feed Flow rate(x3)
Grade(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
20
30
40
50
60
70
Deck Tilt Angle(x2)
Feed Flow rate(x3)
Grade(%)
abc
-1
-0.5
0
0.5
1
-1
0
1
40
45
50
55
60
Wash Water Flow Rate(x1)
Deck Tilt Angle(x2)
Grade(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
35
40
45
50
55
Wash Water Flow rate(x1)
Feed Flow rate(x3)
Grade(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
20
30
40
50
60
70
Deck Tilt Angle(x2)
Feed Flow rate(x3)
Grade(%)
-1
-0.5
0
0.5
1
-1
0
1
40
45
50
55
60
Wash Water Flow Rate(x1)
Deck Tilt Angle(x2)
Grade(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
35
40
45
50
55
Wash Water Flow rate(x1)
Feed Flow rate(x3)
Grade(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
20
30
40
50
60
70
Deck Tilt Angle(x2)
Feed Flow rate(x3)
Grade(%)
abc

Vol.10, No.1 Recovery of Chromite Values 21
Similarly the effect of process variables on recovery of the Cr2O3 to the concentrate fraction of
the wilfley table has explained in Figure 4. Figure 4(a) demonstrates the effect of wash water
flow rate (X1) and deck tilt angle (X2) on recovery of Cr2O3 in the concentrate fraction at center
level of slurry feed rate. It is observed that higher recovery can be achieved at centre level of
wash water flow rate and lower level of deck tilt angle. This is due to an increase in the deck tilt
angle, the transport of the chromite bearing minerals along with the fines at top layer of the flow
will increase, as a result recovery of Cr2O3 to the concentrate fraction increases. It is also noted
that there is marginal effect of the deck tilt angle compared to the wash water flow rate on the
recovery of the concentrate fraction.
Figure 4: Response surface plots showing the effects on recovery (%) of Cr2O3 in the concentrate
fraction: (a) Between wash water flow rate (x1) and deck tilt angle (x2) , (b) Between wash water
flow rate (x1) and feed flow rate (x3), (c) Between deck tilt angle (x2) and feed flow rate (x3).
Figure 4(b) shows the effects of wash water flow rate (X1) and slurry feed rate (X3) on recovery
of Cr2O3 in the concentrate fraction of the wilfley table at center level of deck tilt angle. The
recovery of the concentrate fraction is maximum at intermediate level of both feed flow rate and
wash water flow rate. As the wash water flow rate increases the transport of the fine chromite
minerals to the tailing fraction increases which in turn decreases the recovery of Cr2O3 to the
concentration fraction. Similarly as the feed flow rate increases, the retention time for the
segregation of particles decrea ses.
Figure 4(c) shows the effect of deck tilt angle (X2) and feed flow rate (X3) on recovery of Cr2O3
in the concentrate fraction of the wilfley table at center level of wash water flow rate. The higher
recovery to the concentrate fraction is observed at centre level of feed flow rate and lower level
of the deck tilt angle. It is also noted that, there is no marginal difference in the recovery of
Cr2O3 to the concentrate frac tion as the feed flow rate changes.
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
10
20
30
40
50
60
Wash Water Flow Rate(x1)
Deck Tilt Angle(x2)
Recover(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
10
15
20
25
30
35
40
Wash Water Flow Rate(x1)
Feed Flow rate(x3)
Recover(%)
-1 -0.500.5 1
-1
-0.5
0
0.5
1
0
20
40
60
80
Deck Tilt Angle(x2)
Feed Flow rate(x3)
Recover(%)
abc
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
10
20
30
40
50
60
Wash Water Flow Rate(x1)
Deck Tilt Angle(x2)
Recover(%)
-1 -0.5 00.5 1
-1
-0.5
0
0.5
1
10
15
20
25
30
35
40
Wash Water Flow Rate(x1)
Feed Flow rate(x3)
Recover(%)
-1 -0.500.5 1
-1
-0.5
0
0.5
1
0
20
40
60
80
Deck Tilt Angle(x2)
Feed Flow rate(x3)
Recover(%)
abc

22 Sunil Kumar Tripathy, Y. Ramamurthy and Veerendra Singh Vol.10, No.1
3.2. Optimisation Studies.
In mineral processing grade and recovery are the most important terms for evaluating
performance of any unit operation. It is also common that the grade and recovery of any process
are inversely proportional to each other. For maximizing the grade and recovery of the Cr2O3 in
the concentrate fraction of the wilfley table particular set of process variables are required. For
optimising the separation performances and the corresponding operating parameter values were
determined using the empirical models. For the optimisation point of view the grade and
recovery were set to a minimum of 45% and 40% respectively. Set of optimisation conditions
were obtained under the above condition and are tabulated in Table 6. An interesting finding is
that in most of the cases, the wash water flow rate is of higher value (>5 lpm) compared to the
other variables.
Table 6: Operating conditions derived from the optimisation study.
Test
No.
Predicted Conditions Predicted Values (% Cr2O3)
X1 X
2 X 3 Grade Recovery
1 0.16 -0.80 -0.13 49.89 52.57
2 -0.60 -0.65 -0.17 48.61 53.51
3 0.36 -0.70 -0.67 51.77 49.43
4 0.44 -0.80 -0.37 51.77 49.90
5 -0.12 -0.75 -0.77 49.93 56.33
6 0.44 -0.80 -0.20 50.77 48.95
7 0.28 -0.80 -0.37 51.49 52.44
8 0.28 -1.00 -0.67 52.63 57.63
9 0.28 -0.85 -0.70 52.09 54.10
10 0.16 -0.95 -1.00 51.21 57.39
3.3. Validation of the Model
For validation of the obtained model for predicting the grade and recovery of Cr2O3 content in
the concentrate fraction, numbers of tests were conducted based on the optimized conditions
which were derived from the empirical model. The observed results and the predicted results for
both the responses are shown in Figure 5. Figure 5 shows that the observed values are in good
agreement with the predicted values i.e. R2 values are 0.96 and 0.99 for the grade and for
recovery respectively.

Vol.10, No.1 Recovery of Chromite Values 23
Figure 5: Relationship between observed and predicted values for the grade and recovery (%
Cr2O3).
4. CONCLUSION
Wilfley table is found to be effective equipment for beneficiation of chromite plant tailing. It was
found that the plant tailing can be upgraded up to 61.37% Cr2O3 irrespective of the recovery. The
obtained empirical models for grade and recovery of the concentrate fraction were considerable
and showed good agreement with the observed values which resulted from the ANOVA. It was
also observed that the deck tilt angle dominantly influence the equipment performance compared
to other two variables. However, optimisation of the process parameters were derived for high
grade (>45%) and recovery (>40%) using developed empirical models. It was found that for
achieving the above targets, the wash water flow rate of the wilfley table should be of higher
quantity and the other parameters such as deck tilt angle and slurry feed rate should be of lower
values. The validation of the empirical models had done with set of tests which resulted with
good agreement with predicted values (R2 for grade and recovery are 0.96 and 0.99 respectively).
48
50
52
54
56
58
48 50 52 54 56 58
Observed Value
Predicted Value
 Grade (% Cr
2
O
3
)
Recovery (% Cr
2
O
3
)

24 Sunil Kumar Tripathy, Y. Ramamurthy and Veerendra Singh Vol.10, No.1
ACKNOWLEDGEMENT
Authors are thankful to Tata Steel management for giving an opportunity to work on this project
and permission to publish. The support and services provided by staff of R & D division are also
duly acknowledged.
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