Waste Manage Res 2006: 24: 74–79 Copyright © ISWA 2006

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Waste Management & Research
ISSN 0734–242X

An overview on the production of pigment grade

titania from titania-rich slag

To recover pigment grade TiO2, operating plants all over the Kamala Kanta Sahu
world use chemical processes. Slag-based technology is con- Thomas C. Alex
sidered to be attractive because of low waste generation and
Devabrata Mishra
low chemical cost due to high titanium content and is poised
Archana Agrawal
to replace the conventional technology. This paper provides Metal Extraction and Forming Division, National Metallurgical
a review of the slag-based technology with the specific aim to Laboratory, Jamshedpur, India
produce leachable slag and achieving high titania yield from
recovered wastes. Leachable oxides of the lower oxidation
state, such as TiO and Ti2O3, facilitate the leaching process. Keywords: Titania, slag, pigment, leaching, ilmenite, rutile,
However, during smelting these oxides increase the viscosity wmr 810–4
of the slag. Formation of titanium carbide or carbonitride is
also not desirable as it leads to resistance to the leaching of Corresponding author: Archana Agrawal, Metal Extraction
and Forming Division, National Metallurgical Laboratory,
titanium. This report highlights the problems and their possi- Jamshedpur, India.
ble solutions to obtain leachable slag. e-mail: archana_nml03@yahoo.com

DOI: 10.1177/0734242X06061016

Received 29 November 2004; accepted in revised form 12 October 2005

Introduction

Titanium is the ninth most abundant element and occurs second largest producer and Asia-Pacific region with 21%
mainly in a form that can be mined as ilmenite (FeO.TiO2) capacity, is the third. The remaining 10% are distributed
(95%) and rutile (TiO2) minerals (5%). The current world among the rest of the world.
production capacity of ilmenite and other titanium feed- In India, the installed production capacity for ilmenite is
stocks for production of titanium oxide pigment, titanium 475 000 tpa and the production of ilmenite and rutile during
metal, welding electrodes etc. is around 7 million tonnes per 1997–1998 was over 300 000 and 13 000 tonnes, respectively.
annum (tpa). Approximately half of this is from beach sands There are four synthetic rutile plants in the country belong-
and the balance from ilmenite rocks found in Canada and ing to Indian Rare Earths Ltd, Kerala Minerals & Metals Ltd,
Norway mainly. In view of the limited supply of natural rutile, DCW Ltd and Cochin Minerals & Rutiles Ltd, all of which
its share of consumption in pigment production is rather operate on the Benelite/Wachang process. The total synthetic
small and 55% of its total availability of about 500 000 tpa is rutile production during 1999–2000 was about 60 000 tonnes,
used in the non-pigment applications, predominantly in the more than half of which was exported. There are four pig-
production of welding rod and titanium metal. The present ment-producing plants in the country, three based on the sul-
production capacity of pigment in the world is just above phate process and one based on the chloride process, with a
4 million tpa. North America is the largest producer with total production of about 30 000 tonnes in 1999–2000 (Sivas-
37% of the global capacity. Europe, with 32% capacity is the ubramanian 2001).

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 Production of pigment grade titania from titania-rich slag

Another rich source of titanium is titania slag, which is of the impurities separate as solids. It is then reheated to a gas
generated from a metallurgical process during which iron is and mixed with hot oxygen to form very fine crystalline rutile
removed from ilmenite or titaniferrous ores by carbothermic (raw white pigment). The displaced chlorine gas is recycled
reduction smelting in a DC plasma/electric arc furnace (Wel- to the start of the process.
ham & Williams 1999). The slag is the major source of raw
material supply to the pigment companies providing about TiO2 (impure) + C + 2Cl2 1 TiCl4 + CO2 (2)
39% of the feedstocks in comparison with 33% from natural
ilmenite (Mackey 1994). The titanium slag replaces the nat- TiCl4 + O2 1 TiO2 + 2Cl2 (3)
ural rutile in the same manner as synthetic rutile. Consequently,
the pigment and metal industries worldwide no longer fully Although capital investment in the chloride process is
depend on natural minerals. The major advantages of the slag about 1.7 times more than in the sulphate process, there are
technology are: number of other advantages to the chloride process such as
the yield of high-quality product, more eco-friendly process
• high titanium content; and the generation of smaller amount of waste products.
• low waste generation;
• suitable for both sulphate and chloride processes;
• low chemical cost.
Other processes
In spite of high power consumption, smelting of ilmenite or
There are essentially two process options for the manufac- titanium-bearing ore has several advantages over the synthetic
ture of titanium dioxide pigment from titanium slag, namely rutile route by chemical process. It converts iron oxide as part
the sulphate and the chloride processes. Both process options of the concentrate to value-added iron metal and slag con-
yield final products falling in the category of titanium diox- taining enriched titanium. This technology can use low-grade
ide pigment rutile grade (TDPRG) and titanium dioxide pig- ilmenite with zero waste generation. Several research and
ment anatase grade (TDPAG). development organizations world-wide including India are
involved in improving smelting technologies to bring down
the power consumption and enrich the slag quality with
Sulphate process respect to its TiO2 content as well as leachability. The detailed
In the sulphate process (Mackey 1994), the ground slag or research and development effort on this aspect is briefly
ilmenite is digested with strong sulphuric acid to solubilize reviewed.
titanium, which is later hydrolysed and precipitated to form Brent & Reid (1987) investigated the thermal reduction of
titanium dioxide pigment. The ferric sulphate in the solution ilmenite concentrate to produce a high-grade titania slag and
is reduced with scrap iron to ferrous sulphate after which the pig iron, using a DC transferred-arc plasma furnace. Three
solution is cooled down to crystallize out FeSO4.7H2O. After different concentrates of ilmenite, which were widely differ-
suitable adjustment the remaining solution is boiled to pre- ing in chemical compositions and geological histories, were
cipitate the TiO2.The precipitate is heated, without melting, studied to assess the suitability of the reactor configuration
to about 1000°C to drive off contained water and allow the for ilmenite smelting. The effects of reducing agent, particle
formation of very fine crystals of raw white pigment. size and excess carbon on the slag grade, titanium and iron
recoveries, reducing agent efficiency, etc. were studied, and the
FeTiO3 + H2SO4 1 FeSO4 + TiO2 + H2O (1) DC-plasma furnace was found suitable for ilmenite smelting.
Arc furnace reduction smelting of ilmenite concentrate to
The advantages of the sulphate process are low capital invest- produce pig iron and titania slag on a laboratory scale has
ment and low energy consumption. The disadvantage is the been studied by Poniatowski & Zielinski (1983) using various
formation of a large amount of by-products. amounts and types of carbonaceous reductants. TiO2 and FeO
content could be controlled by varying the amount of reduct-
ant in the charge and they were in the range of 60–80% and
Chloride process 3–8%, respectively.
The chloride process (Mackey 1994) mainly consists of cal- Cui et al. (1984) found that the fusibility and fluidity tem-
cining a mixture of synthetic rutile, coke and chlorine to perature increased with decreasing FeO content. The temper-
form gaseous titanium tetrachloride (TiCl4). Ilmenite cannot ature changes abruptly when changes occur in the nature of
be used as a raw material as its titanium content is too low. the phases in the solidified slag. Basic oxide added as flux
The titanium tetrachloride is condensed to a liquid and most decreases the effect of decreasing FeO content. They also

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K.K. Sahu, T.C. Alex, D. Mishra, A. Agrawal

found that oxygen potential has a large influence on the soluble sulphates. These impurities were subsequently removed
melting temperature of the slag. by water or diluted hydrochloric acid leaching. Lan (1996)
An improved process for production of titania slag was reported a possible process for recovery of titanium dioxide
proposed by Karyazin et al. (1985). This involved the pre- from low grade (∼ 20% TiO2) blast furnace slag by the sulphuric
reduction of powdered ilmenite concentrate with natural gas acid leaching route. Borowiec (1991) investigated the recovery
in a fluidized-bed furnace. The process was much simpler in of titanium as rutile, namely 92–95% TiO2, by sulphiding of
comparison with a rotating furnace and yielded low carbon- solid titania slag at 1100°C. A mixture of H2S + N2 and S2 +
and sulphur-free micro-alloyed steel as a by-product. C was most efficient for the removal of Fe, Mn, V and Cr from
Pre-reduction of ilmenite and titaniferous magnetite in a the slag. For the removal of MgO the sulphation process was
rotary kiln with coal, to assess the feasibility of improving the carried out on the sulphided and leached slag as well as directly
melting operations was investigated by Nafziger & Jordan, on the sulphided slag. The alkali and alkaline earth metals were
(1983). The ilmenite yielded Ti-enriched slag that was suita- removed by leaching with water after sulphation.
ble for further processing by conventional methods. Pre- An economically viable process for extraction of TiO2
reduction decreased electrode consumption during furnace pigment from low titania slag obtained from Panzhihua iron
reduction and also conserved expensive electrical energy that concentrate using the sulphate process has been reported by
otherwise must be used to reduce and melt the entire titanif- Deng et al. (1989). Suitable modifications were incorporated
erous material charge. in the ilmenite–sulphate process to accommodate the differ-
High-quality meltdown slags were produced by Nafziger ences in the characteristics of the titania slag.
(1978) by open-arc operations in a carbon-lined furnace from Rahn & Cole (1981) reported diluted sulphuric acid leach-
re-reduced titaniferous iron sands. The titania slag produced ing of titaniferous material to obtain a stable hydrolysable tit-
was found to be suitable for further processing. He observed anyl sulphate solution. They found that the presence of a
that pre-reduction leads to improved metal and slag quality, reductant in the digestion system greatly accelerated the dis-
higher productivity, and lower energy and electrode con- solution kinetics.
sumption. In a set of investigations, Waldman et al. (1981) found that
An improved process was reported by Denton & Schoukens the presence of an excessive amount of iron in the digested
(1994) for the production of titanium-rich slag and pig iron solution of titanium ore causes premature hydrolysis and sub-
from ilmenite. The ilmenite was fed continuously, together sequent loss of titanium in the gangue, and furthermore this
with carbonaceous reductant in the absence of fluxes to the causes waste disposal problems. Taking care of such problems,
molten bath of a circular DC plasma arc furnace, wherein the a stable titanyl sulphate solution was successfully prepared.
molten bath formed the anode and one or more electrodes in Thus titaniferous material was reacted with dilute sulphuric
the furnace roof formed the cathode. Titanium-rich slag tapped acid to provide titanyl sulphate at a temperature below 140°C.
off continuously or intermittently, could be used as a feed to The resulting reaction mixture was cooled to a temperature
the chlorine-based titanium dioxide production process. below 110°C with titanyl sulphate in the dissolved state and
In order to avoid the by-product disposal problems of the diluted with water to produce a reaction mixture having an
sulphate process, Raddatz et al. (1979) conducted flux smelt- iron to titanium dioxide weight ratio of about 0.5–1.2 : 1.0,
ing studies of ilmenite. They smelted a charge comprising of titanium dioxide content of about 120 to 180 g L–1, specific
ilmenite, soda ash and carbon to produce a high soda titanate gravity between 1.4 to 1.8, and an active sulphuric acid to
(HST) and pig iron. Upon calcination this HST, after mixing titanium dioxide mole ratio of 1.4–1.9 : 1. Undissolved solids
with sulphuric acid followed by water leaching, yielded a low were then separated and a stable titanyl sulphate solution was
titania slag (LTS). Both the slags (HST and LTS) were found obtained. The titanyl sulphate was then hydrolysed and cal-
to be effective substitutes for ilmenite/rutile for TiO2 manu- cined to give a titanium dioxide and spent sulphuric acid solu-
facture using the sulphate/chloride process. TiO2 extraction tion.
of 94–97% was obtained with 50 wt% sulphuric acid. Dissolution of titanium to hydrolysable titanyl sulphate
solution by decomposition of the ternary raw material mix-
ture was studied by Rieck et al. (1981). The process involves
Titania recovery from titania slag simultaneous decomposition of ilmenite mixed with slag,
Several methods have been developed to recover titanium containing various properties of TiO2 and Ti(III) compounds.
dioxide from slag. Elger et al. (1977, 1986) reported the sul- Titanium recovery was found to be more than 90% of the
phation process to produce high-purity slag suitable for chlo- total input.
rination. The slag was reacted with a SO2 and air mixture at Gueguin (1982) reported the sulphuric digestion of titani-
700–900°C, which selectively converted the impurities to ferous slags and more specifically of a method to decrease the

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 Production of pigment grade titania from titania-rich slag

reduced titanium concentration in the resulting sulphate liq- son 1982, Skillen 1992) Each of these three-phase open-arc
uor. The process uses lignin products to assist in the oxida- furnaces is rated at 69 MW. The process technology, origi-
tion of the Ti+3 content of the slag to Ti+4. nally developed by Quebec Iron & Titanium (QIT Fer et
Water-soluble titanium compound was recovered by Hall Titane) of Sorel, Canada, was supplied to RBM in the mid-
et al. (1981) in solid forms from titaniferous concentrate or slag 1970s, with the first furnace starting up early in 1978. The
by reacting with sulphuric acid inside a heated pelletizing appa- fourth furnace started up in mid-1991. The process has
ratus. Titanium recovery of up to 88% was achieved. been adapted to smelt fine ilmenite obtained from a beach-
Significant enhancement in titanium dioxide recovery from sand deposit on the north-eastern coast of South Africa.
a titaniferous ore was reported by Sheehan & Knapp (1982) An open-bath approach is employed for which careful con-
by improving the efficiency of the comminution of the ore, trol is required to avoid erosion of the refractories of the side-
using polyols. and end-walls by the very reactive titania slag. The installed
electrical capacity possibly makes these furnaces the largest
scale AC transferred-arc plasma operation to date. RBM is
Process followed by different companies currently the only South African producer of titania slag, and
in the world has an annual production capacity of some 2 million tonnes
There are four major companies in the world, other than Rus- of ilmenite (FeO.TiO2) and 125 000 tonnes of rutile (TiO2).
sia, engaged in the recovery of TiO2 from ilmenite (Mukher- The ilmenite is smelted to produce about 1 million tonne of
jee 1998) by the slag route (Table 1). slag and 550 000 tonnes of pig iron per annum (Robinson
1992). RBM’s ilmenite is of too low a grade to be used
QIT-Feret Titanate Inc. of Canada, a subsidiary of the RTZ (Rio directly for the production of pigment or synthetic rutile.
Tinto-Zinc) corporation produce slag and iron at its Sorel Therefore, RBM followed the slag-beneficiation route, and
plant by smelting rock ilmenite analysed as 34% TiO2 and currently produces about half of world titania slag output.
53% iron oxides in its maiden electric furnace. The plant Tinfos Titan and Iron KS of Norway produces sulphatable slag
has incorporated roasting and beneficiation processes for the from hard rock ilmenite analysed at 44% TiO2 and 46.5%
upgrading of the ore, thus producing 1.05 million tonnes FeO. The process based on AC smelting technology, is
of sulphatable slag analysed as 72% TiO2. The company slightly different from QIT/RBM which involves additional
also produces 0.2 million tonnes of upgraded slag analysed steps such as pelletization and pre-reduction at 1150°C prior
at 95% TiO2, 0.6% Al2O3, 1.95% SiO2, 0.14% CaO, 0.6% to smelting. Presently, about 0.3 million tonnes of slag ana-
MgO, 0.05% MnO, 0.46% P2O5, and 0.03% Cr2O3. lysed at 75–85% TiO2 and 0.1 million tonnes of pig iron
Richards Bay Minerals (RBM) of South Africa employs four are produced per year.
rectangular six-in-line graphite-electrode furnaces for the Namakwa Sands of South Africa in collaboration with
smelting of ilmenite. Each furnace is 19 m long, 8 m wide, MINTEK successfully commercialized DC smelting tech-
and has a power supply rated at 105 MVA (each pair of elec- nology for processing sand ilmenite containing 47.3% TiO2
trodes being supplied by a 35 MVA transformer) (MacPher- and 46.7% iron oxides.

Table 1: Metallurgical process options for beneficiation of ilmenite to titania slag.

Process Description Plant status

QIT – Electrosmelting Carbothermic smelting of hard rock ilmenite at SOREL Quebec

1700°C to pig iron and sulphatable titania slag 1 000 000 tonnes year–1 slag
(86–87% TiO2)
RTZ Iron & Titanium Same as above: but customized to suit beach Richards Bay, South Africa, 1 000 000 tonnes year–1
Electrosmelting sand ilmenite of slag and 500 000 tonnes year–1 pig iron
Submerged arc smelting process Pre-reduction of ilmenite (50% TiO2), followed TINFOS, Norway,
by smelting in arc furnace to produce pig iron 200 000 tonnes year–1 of slag and
and titania slag (87% TiO2) 100 000 tonnes year–1 of pig iron
Plasma DC arc smelting Carbothermic smelting of ilmenite in DC arc NAMAKWA Sands Ltd., South Africa,
technology (South Africa plasma furnace to pig iron and titania slag 1 100 000 tonnes year–1 of slag and
Anglo American Corp.) 450 000 tonnes year–1 of pig iron
ISCOR, South Africa 220 000 tonnes year–1 of slag
from 1999
Upgraded slag Conditioning of the SOREL slag and its leaching SOREL, Quebec 200 000 tonnes year–1 of slag to
technology followed by calcination to UGS(up-graded slag) be expanded to 600 000 tonnes year–1
(95% TiO2)

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K.K. Sahu, T.C. Alex, D. Mishra, A. Agrawal

Table 2: Synthetic rutile plants in India

Present production
Name of the company Process
(metric tonnes year–1)

Cochin Minerals & Rutile Ltd. 18 000 (95% TiO2 for export) Wah-Chang process: reduction roasting, two-stage leaching
with 30% HCl, oxidation of first leach liquor to FeCl3 for sale.
TiO2 recovery: 88%.
Kerala Mineral & Metal Ltd. 18 000 (89% TiO2 for in-house use) Benelite process: reduction roasting, two-stage leaching with
regenerated acid.
Dhrangadra Chemicals Ltd. 20 000 (95% TiO2 for export Wah-Chang process: reduction roasting, leaching with 30%
HCl, disposal of leach liquor, TiO2 recovery: 90%.
IREL (OSCOM) 7000 (92% TiO2 for export) Benelite process: reduction roasting, three-stage leaching with
regenerated acid, TiO2 recovery < 70%.

The production of synthetic rutile through acid leaching should be avoided. To avoid the formation of titanium car-
of various grades of ilmenite (Mukherjee 1998) by Indian Rare bide, reduction of TiO2 in the system to Ti during smelting
Earth Ltd. (IREL) is given in Table 2. Large quantities of ilmen- has to be controlled, because the presence of metallic tita-
ite are exported to foreign companies for the production of nium will lead to the formation of TiC as per the following
synthetic rutile, slag and pigment. reactions:
Furthermore, KILBURN, TUTICORIN and KOLMAK,
Calcutta, produce titanium dioxide pigment rutile grade TiO2 + 2C 1 Ti + 2CO (4)
(TDPAG) with capacities of 1800 and 3000 tonnes year–1,
respectively, by the sulphate route. Ti + C 1 TiC (5)
Leachability of titanium in the slag mainly depends on
the phases present, grain size and the oxidation states of the As titanium metal cannot exist in the presence of iron oxide,
corresponding oxides. Leaching efficiency will decrease if the the formation of TiC can be avoided by keeping a small amount
slag is rich in titanium dioxide, titanium carbide/nitride or of FeO in the slag. This can be achieved by smelting ilmenite
carbonitride, because these are practically insoluble even in and deficient carbon to have TiO2, Ti2O3, TiO and a rela-
hot sulphuric acid. Hence, to get higher leaching efficiency, tively higher concentration of FeO at equilibrium in the
the contents of these phases have to be minimized by con- melt. Further reduction of FeO, if needed, could be achieved
trolling the process parameters, which will facilitate the for- by slow injection of additional carbon.
mation of leachable oxides of lower oxidation states such as It has been reported (Miller 1957) that rapid cooling by
TiO or Ti2O3. water quenching causes slag to have a small grain size result-
Viscosity of the slag plays a major role during smelting of ing in the formation of fairly large amounts of TiO2, which is
titanium-containing ores. In this context the effect of slag com- insoluble in sulphuric acid. Block casting and slow cooling of
position on the viscosity of the melt during the smelting oper- slag produced better results. In this operation the slag is poured
ation is discussed by Ross (1958). It was found that titanium into large moulds, which are slowly cooled by spraying water
sesquioxide (Ti2O3) and titanium monoxide (TiO) increased inside the quenching chamber. Slow cooling of slag resulted
the viscosity of both acidic and basic slags. High slag viscosity in the formation of larger grain size and the formation of a
causes problems in working of the furnace and in handling of smaller amount of insoluble TiO2, which may give a compar-
the slag. An increase in viscosity also occurs due to the for- atively higher leaching efficiency.
mation of titanium carbide or titanium carbonitride, which The results of solubility tests show that when the TiO2
are formed during carbothermic reduction at high tempera- content of ilmenite and its alteration products goes over 60%,
tures. The presence of titanium carbide and carbonitride in the solubility decreases below the value generally accepted
the slag provides resistance towards leaching efficiency of (98% TiO2 solubility) by the pigment industries. Acid-solu-
titanium. Hence, the formation of these compounds is not ble slag with TiO2 content around 90% has been produced
desirable during smelting. from ilmenite. It has been suggested (Sinha 1979) that to
Therefore, by controlling the operating conditions, namely achieve the required high degree of solubility, the titanium
the reducing environment, leachable slag containing lower originally present in the feed material has to be converted to
oxides such as Ti2O3 or TiO can be obtained, but that will the anosovite phase. Anosovite is a solid solution structure
lead to various other problems as mentioned above. based on Ti3O5. High-temperature oxidation of ilmenite using
To obtain leachable slag, formation of titanium carbide the Beecher process developed in the Government Chemical
or carbonitride and a large proportion of titanium dioxide Laboratories of Western Australia (Mackey 1994) produces

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 Production of pigment grade titania from titania-rich slag

a pseudobrokite structure, which upon solid state reduction from slag are still at laboratory scale. These processes have to
with carbon forms anosovite, minor sub-rutile and metallic be tested and transformed into commercial-scale operations
iron. using the indigenously produced raw material.
Several companies (Mackey 1994) are currently using ther-
mal reduction of ilmenite to produce pig iron and titania slag.
All the Fe2O3 and FeO is reduced to metallic iron with a small
Conclusions
amount of iron in the slag. A pseudobrokite phase is pro- The following conclusions can be drawn:
duced which is suitable feed for the sulphate process. There-
fore, further reduction of the slag having pseudobrokite phase • Smelting of various types of ilmenite results in slag that is
may yield the anosovite phase, which is suitable for titanium rich in TiO2.
recovery. • In smelting, the desired slag chemistry and phase can be
A lot of literature is available on the generation of high obtained by the proper control of the process parameters
titania slag, its quality and leachability. However, due to the (i.e. temperature, reductant, time, flux, etc.).
high power consumption, India just has started to exploit the • A clear separation of slag and metal can be achieved by
indigenous raw material on a commercial scale. Therefore controlling the viscosity.
more research and development efforts are needed not only • The rate of cooling of the slag is an important factor in
on minimization of the power requirement but also on the achieving the desired leachability of the slag.
study of the slag quality and its physical properties and leach- • Detailed characterization of the slag is essential to under-
ability. Most of the processes developed for titanium recovery stand the leaching behaviour.

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