SYNOPSIS
The western plateau of West Bengal covering a large part
of Purulia and Birbhum districts and a smaller part of Midnapur
and Murshidabad districts has quite extensive deposits of late**
rite mineral*
A conservative estimate has shown that the mate­
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rial is distributed over an area of about 1000 sq*kms* running
with an approximate north-northeast trend from Farakka through
Kharagpur and merging with costal laterite tract of Orissa*
The
laterite deposit is found mostly on the surface going down to an
average depth of about 2 metres.
This material has not so far
drawn enough attention for exploitation, precisely because India
is endowed with extensive reserves of high quality iron ores
relative to which this low grade ore with barely 33$ total iron
content is yet premature for a consideration.
But since its con**
sumption as a sheer road construction material is not at all
justifiable, this investigation ' s
undertaken in order to seek
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some other utility route for this material - for its iron con­
tent or otherwise*
The major long term objectives for developing
processes for its utilization were s stoppage of a wasteful pro­
cess by which a mineral is being consumed for purposes other than
metallurgical, making available large arable lands for cultiva­
tion and bringing into open new areas for industrial development,
particularly iron, steel and ancillary ferrous industries thus
rendering possible new avenues for manpower utilization*
The
study was desired to be comprehensive and was therefore divided
into two parts s One dealing with electrolytic iron powder production and the resulting characteristics for powder metallurgy
applications and the other consisting of pellet production and
direct reduction of the same.
For the powder production aspect of the work* the ore,
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washed, dried and crushed to -100 mesh was leached in concentrated
sulphuric acid under boiling condition with stirring.
Using acid
30$ excess of stoichiometry, maximum leaching efficiency of 92$
oould be achieved and after filtering off the residue, the leached
liquor had to be reduced from ferric to ferrous state by using the
required amount of iron scrap, cleaned and cut to small pieces.
This ferrous sulphate solution, after filtration was crystallized
twice to obtain very pure FeS04 crystals, utilizing as muoh of
the mother liquor as possible*
Most of the unwanted contaminants,
specially AlgOg which constituted a large part of the ore (19.75$)
and SiOg (13.75$) could be eliminated by this process together
This additional step,
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with other impurities like Cr, Ti etc.
prior to electrolysis, was decidedly the only means instrumental
in delivering the purest possible iron powder on electrodeposi­
tion.
This step was a ready way out for obtaining pure, lustrous
and steely metallic iron powder from this low grade ore because
numerous other alternative attempts had failed in someway or the
other * The crystals (FeS04t7H20) thus obtained were employed to
prepare the ferrous sulphate bath which was electrolysed with
stainless steel cathodes and graphite anodes in a 2-litre capacity
cell (made of perspex glass) having provision for electrode adjust­
ment in all required directions*
A hot water-bath, magnetically
stirred, was employed with proper controls to attain the required
operating temperatures recorded by a thermometer inserted into
the bath*
A suitable rheostat was put in the circuit to obtain
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the desired current densities and the pH was varied by additions
of either H2$0d or sodium bicarbonate addition as the case might
be*
As an addition agent NaCI was used in amounts of 45 gm/litre•
The iron powder electrodeposited was thoroughly washed* dried and
reduced in Hydrogen at 800°C for 1 hour, to obtain the powder in
pure metallic form.
This was processed into standard compacts of
0*5 cm length and 1 cm diameter, sintered to 1100°C and polished
and these samples were scanned under electron probe microanalyser
to detect impurities, if any*
During this powder production, the
variables studied were concentration of ferrous sulphate bath,
temperature, pH of the bath and current density*
After optimiza­
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tion, the conditions leading to a current efficiency of 90$ were *
bath (FeS04 ) concentration of 140 g/1, bath pH of 4*5, operating
temperature of 45°G and CUD* of 0*055 Amp/os2 *
With these conditions of operation, attempts were made to
raise the yield of powder as far as practicable*
A maximum yield
of 49*8$ could be achieved with a 4 hour duration of electrolysis
utilizing 5 electrodes*
The powder thus obtained was thereafter
subjected to various standard tests to study its powder charac­
teristics,
The following properties were determined * Hydrogen
loss, apparent density, flow rate, particle size distribution,
green density, bending strength, densification parameter and
chemical analysis*
powder having a
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Krom these, it was concluded that the
total iron content of 99*3$, metallic iron content =» 97,4$,
Al = 0 ^ } Si s 0*08$, Ti » 0.12$ and Or » 0*14$, when compared
to the electrolytic iron powder market ted from Japan and the
Hoganas reduced iron powder, showed a favourable position as
regards apparent density and total iron content#
The flow rate,
particle distribution, green density and bending strength have
been slightly inferior#
The sieve analysis data showed that the
maximum fraction <30*15$) belongs to the mesh size of -100 to
+150 i .e . relatively coarser due to which the flow rate was a
little higher being 33*7 secs/50 pis*
The apparent density in
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the present case was 2.849 gas/c *cM densification parameter
0*211 and H2-loss was 0*525$#
This last value being comparatively
higher and particles being coarser, the values of compressibility
(green density) and bending strength were relatively lower, being
5*895 gm/c.c# and 14*1 kg/mm3 respectively.
The other aspect of this powder extraction process was
treatment of the residue after leaching#
analysed
This residue finally
to be 87*46$ Si02 and 2*02$ A1203 showing that about
90}i of the AlgOg was eliminated from the ore#
This material was
tested for its refractoriness and the standard P*CVE* tests were
performed*
Results indicated that this residue part has a P*C*B*
between ORTON cone 38 and 30, i*e* has a melting point between
1646° and 1665°C.
Remembering that pure silica has a softening
temperature of 1695°C and that the residue here contains some
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iron oxide too* this material could be utilized as a refractory
material#
In the second part of the study, the ore of -100 mesh
size has been pelletlzed in a laboratory rotation drum ;<2? cms
dia* and 15 cms length)t selecting a lower speed of 24 rpm* During
this, the ore was thoroughly mixed with moisture and bentonite*
the percentages of which were varied from 7 to 11 and 0*5 to 1*2,
respectively. \The optimum conditions as regards recovery of sound
pellets were found to be 10^ moisture and 1$ bentonite on the basis
of which lime addition was varied In the charge from 1 to 4$, prior
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to firing*
temperature*
Pellet firing consisted of two variables viz* time and
Time was varied from 1 to 3 hours while the tempera­
ture from 900 to H00°C*
The pellets were taken in rectangular
steel trays, dried at 120°C for about it hour and then fired at the
desired temperature in an electric furnace having silicon carbide
heating elements*
The fired pellets were thereafter tested for
their crushing strength and individual permeability*
A special
loading device was employed for strength measurement and a standard
permeability meter was used for noting the permeability*
For the
latter case, the pellet was given a cylindrical shape of 0*8 cm:
dia. and 1 car height and was kept in position above the nozzle
of the instrument* in a specimen holder or attachment, specially
fabricated for the purpose*
In general, it has been
observed that the effect of lime,
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except at higher temperature has not been very significant in
increasing the strengths
Lime usually fluxes the refractory slag-*
forming constituents leading to an increase in slag bonded matrix;
it generally has a tendency to form calcium ferrite having a good
bonding network*
Here, owing to profusion of quartz, as was petro*
graphically detected by analysing thin sections, hedenbergite
£ (Ca, Fe) Si03J
was forming and moreover due to high silica,
proper basicity could not be maintained,
Temperature and time of
firing, on the other hand, have been found to have a marked effect
on strength and permeability.
The rapid rise in strength at tem­
peratures above 1000°G was attributed to a possible mechanism
The strength
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of sintering in the presence of a liquid phase.
versus temperature showed a change of slope i .e . a knee, in
general, indicating that two types of mechanism were possible
operative.
With the rise in time and temperature of firing there
has been a steady decline in permeability.
Such closure of pores
by diffusion and by the difference in the degree of grain growth
(accretion) continues with rise in time and temperature, thus
decreasing permeability with increasing strength*
However, the
maximum strength values obtained were appreciably low compared
to what is generally recommended for use*
This was attributed to
the facts that firstly, unlike the usual finer size of -325 mesh
used for pelletizing, here the ore was of -100 to +150 mesh;
secondly, the ore was not in any m y concentrated and thirdly,
it was very high in A1203 and Si02 contents, thus making diffusion
bonding and slag bonding inefficient and maintenance of proper
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basicity impossible. Still higher temperatures of firing or pro­
longed drying techniques evidently would not have added much*
A regression analysis was done for both production and
firing of pellets*
Results of the calculations indicated greater
effect of bentonite on pellet recovery (or pelletizability) than
moisture*
The following equation was formed *
1
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where,
1
»
»
40Xx + 6X2 - 22
...
(1)
% Recovery of pellets*
X-^ « '■$ Bentonite.
Xg «
$ Moisture*
>
In case of firing, temperature was found to have a more pronounced
effect on strength than time - the combined effect being encouraging.
The equations (Regression) in this case were
Y
«
4.48 + 1*78XX + 0 . 58X3 + 0*4X 1X2
t
«
Strength <kg/pellet)
...
<2)
Temperature * 1050
50
Time - 2.5
0.5
and
«
47.2 - 2 0 .SX^ - l*2Xg +
Y*
«
Permeability'
„*
X-i
=
***
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where,
Y*
x
and
v» _
2 ~
Temperature * 1050
........ ... . '■■
■■.
50
Time - 2.5
" oT5
Drastio reduction in strength had to be accepted to keep permea­
bility high.
Reduction of pellets was (tone subsequently by changing
the reducing agents time and temperature of reduction.
agents used were Hg, charcoal and graphite.
Reducing
Temperature used
were 800} 900 and 1000°C and reduction times of 1, 2 and 3 hours*
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Though Eq was found to be the most efficient of the three* it
was not pursued.
Even i f the cost of hydrogen, as a reducing
agent, yis^a-vis, that of charcoal is lower, the total reduction
cost is likely to be more in view of the electrical power consump­
tion and installation of the special hydrogen reduction furnace
when singly used for the reduction of pellets alone.
Graphite
reduction having proved unworkable, charcoal was finally seleoted
for reduction*
The pellet samples selected for the
purpose were
packed securely with charcoal powder in a Nickel crucible and
introduced into the furnace for reduction at the desired time
and temperature*
The percentage metallization was found to fall
with rising strength and lowering permeability, showing that the
more well-oxidised and porous the pellet} greater was the reduc­
tion leading to metallization*
Sinae pellets of intermediate
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strength and permeability <i*e* those fired at 1000°C for Z hours)
showed greater metallization, taking this category of pellets,
the reduction time was varied*
While the rise in metallization
was remarkable for a change from 1 hour to 2 hours it was not so
well marked for a change from 2 to
3 hours <i*e* 43$ to 43$)*
reduction temperature in these cases was 1000°C*
The
When temperature
was varied from 800 to 1000°C, the rise in metallization was found
to be very steep.
This was expected since the reductioa was gene­
rally governed by diffusion of products and reactants through a
reacted shell and moreover, the solid state reduction being endothemic in character, higher temperatures helped*
But an indis­
criminate rise in temperature might retard reduction at the centre
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owing to fusion*
However, with these operating condition^, if
the reduotion were carried out under vacuum, metallization values
would obviously have improved manifold* Metallization
(fa )
values
were calculated by X-ray diffraction analysis on the basis of peak
heights obtained at a selected 29
angle of 57*10° with iron target
Finally a brief estimate of the economics of the process
was made in order to seoure informations for throwing some light
on any production work that can be attempted with this material*
After detailed consideration of the approximate market price of
the various raw materials, power, gas etc., and the respective
amounts consumed for the powder produced on a per kg basis, the
cost was found to match favourably with the present market rate
of imported electrolytic iron powder*
Certain major theoretical
assumptions like very efficient acid regeneration during electro­
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lysis, replacement of power for only the heating purposes like
leaching, scrap reduction, crystallization by superheated steam
and joint use of Hg-reduotion set up have been made to lower the
running cost•
Since it was beyond the scope of the present investi­
gation to consider in details, how the pellets having a comparative­
ly low degree of metallization would fare in comparison with the
normal pellets with high degree of metallization, the actual cost
considerations for the pelletizing part were not attempted.
The author wishes to conclude that considering the abun­
dance of gangue contents, a centralized well-equipped beneficia-
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tion and concentration plant for rendering the ore much more
workable by ridding it of the degrading impurities would be a
common solution for either or both of the processes for utilizing
this ore*
Moreover, since Hg-reduotion of pellets is comparative­
ly much more efficient, a combined plant using Hg-reduction for both
the processes i»e* powder as well as pellet reduction, would be
worth a consideration*
It will be worthwhile to consider the reduc­
tion of powder and/or pellet in a tubular furnace, heated by high
calorific value fuels instead of an electrical resistance furnace
with associated high power costs.