Journal of Minerals & Materials Characterization & Engineering, Vol. 6, No.2, pp 143-150, 2007
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143
CHARACTERIZATION AND BENEFICIATION OF ANKA
CHROMITE ORE USING MAGNETIC SEPARATION PROCESS
O.K. Abubakre
1
, R.A. Muriana
2
and P.N. Nwokike
3
Mechanical Engineering Department, Federal University of Technology, Minna
1
-
2
-
3
- Deceased
ABSTRACT
Chromium is largely used as alloying elements in steels for production of stainless steels.
It is added in form of ferro-chromium, which is presently imported from Russia and
Germany to meet the need of Nigerian Steel Industry. Chromite ore (FeOCr
2
O
3
) is the
most important mineral occurrence of chromium, and some deposits are reported in Anka
Local Government Area of Zamfara State. This work involves the analysis of collected
ore samples from Anka in Zamfara State. The samples assaying 36.84% of Cr
2
O
3
was
subjected to beneficiation process in order to enrich the ore, in terms of the Cr
2
O
3,
using
magnetic separation. The results obtained after the analysis of the beneficiated ore using
wet chemical analysis and Atomic Absorption Spectrometer shows that Cr
2
O
3
content
increased to 48% with a maximum Cr:Fe ratio of 6.2:1. The content of Cr
2
O
3
is high
enough and can be used for ferro-chromium alloy production.
Keywords: Ferro-alloys, Chromium recovery, Chromium Iron Ratio, Alloy steel, Atomic
Absorption Spectrometer.
1.0 INTRODUCTION
Ferro-alloys are alloys of iron as the base metal and other leading alloying
elements such as Silicon (Si), Manganese (Mn), Tungsten (W), Vanadium (V), and
Chromium (Cr). They are mainly used as additives in steel making processes and many
other refining processes. Based on the quality of steels desired, the following ferro-alloys
are commonly used in the production of iron and steels in melting shops: Ferro-silicon
(Fe–Si), Ferro-manganese (Fe-Mn), Ferro-Tungsten (Fe-W), Ferro-chromium (Fe-Cr);
Ferro-vanadium (Fe-V). The use of ferro-alloys in steel making is very important because
of their effect on the quality of steels in terms of physical and mechanical properties and
other alloying effects such as deoxidation capability, desulphurisation and
dephosphorisation potentials [1]

144 O.K. Abubakre, R.A. Muriana and P.N. Nwokike Vol.6, No.2
In Nigeria, the demand for ferro-alloy is expected to be on increase annually
following the country’s emphasis on accelerated industrialization hinged on fast growth
in iron and steel industries. At the peak of capacity utilization of Aladja Steel Company,
the annual demand for the various grades of ferro-alloys is about 15,000 tones per annum.
[2]. The prospect of rapid increase in the demand for ferro-alloys is envisaged with the
success of the current reforms of present regime aimed at attracting investors with
technical expertise, managerial skill and resources to turn the public Iron and Steel plants
around. This couple with the emphasis of the present regime on the development of the
solid minerals in the country has made investigation into the locally available sources of
chromite ore and other ferro-alloys inevitable [3 and 4].
This present work is aimed at investigating the quality, metallurgical and general
engineering properties of locally sourced chromite ore from Tunga Kuduka, Anka Local
Government Area of Zamfara State. Chemical analysis of the ore was carried out and
various ore processing techniques such as mineralogical studies, ore dressing, particles
size analysis was employed. The beneficiation of the ore was carried out using magnetic
separation technique. The results obtained after the chemical analysis of the beneficiated
ore using Atomic Absorption Spectrometer shows that Cr
2
O
3
content increased to 48%
with maximum Cr:Fe ratio of 6.2:1. The content of Cr
2
O
3
is high enough and can be used
for ferro-chromium alloy production.
2.0 LITERATURE REVIEW
Minerals are natural inorganic substances possessing definite chemical
compositions and atomic structures. An ore is described generally as an accumulation of
mineral in sufficient quantity as to be capable of economic extraction. The minimum
metal content required for a deposit to qualify as an ore varies from metal to metal while
the suitability of a deposit for economic mining and processing is controlled by location
and size of the deposit, the ore-feed grade, mineralogy, and texture of the ore, mining
cost and the cost of ancillary services such as power –supply, water, roads, tailings
disposal and amenability of the ore to economically viable treatment and the demand for
and value of the metal and its concentrates [5].
2.1 Chromium in Steel Industry
The only important ore mineral of chromium is the chromite – FeO.Cr
2
O
3
(68%
Cr
2
O
3
, 32% FeO) which crystallizes in the isometric system. The hardness value is 5.5 on
Mohr scale and its specific gravity is 4.6. In colour, it is iron – black to brownish –black.
Pure chromite with composition FeCr
2
O
4
is rare because magnesium usually substitutes
for some ferrous ion and aluminum and ferrous ion substitutes for chromium.
Chromium is produced in the form of ferro-chromium by various methods
ranging from reduction in an electric furnace, electrolysis, thermal dissociation, thermal
decomposition. Ferro-chromium has been produced by reduction of chromite ores with
carbon or silicon in an electric furnace. It can also be produced by a silicon thermic

Vol.6, No.2 Characterization and Beneficiation of Anka Chromite Ore 145
reaction in the presence of suitable oxidizing agents such as calcium chromate CaCrO
4
,
sodium nitrate NaNO
3
, or manganese dioxide MnO
2
in an exothermic reaction. It can
equally be produced by exothermic reduction of chemically produced Cr
2
O
3
using
powdered aluminum as the reductant. The use of aluminum is associated with explosive
hazards and with considerable losses of chromium while molten aluminum in an arc
furnace at 1493
0
C reacts vigorously with Cr
2
O
3
. To avoid this explosion the molten
aluminum is best poured at a lower temperature into a melt of Cr
2
O
3
and with vigorous
stirring, nearly 94% of chromium recovery has been reported.
Chromium metal is also produced on a commercial scale by electrolysis of an
ammonium chromium alum solution prepared either from chromium ore or from high
carbon ferro-chromium. In addition it is produced in more limited quantities by thermal
dissociation of chromium iodide in contact with a suitable heated deposition surface
under vacuum conditions. This gives the purest chromium presently available [6].
2.2 Beneficiating Chromite Ore
Beneficiation operations typically serve to separate and concentrate mineral
values from waste materials, remove the impurities or prepare the ore for further
refinement. Beneficiation activities do not change the mineral values themselves other
than by reducing (crushing and grinding) or enlarging (pelletising and briquetting)
particle size to facilitate further processes.[7]. Chromite ore is beneficiated for processing
using several methods, depending on the ore source and the end use requirements. Coarse
clean ore is hand sorted, while the fine clean ore is gravity separated. Lumpy ore mixed
with host rock may require heavy medium separation. If chromite minerals occurs in fine
grains intermixed with host rock, crushing, gravity separation and magnetic separation
may be used [8].
Various approaches have been used by different researchers to upgrade chromite
ores. Thermal beneficiation was successfully carried out on a low grade chrome ore from
Vagda deposits, Ralnagiri District, Maharashtra India. The method employed was
preferential roasting reduction and acid – leaching. The initial chromium – iron ratio of
1.53:1 was increased by this approach to 8:1 with more than 87% chromium recovery.
The preferentially reduced ore from 72 mesh to 170 mesh (B. S. S) was roasted at 1250
0
C
for a period of two and a half hours with subsequent leaching with dilute tetraoxosulphate
(VI) acid. The effect of temperature on reduction was investigated within the range
1,100
0
C to 1,300
0
C. It was observed that the percentage of ore reduced and dissolved on
leaching sharply increased from 22.84% to 80.78% at 1,200
0
C and with no improvement
noticed at higher temperature. (Ives, 1972). Beneficiation of chromite ore using selective
chlorination was successfully employed to improve the low grade ore in South Africa.
The Cr:Fe ratio attained, using this approach was 20:1 [9]
2.3 Magnetic Separation Technique
Magnetic separation process exploits the difference in magnetic properties
between the ore minerals and is employed to separate valuable mineral from non-
magnetic gangues. All materials are affected in some way when placed in a magnetic

146 O.K. Abubakre, R.A. Muriana and P.N. Nwokike Vol.6, No.2
field, though with most substances, the effect is too slight to be detected. Materials are
classified into paramagnetic and diamagnetic depending whether or not the effect of
magnetic field on them are strong respectively.
The magnetizing force which induces the lines of force through a material is
called the field intensity. The capacity of a magnet to lift a particular mineral is
dependent not only on the value of the field intensity, but also on the field gradient,
which is the rate at which the field intensity increases towards the magnet surface.
2.2
1.2;
−−−−−−−−=
−−−−−−−−−∝
dt
dH
HF
dt
dH
HF
o
µ
where F is the force on the particle, H is the field intensity, and dH/dt is the field gradient
Thus in order to generate a given lifting force, there are an infinite number of
combinations of field and gradient which will give the same effect. Production of a high
field gradient as well as high intensity is therefore an important aspect of separator
design.
3.0 EXPERIMENTAL PROCEDURES
3.1 Size Analysis
A sample of the ore was crushed and ground to fine particle size ranges and
subjected to sieve analysis. At the end of five minutes, the nest was taken apart the
amount of material retained on each sieve was weighed. The result of the sieve analysis
of the sample was recorded.
3.2 Chemical Analysis of the Anka Chromite Ore for Chromium: Iron Ratio
The size fractions obtained from the particle size analysis were subjected to
chemical analysis. Weighed amounts of the chrome ore concentrate and coke were
briquetted with 1% dextrine in the presence of 2% moisture under a pressure of 5.0 PSI.
The coke sample used for the investigation contained 75.52% fixed carbon, 22.51% ash
and 1.95% volatiles.
The dried briquettes were placed in a tightly covered crucible and heated at a
desired temperature for a specific period in a crucible furnace. The briquettes were cooled
under cover after reduction and crushed to pass through the size ranges used for sieve
analysis. A weighed amount of the reduced ore powder was then leached with excess of
10% by volume tetraoxosulphate (vi) acid solution under boiling condition for an hour.
The slurry was then filtered and the residue free from acid. The leach cake was air dried
at 110
0
C and the Fe: Cr ratios was calculated using leach liquor analysis and weight loss
after reduction.

Vol.6, No.2 Characterization and Beneficiation of Anka Chromite Ore 147
3.3 Wet Chemical Analysis
Standard chemical procedure [10] was employed to determine total iron content
and contents of Silica (SiO
2
), Alumina (Al
2
O
3
), Calcium Oxide (CaO) and Magnesium
Oxide (MgO) in the sample.
3.4 Magnetic Separation of Anka Chromite Ore
In this operation, use was made of the cross-belt separator. This technique is a
continuous process carried out on a moving stream of particles passing into and through
magnetic field. In this technique close control of the velocity of the passage of the
particles is ensured to drastically reduce the chances of free-fall. The dry material was fed
in a uniform layer on to the conveyor belt and was carried between the poles of the
magnetic system, which consists of two or more horse-shoe electromagnets, the poles
being arranged one above the other. This arrangement ensures the concentration of the
force fields which then attract the paramagnetic materials towards the poles. The cross-
belt serves to prevent the magnetic particles from adhering to the poles and carry them
out of the field.
For this work, two kilograms (2kg) of the ore was used and at the end of the
operation, the magnetic material obtained weighed 739.94g representing 36.997% while
the balance weighed 1260.06g representing 63.003% of the total material. The wet
chemical analysis of the concentrate was also carried out.
The concentrate was pulverized and size fractions 72,100,120,170,200, and 250
BSS mesh were subjected to the Atomic Absorption spectroscopy and the result for
percentage chromium recovery and Cr:Fe ratios obtained is presented in Figure 2 while
the result of the analysis of the concentrate is shown in Table 2.
4.0 RESULT AND DISCUSSIONS
The result of the chemical analysis study of the Anka chromite ore using wet
chemical analysis is presented in the Table 1. From the result shown in the table it is seen
that the Anka deposit is rich in Chromium assaying 36.84% Cr
2
O
3,
It also shows that
other compounds such as FeO, MgO, Al
2
O
3
, SiO
2
and CaO, which are of industrial
values, are in association.
The ore could be classified to be of medium grade when compared with other
worlds deposits such as the Turkish ore (Gostar and Vagda deposits) containing 30%
Cr
2
O
3
, Ratnagiri District Maharashtra which contains 33.14% Cr
2
O
3,
and Central Ural
deposits in Russia containing 30 - 39% Cr
2
O
3
[6 and 11].
The result of the sieve analysis of the ore sample is presented in the Fig 1. The
coarse component of the ore is quite significant. More than 50% weight falls within sieve
aperture size of 1000 – 500 µ m. The proportion of the extremely fine particles is also
considerable constituting about 10% weight of the sample.
The variation in percentage chromium recovery and the change in Fe:Cr ratio at
different mesh size is presented in Figure 2. The result shows that Cr:Fe ratio appreciated
progressively above particle of mesh size of 100 and falls again as the particles become
finer. The fall in Chromium: Iron ratio can be attributed to losses of the valuable minerals

148 O.K. Abubakre, R.A. Muriana and P.N. Nwokike Vol.6, No.2
as the fineness of the particle increases. An average sample of the enriched ore taken
from mesh sizes ranging between 70 – 250 BSS were subjected to both wet chemical
analysis and Atomic Absorption Spectroscopy and results are as shown in Table 2.
The results in the table shows again that the maximum chromium recovery lies in
the size ranges considered as any progress on either side of the size ranges results to a
noticeable drop in chromium recovery.
Table 1: Chemical Analysis of raw sample of Anka Chromite Ore
Molecules FeO MgO Cr
2
O
3
Al
2
O
3
SiO
2
CaO Others
Composition % 21.51 14.00 36.84 11.00 13.74 2.24 0.67
Table 2: Chemical Composition of the Enriched Ore
Compound FeO MgO Cr
2
O
3
Al
2
O
3
SiO
2
CaO Others
Analysis based on AAS 13.80 10.00 44.68 16.76 2.46 1.34 10.96
Wet Chemical Analysis 11.57 11.35 43.97 16.38 3.56 2.21 10.96
Figure 1: Result of Sieve Analysis of the Chromite Ore Sample.
0
5
10
15
20
25
1000
710
500
355
250
180
125
0.09
0.06
-0.06
Sieve Size
Percentage Weig
ht
% Weight

Vol.6, No.2 Characterization and Beneficiation of Anka Chromite Ore 149
Figure 2: Variation of Chromium Recovery and Cr:Fe ratio with particle size.
5.0 Conclusion
Magnetic separation is known to be more appropriate for treating fine particles.
However, the greater tendency of the associated gangue materials to form dust more
readily than chromite ore may be responsible for poor chromium recovery as the fineness
of Anka chromite deposit increases.
The characterized ore was successfully upgraded to meet specification for
metallurgical use by conventional ore dressing methods.
It was also established that chromium ore can best be beneficiated for maximum
chromium recovery in fraction ranges 100 – 200 mesh (BSS).
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0
10
20
30
40
50
60
70
80
90
100
70
100
120
170
200
250
Sieve Size
% Cr Recovery
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Cr:Fe Ratio
% Cr Recov
Cr:Fe

150 O.K. Abubakre, R.A. Muriana and P.N. Nwokike Vol.6, No.2
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