processing of alumina-rich indian iron ore slimes - TRDDC

Reprinted from International Journal of Minerals, Metals 

and Materials Engineering, 59(5), 2006, pp 551-568 

PROCESSING OF ALUMINA-RICH INDIAN IRON ORE SLIMES 

Introduction 

Pradip 

Tata Research Development and Design Centre, Pune, India 

Email: pradip.p@tcs.com 

Indian iron ore industry is one of the world’s largest and growing at a rapid 

pace. Out of a total production of 142.7 million tones of iron ore produced in 

the year 2004-2005, a record 78.1 million tonnes (consisting of 13.5 tonnes of 

lumps and 64.6 tonnes of sinter fines) were exported. As per the national steel 

policy announced recently, India will be producing 110 tonnes million tonnes 

(more than double of the current level of around 42 million tonnes only) of 

steel per anum by 2020 requiring around 190 million tonnes of iron ore. In 

addition the iron ore exports are being projected to reach a level of 114 million 

tonnes and thus the total production of iron ore is expected to cross 300 

million tonnes by 2020. It is an achievable target provided we employ the best 

in science and technology to harness this valuable but non-renewable 

resource for the benefit of our country (1). 

Powered by the steel boom in the country, multi-billion dollars (US$ 70 billion) 

worth of new investments by global steel producers like Mittals, Pohang Steel 

(POSCO), Tata Steel, Jindal and Essar groups, in green field projects in the 

states of Chattisgarh, Jharkhand and Orissa have been announced. The 

public sector giant, Steel Authority of India (SAIL) plans to invest close to USD 

10 billion in next few years to achieve a production capacity of 22 million tons 

per year from the current level of 13 million tons only. There will thus be a 

proportionate quantum jump in the mining and processing of iron ore, 

chromite and coal to meet the demands of the steel industry in the country. 

POSCO alone for example will be developing a 30 million TPA iron ore mine. 

BHP and Orissa Mining Corporation have announced a joint venture project to 

beneficiate iron ores in Orissa. Optimum utilization of iron ore resources of the 

country has thus become a national priority. It has also opened up several 

national policy questions regarding mine ownership, export, waste utilization 

and disposal strategies. 

India is currently producing all the possible marketable products of iron ore, 

namely iron ore lumps, ore concentrates, pellets, iron oxide powder and iron 

ore sinter. One of the most immediate technological challenges facing the 

industry is to deal with the problem of processing alumina rich iron ore fines 

and slimes. For the sustainable growth of iron ore industry which is beset with 

serious problems of shortage of land and water, it is absolutely imperative that 

the state-of-the-art mineral processing technology is utilized to take the 

industry closer to a position of zero waste production. 

India is endowed with rich resources of iron ores in the form of hematite and 

magnetite ore deposits. Hematite reserves estimated to be 12.9 billion 

tonnes, located primarily in the states of Jharkhand, Chattisgarh, Orissa and 

1

Madhya Pradesh are high grade (> 62% Fe) but also high in alumina content. 

Total magnetite reserves, estimated at around 10.6 billion tonnes with an 

average grade of 35-40% Fe are found in the states of Karnataka, Goa, 

Andhra Pradesh and Kerala. According to Indian Bureau of Mines, Indian iron 

ore reserves are over 24 billion tonnes (1). 

Indian hematite ores are typically rich in iron but contain unusually high 

alumina (as high as seven percent) and in some cases, problem of high 

phosphorous content is also noted. The current practice of iron ore washing 

in India results in three products, namely coarse ore lumps, directly charged 

to blast furnace, the classifier fines, (3-5% alumina) which with or without 

beneficiation are fed to sintering plants and the slimes (6-10% alumina) which 

are currently discarded as waste. 

It is a well-recognized fact that in order to enhance the competitive edge of 

Indian iron and steel industry, an efficient alumina removal technology for 

Indian iron ores is absolutely essential. It is worth noting the following facts in 

this context: 

• The alumina content in iron ore fines used in sinter making all over the 

world is less than 1%. In contrast, iron ore fines in India assay as high as 

3.0-5.5%. The sinter quality produced from such alumina-rich ore fines, is 

thus much poorer. The adverse effect of alumina on sinter strength 

productivity and its reduction – degradation characteristics (RDI) are well 

documented and conclusively established (2-4). 

• Whether in the form of alumina-rich lumps or sinter, the blast furnace 

productivity is significantly affected due to the presence of alumina in the 

feed. High alumina slag which is highly viscous, requires larger quantity of 

flux (10% MgO) and relatively larger slag volumes resulting in an increase 

in coke consumption and a decrease in blast furnace productivity. 

According to one estimate, a decrease in alumina content in the sinter 

from 3.1 to 2.5% will improve the RDI by at least six points, lower blast 

furnace coke rate by 14 kg per tonne of hot metal and increase its 

productivity by about 30% under Indian operating conditions (4-7). 

• The generation of iron ore slimes in India is estimated to be 10-25% by 

weight of the total iron ore mined – the iron ore values are lost to the tune 

of 15-20 million tonnes every year. In addition, these slimes stored in 

massive water ponds pose enormous environmental hazard. SAIL alone 

has more than 50 million tonnes of slimes accumulated over the years. 

Considering the fact that iron ore production will more than double and rise 

to at least 300 million tonnes soon, finding suitable means of safe 

disposal/utilization of slimes is indeed urgent. 

Motivation for the Beneficiation of Indian Iron Ores 

Iron ores are being beneficiated all around the world including at Kudremukh 

in India. Several techniques such as spirals, floatex density separators, jigs, 

2

multi-gravity separator, low and high intensity magnetic separator, 

conventional as well as column flotation, selective dispersion – flocculation 

are all part of current industrial practice (8 -19). Recent advances include 

Batac jigs (20), packed flotation column (21) packed column jigs (22) and 

centrifugal concentrators like Falcon Concentrator (23) Kelsey jigs (24) 

Knelson Concentrator (24) for the beneficiation of iron ore slimes. Processing 

of hematitic ores in India at present, however, does not involve any 

beneficiation except for whatever rejection of silica (and to some extent 

alumina) occurs during washing and classification of crushed ores. Several 

commendable efforts have been made by Tata Steel to come up with an 

economically viable beneficiation flow sheet for processing classifier fines (25- 

28). More recently at least two beneficiation plants have been set up to 

process sinter fines for value addition. 

Tata Steel has commissioned a Batac jig of 300 tonnes per hour capacity at 

Noamundi to beneficiate sinter fines to achieve alumina content less than 2% 

(28). Essar (29) has pioneered the pellet route of utilizing fines. A state-of-theart 

iron ore fines processing plant including a 267 kilometers long slurry 

pipeline has been commissioned recently. Essar is operating an 8 million 

tonnes per anum pellet plant in Visakhapatnam based on the fines and slimes 

being pumped from NMDC’s Balladilla iron ore mine. The beneficiation 

flowsheet includes gravity separation (spirals) of the coarser fraction after 

desliming and high intensity magnetic separation of the finer fraction. The 

concentrates thus produced assay less than 2.5% by weight of combined 

silica and alumina content and are further ground to produce a feed 

acceptable for the pelletization circuit. The Essar group has also announced 

to set up another 8 million tonnes per anum pellet plant as a part of its 

integrated steel unit in Orissa. 

A comprehensive technology development programme needed to delineate 

the appropriate beneficiation/utilization strategy for Indian iron ore deposits 

must include (i) the nature of occurrence, association and liberation 

characteristics of the alumina containing minerals; (ii) a comparison of the 

separation efficiency of various unit operations for both hematitegoethite/kaolinite/gibbsite 

separation in terms of recovery-grade plots 

(separation characteristics) and as a function of particle size (iii) a preliminary 

techno-economic assessment of the various technology options for a typical 

iron ore mine in the country. A schematic diagram showing the basic 

technological elements of an integrated strategy to utilize alumina rich iron ore 

deposits in the country is presented in Fig. 1. The readers are referred to our 

earlier publications for more details (8-10). The discussion in this paper is 

confined to the possible beneficiation strategies for alumina- rich iron ore 

slimes and that also, primarily to separation by flotation and selective 

flocculation techniques followed by semi-dry disposal of residue produced 

during the beneficiation. It must however be stressed that with ever increasing 

premium on bringing down the alumina content of Indian iron ores to less than 

2%, preferably less than 1%, the generation of slimes as a proportion of the 

total mined output is likely to increase further. The development of an 

appropriate strategy to deal with alumina-rich iron ore slimes is therefore of 

great relevance and it is an urgent necessity. 

3

Blast furnace charge 

-40+10 mm 

Lumps 

1.9 % Al 2 O 3 -10+0.15 mm 

Tailing pond 

Slimes 

-0.15 mm 

~10 % Al 2 O 3 

Iron ore 

(ROM) 

Fines 

2.7 % Al 2 O 3 

Sinter plant 

High alumina fines & slimes 

Enriched low 

alumina product 

Size 

enlargement 

Appropriate 

beneficiation strategy 

Blast furnace 

charge material 

• Sintering 

• Pelletization 

• Briquetting 

• Cold bonded - 

pelletization 

• Gravity separation 

• Magnetic separation 

• Froth flotation 

• Selective dispersion / flocculation 

• Bio - beneficiation 

Residual Waste 

• Semi-dry disposal 

Waste 

• Iron rich cements 

management 

• Thermal conversion 

strategy 

to iron 

• Glass ceramics 

Value-added products 

Eco-friendly storage 

Fig. 1: An integrated approach to high alumina iron ore 

beneficiation 

Beneficiation Strategies for Indian Iron Ore Slimes 

Considering the present magnitude of the iron ore slimes generation annually, 

the quantities of slimes accumulated over the years, the fact that these slimes 

are available in already ground form and assaying reasonably high %Fe, it is 

obvious that if properly beneficiated, these slimes can be considered a 

national resource rather than a waste of nuisance value. 

The alumina content of the slimes, if brought to less than 2% Al 2 O 3 in the 

beneficiated product will (a) lead to better utilization of national resources (b) 

4

achieve higher mine output (enhanced production) with not much additional 

costs (c) reduce environmental hazards associated with storage and disposal 

of slimes and (d) result in higher blast furnace and sinter plant productivity 

A number of research groups in the country have explored the possibility of 

reducing alumina in iron ore slimes (8-10, 18, 19, 25-43) A critical review of 

the earlier R&D investigations, as presented and discussed in greater detail in 

our earlier publications (8 -10), clearly indicates the need to carry out a 

comprehensive study targeted to establish an integrated strategy of utilization 

of Indian iron ore slimes. It must address the following important issues: 

• A quantitative and definitive assessment of the extent of alumina reduction 

possible with state-of-the-art beneficiation technology backed up by 

reliable scientific data 

• A conclusive evaluation of the state-of-the-art agglomeration (sintering/ 

pelletization/ briquetting) technology which can successfully convert the 

beneficiated slimes into a product acceptable to blast furnace without 

adversely affecting its productivity 

• A techno-economic assessment of the various options available to utilize 

the residual “waste” product (it could be as high as 50%) containing high 

amount of alumina and iron oxide – for example, in the production of ironrich 

cements, recycling of iron-rich wastes by thermal treatment including 

conversion into metallic iron and in the production of glass ceramic 

materials from iron-rich waste 

• A comprehensive eco-friendly and safe technology (for example, semi-dry 

disposal technology) to replace the conventional practice of storing iron 

ore slimes in tailings ponds 

One of the more important findings of earlier investigations is that alumina in 

Indian iron ore slimes occurs in the form of two distinct mineral constituents 

namely, gibbsite (hydrated aluminium oxides) and kaolinite (and other clay 

minerals in minor quantities). Even though not adequately quantified, the 

liberation studies also indicate that a substantial portion of alumina is present 

in the liberated form and hence amenable to separation by physical means. 

The work conducted by Tata Steel on Noamundi iron ore slimes is by far the 

most comprehensive study available at present. Based on their data, Pradip 

(9) compared the efficiencies of different unit operations including wet high 

intensity magnetic separation (WHIMS) and multi-gravity separator (MGS). As 

illustrated in Fig.2, it was possible to produce concentrates, at least on a 

laboratory scale; assaying less than 2% alumina at an overall yield of around 

50% from slimes feed analyzing 7-8 % alumina. Another noteworthy 

observation is that the separation achieved in MGS is remarkably close to the 

theoretical yield predicted based on the sink-float tests done on the same 

sample of slimes. The availability of high capacity MGS units is currently a 

serious limitation. 

5

Based on the published literature on the beneficiation of Indian iron ore 

slimes, it is not possible to conclude the reasons why the yield is limited to 

50% only. Is it because of interlocking and/or because of lack of a suitable 

separation device? Any future work on this topic must address this question. 

More importantly, the efficiency of various unit operations must be compared 

on the basis of such plots rather than a single point result (recovery at a 

specified grade) which is often misleading. Another benefit of such plots is 

that one can also assess the pros and cons of specifying a particular grade of 

the concentrate. In case there is a sharp drop observed in recovery with 

grade, one has to be careful in targeting a particular concentrate grade since 

it may or may not be possible to achieve it through a robust industrial 

separation circuit. Sensitivity of these results with respect to different feed 

grades, for feeds from different mine areas and feeds of different size 

distributions must be studied since all physical separation processes are 

known to be sensitive to particle size, mineralogy, liberation and feed grade. 

Fig. 2: Beneficiation of Noamundi iron ore slimes – the separation efficiency 

(yield as a function of alumina content of the concentrate) of various 

unit operations compared (After Ref. 9) 

Separation processes based on the surface-chemical differences between 

iron and alumina containing minerals, for example, froth flotation and selective 

dispersion – flocculation are also promising but have not been investigated 

adequately for processing Indian iron ores. These two separation techniques 

are therefore discussed in greater detail in this communication. The 

availability of selective reagents capable of achieving the desired separation 

efficiencies is a serious limitation. It has been addressed by us in our work on 

iron ore slimes (8-10, 37-41). 

6

Froth Flotation of Iron Ore Slimes 

Flotation process for concentrating iron ores received a big impetus in USA 

immediately after the Second World War due to the dwindling resources of 

direct shipping iron ores in the Lake Superior District. Flotation of iron ores 

essentially for silica removal has been reviewed extensively in literature (8-10, 

13, 16, 44-53). The iron ore industry in Minnesota and Michigan in US uses 

cationic flotation of silica from magnetic taconites at a rate of 40 million tons 

per annually (44). Column flotation technology for rejecting fine silica using a 

variety of cationic amines has also been commercialized in iron ore industry 

including at Kudremukh in India. 

Several iron ore producers in Brazil employ reverse flotation separation of 

silica from iron ore minerals for producing pellet quality concentrates. It has 

been reported that the presence of gibbsite and/or kaolinite as the major 

alumina containing minerals in iron ores does dictate the choice of 

beneficiation flow sheet (47) While kaolinite does not interfere with flotation 

selectivity, gibbsite tends to contaminate the flotation concentrate as it is 

depressed together with iron oxides and hydroxides during the reverse 

cationic flotation process (47). These observations are remarkably close to 

what Pradip and co-workers (8-10, 41) have reported based on their 

investigations on the beneficiation of Indian iron ore slimes by flotation and 

selective flocculation. Gibbsite tends to go with iron oxide because of its 

surface chemistry and chelating chemistry being very similar to iron oxide 

minerals. In fact the separation of iron oxide from gibbsite continues to remain 

a challenge before mineral engineers. 

In order to process finely disseminated large deposits of oxidized taconites 

containing predominantly hematite and goethite, US Bureau of Mines in the 

late sixties developed a process involving selective flocculation and desliming 

followed by cationic flotation of coarse silica. It was commercialized for the 

first time in 1974 at the Cleveland Cliffs Iron Co’s Tilden Concentrator in USA 

(17). The 4.1 million tons per year capacity plant was later expanded to 

produce 8.2 million tons per year of pellets assaying 64%Fe. The plant flow 

sheet involves dispersion of minerals using sodium hydroxide in combination 

with sodium silicate/ lignosulfonates / hexametaphosphate or 

tripolyphosphates during grinding followed by selective flocculation of iron 

minerals using starches (for example, tapioca flour starch). The settled 

(flocculated) concentrate is then subjected to reverse flotation with cationic 

amine reagents in order to remove coarse silicates. Starch thus works both as 

a selective flocculant and as a depressant for iron minerals. 

Samarco, a joint venture between BHP Billiton and CVRD operates a flotation 

concentrator producing an iron ore concentrate which is pumped via a 396 

kilometer pipeline to a pellet plant in Brazil. In order to reduce the silica 

content from present level of 3.1 % to less than 1% (super concentrate), 

regrinding to 100 % passing 105 microns was found to be most appropriate 

for enhanced liberation before flotation (53). 

7

In summary, a critical review of the published literature on the flotation of iron 

ore minerals suggests that it is likely to be a cost-effective commercial 

proposition for the beneficiation of Indian iron ore slimes provided one is able 

to design a selective flotation collector for iron oxide – gibbsite separation. 

Flotation of Iron ores for Phosphate Removal 

So far as the separation of apatite from iron ore minerals is concerned, 

flotation has been observed to be the most appropriate process. Depending 

on the mineralogy of occurrence of phosphate in iron ores, appropriate 

selective reagents have to be selected and/or designed to achieve acceptable 

grades of flotation concentrates. The magnetite ore available at LKAB’s 

Kiruna deposit in Sweden contains about 1 wt % of phosphorous. The 

phosphate minerals are floated with the help of a proprietary reagent 

developed by AKZO Nobel for this separation (50-52). The flotation collector, 

ATRAC-1562, found to be most effective in this flotation separation is a 

modified fatty acid reagent. It is possible to achieve a flotation concentrate 

grade assaying 0.025 % P and 71% Fe (acceptable for pellet plant) from a 

feed assaying 61% Fe and 1% P at a throughput rate of 3.8 million tons per 

year. A process control system is installed to be able to maintain the 

concentrate grade. Besides reagent dosage, and pH, the pulp temperature 

(varies between 10 to 40 0 C) is also observed to affect the flotation kinetics as 

well as the selectivity of separation (50). 

Cleveland Cliffs is the largest producer of iron ore pellets in North America 

with a combined production capacity of 38 million tons and operating six 

mines located in Michigan, Minnesota and Eastern Canada (54). While the 

majority of pellets assay in the range of 0.01 to 0.02% P, the pellets produced 

from martite ores (Tilden Mine) tend to have higher phosphorus and hence its 

reduction is an ongoing challenge. The Tilden plant flow sheet consists of very 

fine grinding to 80% passing 25 microns followed by selective flocculation – 

desliming process step whereby a significant portion of siliceous gangue, 

goethite slimes and phosphorous containing minerals are rejected. Reverse 

amine flotation is utilized to further bring down the silica content of the 

concentrate to meet the pellet quality grade. pH is maintained near 11. 

Previous research indicated two modes of phosphorus occurrence, namely in 

the form of discrete apatite grains as well as in the form of solid solution in 

goethite grains (54). Since apatite grains exhibits strong 

cathodoluminescence, the cathodoluminescence microscopy (CLM) technique 

has been successfully utilized to investigate the nature of phosphorus 

occurrence in iron ores (54). A CLM detector is added to the SEM for ore 

characterization. 

8

Selective Dispersion-Flocculation Studies on Iron Ore Slimes 

Pradip and co-workers (8-10, 37-41) have systematically investigated the 

possibility of achieving selective separation amongst hematite- aluminakaolinite- 

montmorillonite minerals, the mineral constituent’s representative of 

Indian iron ore slimes. 

Two classes of commercially available flocculants namely starch based 

natural polymers and polyacrylamide (PAM) - polyacrylic acid (PAA) family of 

synthetic polymers were extensively tested. Statistically designed experiments 

were conducted in order to compare the efficiencies of the two polymers 

namely starch and PAA for selective separation of hematite from kaolinite. As 

illustrated in Fig. 3. Starch at pH 10 is found to be a much more selective 

reagent for this separation (41). 

Pradip et al. (8, 38, 41) have also established that as compared to the 

commonly used dispersant, sodium silicate, low molecular weight synthetic 

polymers such as polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) 

are more selective dispersants for hematite-kaolinite separation with PAA as a 

flocculant. It is interesting to note that this effect of various dispersants on 

selectivity is not observed in case starch is used as a flocculant. 

Fig. 3: A comparison of the efficiency of polyacrylic acid (PAA) and starch 

flocculants in the separation of hematite from its mixtures with kaolinite 

in the pH range 6.5-9 for PAA and 9.5-11.5 for starch (After Ref. 41) 

Pradip and co-workers have also modified starch as well as polyacrylamides 

by incorporating more selective functional groups. Modified polyacrylamides 

containing iron chelating groups such as hydroxamates, (PAMX) were, for 

example are found to be much more selective than PAA or PAM. The 

9

epresentative results on the separation efficiency of PAMX in hematitekaolinite 

separation are presented in Table 1 (37). 

Table 1: Enhancement in selectivity in iron oxide/kaolin separation by the 

introduction of hydroxamate functional groups in polyacrylamide 

Feed grade: 35% Fe (1:1 mixture) 

Pulp density: 1% Dispersant: 40 mg/l Na-Silicate 

Test 

Results 

Conditions 

Flocculant 

pH Dosage Grade Recovery 

(mg/l) 

Name CONH 2 COOH CONHOH (%) (%) 

None 10 - 63.6 50.0 

PAM 100 - - 9 1 55.5 57.5 

PAA - 100 - 9 2 61.6 74.0 

PAMX* 68.7 23 8.3 9.2 5 66.5 72.0 

* 8.3% - CONHOH (hydroxamate) and 23% COOH (carboxylate) and rest 

CONH 2 (acrylamide) functional groups 

[ CH 2 CH] x 

[ CH 2 CH] y 

C O C O 

NH 

O - H + 

PAMX 

O - H + 

[ CH 2 CH] z 

C O 

NH 2 

The deleterious effect of the presence of montmorillonite in the system was 

also studied in detail (55). Flocculation experiments conducted on the 

synthetic mixtures of hematite-kaolinite-montmorillonite indicated that as little 

as 5% of montmorillonite when introduced in hematite-kaolinite mixtures could 

lead to a marked deterioration in the separation efficiency due to relatively 

less flocculation of hematite in presence of montmorillonite. Kaolinite 

flocculation remained largely unaffected. This deleterious effect of 

montmorillonite could be partially mitigated by the use of modified flocculants 

containing more selective functional groups. The selectivity could be partially 

restored when modified starch-mercaptan was used as a flocculant instead of 

conventional maize starch. The beneficial effect of a more selective flocculant 

like starch mercaptan was observed with synthetic dispersants as well (55). 

An interesting finding of our work is that starch, modified starches and 

modified polyacrylamides (PAMX) which were found to be excellent 

flocculants for selective separation of hematite from kaolinite and even 

montmorillonite, turned out to be disastrous in hematite-gibbsite separation. In 

fact, we established that starch flocculated both alumina (gibbsite) and 

10

hematite (iron oxide) equally well. It is well known that iron oxide and alumina 

have identical crystal structures. The results for hematite (iron oxide) – 

kaolinite and hematitie – alumina separation using starch are shown in Fig. 4 

for illustration (8, 56). 

100 

Fe2O3 - KAOLINITE 

80 

WEIGHT PERCENT SETTLED 

60 

40 

20 

MAIZE STARCH 

1% SOLIDS 

1:1 MIXTURE 

pH 10.5 

Fe 2 O 3 

KAOLINITE 

Fe 2 

O 3 

Al 2 

O 3 

SODIUM 

SILICATE (mg/l) 

40 

80 

Fe2O3 - Al 2 

O 3 

0 

0 10 20 30 40 50 

0 10 20 30 40 50 

FLOCCULANT DOSAGE, mg/l 

Fig.4.: Selective flocculation of hematite (Fe 2 O 3 ) from its synthetic mixture with 

alumina (Al 2 O 3 ) and kaolinite using a natural maize starch flocculant at 

pH 10.5 + 0.1 in presence of sodium silicate dispersant (After Ref. 8) 

We have also proposed a molecular recognition mechanism underlying starch 

interaction with iron oxide and alumina which explains this observation (41, 

56). The adsorption mechanism for starch end group having O - O distance of 

2.85 A° interacting with iron oxide substrate having Fe - Fe distance of 2.85 

A° on its cleavage plane is schematically shown in Fig.5. Al-Al distance on 

the cleavage plane of corundum (Al 2 O 3 ) is exactly same and hence strong 

interaction and flocculation of alumina (Al 2 O 3 ) is observed with starch. 

Fig.5: A schematic diagram illustrating the mechanism of starch adsorption 

through binuclear complexation with Fe sites on hematite surface or Al 

sites on Al 2 O 3 surface (After Ref. 41) 

11

Various research groups including ours have reported on the lack of any 

worthwhile success in the beneficiation of natural iron ore slimes using 

commonly used dispersant–flocculant combinations. Based on our 

systematic work on delineating the reasons for this observation, we attribute it 

to the remarkable similarities in the crystal structure as well as the surface 

chemical properties of the two constituent minerals of iron ore slimes namely 

hematite (iron oxide) and gibbsite (alumina). 

While kaolinite separation is possible with certain modified flocculants 

synthesized by us, even in presence of minor quantities of montmorillonite, it 

is not enough to reduce alumina to the desired levels in the concentrate 

because gibbsite remains with hematite. The key to solving the problem of 

iron ore slimes thus lies in developing selective reagents (flocculants, 

dispersants and flotation collectors) for iron oxide – gibbsite separation. This 

is by no means a trivial task but any breakthroughs on this front, in the 

author’s opinion will have far-reaching impact on the techno-economics of 

processing alumina-rich iron ore slimes. These reagents are not only essential 

for solving the problem of alumina rich iron ore slimes but those will also have 

a significant impact on the beneficiation of iron-rich bauxite deposits in India. 

Processing of Indian red muds, rich in alumina and iron oxide will also 

become economically more attractive with the availability of reagents capable 

of iron oxide– gibbsite separation. A sustained and systematic interdisciplinary 

effort by Indian researchers in this direction is therefore urgently needed. 

State-of-the-art molecular modeling techniques appear to be promising for the 

design of highly selective, tailor-made reagents for industrial applications (56- 

62). 

Pelletization and Sintering of Beneficiated Iron ore Slimes 

In India, Kudremukh Iron Ore Co. (KIOCL), Mandovi Pellets, Jindal 

Vijayanagar Steel Ltd (JVSL) and Essar are producing iron ore pellets of 

marketable grade. While a discussion on the overall techno-economics of 

pelletization vis-à-vis sintering of iron ore slimes is outside the scope of this 

paper, it is worth mentioning that major technological advances have taken 

place in both the processes leading to improvement in productivity and quality 

of pellets and sinter. 

One of the more recent innovations has been the demonstrated utility of High 

Pressure Grinding Rolls (HPGR) for the production of pellet feed. Modern iron 

ore pellet plants consist of HPGR for grinding the feed to acceptable levels of 

fineness with comparatively less consumption of energy (63). One of the very 

first installations of HPGR in the iron ore industry was at LKAB in Sweden in 

1994. Currently there are 29 HPGR units operating worldwide in the iron ore 

industry including one at Kurdremukh in India (63). Besides low specific 

energy consumption and high throughput per machine, the pellet specific 

advantages of HPGR machine include increase in fineness, more appropriate 

size distribution and shape of HPGR product compared to grinding in 

conventional ball mills, increase ultrafines in the product, more uniform pellets 

12

esulting in reduction in circulating loads, better moisture control in filtration, 

higher density of pellets (average porosity decreased from 33 % to 25 %), 

lower binder requirement and increased green pellet strengths (upto 50% 

higher than conventional pellets), reduced fuel consumption, higher loading, 

better bed permeability and increased productivity during induration (63). 

Some of these benefits are illustrated with data reported by Ehrentraut and 

Ramachandra Rao (2001) on the quality of pellets obtained at Kudremukh 

Iron Ore Pellet plant in India (64). 

Table 2: A comparison of pellet properties with and without roller press as 

obtained at Kudremukh Iron Ore Company (Ref 64) 

Parameter 

Without 

Roller 

Press 

With Roller 

press 

Blaine value of feed on pelletizing disc (cm 2 /g) 1450 1800 

Green Pellets (wt%) of 9-16 mm 86 90 

Green Pellets (wt%) of < 5 mm 2 1.5 

No of drops – Green Pellets Strength 7 10 

Tumbler Index after induration, wt% of > 6.3 mm fr. 94.5 95 

Abrasion index, wt% of < 0.6 mm 4.5 4.0 

Porosity (%) 29 26 

Swelling Index (%) 17 15 

Reducibility (%) 64 62 

Bulk Density (t/m 3 ) 2 2.24 

Significant improvements in terms of quality and productivity in iron ore pellet 

plants have been achieved through the introduction of advanced process 

control systems including expert systems (65-67). 

It is well known that sintering plants can be appropriately modified to accept 

higher levels of fines/slimes, for example by micro-pelletizing the slimes 

before sintering. To the best of my knowledge I have not come across any 

investigation on Indian iron ore fines, carried out to establish the proportion of 

slimes one can accept in sinter plants without adversely affecting their 

performance. Such investigations must be conducted as a part of the R&D 

program on beneficiation of iron ore slimes. 

Next generation Technology for Safe Disposal of Iron Ore Slimes 

Residue after Beneficiation 

No discussion on processing of Indian iron ore slimes is complete without a 

mandatory section on the environmentally acceptable recycling and/or storage 

of residual waste after beneficiation. No matter which process of beneficiation 

is employed, it is clear that with existing technologies one is able to recover 

only 50% of the slimes as a valuable product. The iron rich alumino-silicate 

residue is thus available for further processing and/or safe disposal. Current 

practice of storing slimes in huge dams built for this purpose not only occupy 

precious land but also hazardous to the population. In response to this need 

to find an alternative to current tailings disposal schemes, an innovative 

13

thickened tailings disposal technique has been invented and commercialized 

recently. It is very much relevant to the present discussion on processing of 

iron ore slimes. 

The conventional technology of tailings disposal/storage involves building an 

expensive tailings dam or a tailings pond where million of tons of very dilute 

and fine particulate suspensions are allowed to settle under gravity for several 

years. The industry is well aware of the inherent problems/hazards associated 

with this strategy. 

The widely prevalent practice of disposal and management of tailings entails 

dilution with water, pipe line transportation of relatively dilute slurry and 

impoundment in a pond or tailings dam. This so-called third generation 

technology has associated with it immense hidden costs and potential for 

serious environmental damage (68). Apart from locking up vast quantities of 

water and valuable land for generations on end, it poses a man-made hazard 

to the soil and water sub-systems of the environment with unpredictable and 

far-reaching consequences for the ecosystem and the habitat. There is then a 

need for the development of an environmentally benign and cost effective 

technology for disposal and management of wet particulate wastes. The 

imperative for an alternate approach is even more urgent in the Indian context 

where water is a scarce resource, land is at premium, and environmental 

degradation has already reached alarming proportions. 

Thickened tailings disposal (TTD) is a concept introduced by Robinsky (1975) 

and commercialized only very recently in the mining industry. The success of 

a TTD system depends on the extent the tailing can be thickened to 

concentrated but pumpable slurry. Once thickened, the tailings can be 

pumped and discharged in the form of a self-supporting ridge/cone designed 

to attain a slope of 2% to 6% only. This new technology obviates the need of 

building dams/ponds and is therefore going to have a major impact in the 

industry (68 - 80). 

Excellent work of David Boger and coworkers (73-80) has provided the 

necessary scientific foundations to this new concept of tailings disposal. The 

basic science underlying the dewatering, pumping and stacking of mineral 

slurries (also called “particulate fluids”) is thus understood to a great extent 

and hence, this “fourth” generation technology of tailings disposal is ready for 

wider application in the industry. This concept has been adopted and 

perfected as a semi-dry disposal technology treating red-mud (a residue of 

bauxite processing) by Alcoa of Australia (75, 80). 

The results obtained after a comprehensive characterization of the Alcoa ‘red 

mud’, were utilized to optimize design and the semi-dry disposal system for 

the bauxite residue produced by Alcoa in Western Australia (75). Depending 

on the rheological characteristics of the given red-mud, it was shown that 

there exists an optimum solid concentration (wt% solids) in the slurry at which 

it should be pumped. 

14

The rate and extent of thixotropic break down of the red mud slurry as a 

function of mixing time are gainfully exploited in this process. The results 

clearly indicate that destruction of network structure by mechanical agitation 

leads to the reduction in yield stress (a measure of the slurry’s resistance to 

flow) by several orders of magnitude. It is even more interesting to note the 

rate of recovery of the structure (and hence rise in yield stress) of red mud as 

a function of time in the absence of shear. Recovery is slower by an order of 

magnitude than degradation. This property of red mud is exploited in pipeline 

transport at high percent solids followed by semi-dry discharge at the disposal 

site and subsequent consolidation of red mud slurries (75). 

Based on the work of Boger and coworkers, Alcoa commissioned a 

demonstration plant in Kwinana in 1984. Subsequently, a semi-dry disposal 

method was commissioned at Pinjarra refinery in 1985. Later three high 

density super thickeners to produce highly concentrated slurries were built 

and are in use. The largest of these operations consists of a thickener having 

a diameter of 90 metres with 10 metres deep compression zone, a capacity of 

450 tons per hour, use 60 g flocculant per ton of red mud and produces a 

50% solids underflow which is pumped to a 70 hectares drying area where by 

it is discharged by the slope thickened tailings disposal (TTD) method (75). 

Recent developments in multi-disciplinary researches at the intersection of 

colloid science, yield rheology and processing of particulate fluids suggest 

that it should now be possible to develop/optimize the fourth generation 

technology, which can be optimally tailored for a specific waste management 

problem, for example disposal/storage of iron ore slimes/residue. The subprocesses 

that need to be embedded in this novel technology for 

disposal/storage of iron ore slimes/residue include one or more of the 

following: 

• Dewatering for efficient recovery and recycle of water to the plant by 

thickening and high pressure filtration operations, as dictated by the 

compressive yield stress of the iron ore slimes/residue. 

• Pipeline transportation of highly concentrated suspensions, based on timedependent 

and time-lag thixotropic flow of the particulate fluid for example, 

iron ore slimes/residue 

• Above ground stacking, so called semi-dry disposal, which takes 

advantage of the shear yield stress of the residue/slimes 

It will be appreciated that, unlike the conventional technology, the new 

approach is based primarily on exploiting the flow and rheological 

characteristics of the particulate fluid (for example, red-muds), which in turn 

depend on surface forces, particle interactions and the resulting structure of 

the particle network in suspensions. The immediate need is to first 

characterize iron ore slimes in India to ascertain whether it has thixotropic 

properties similar to red muds. If not, one can suitably design appropriate 

additives in order to manipulate the yield rheology as required. 

Based on the recent advancements in our understanding of the rheology of 

suspensions, particularly its manipulation/control with the help of appropriate 

15

surfactant and polymeric additives (78-89) it should now be possible to design 

a semi-dry disposal strategy tailor-made for Indian iron ore slimes. 

A mission mode R&D program aimed at customization and demonstration of 

this semi-dry disposal technology for iron ore slimes must be immediately 

undertaken in India so that one can replace the existing tailings dam 

technology with a more robust, more eco-friendly and more cost effective 

fourth generation technology of semi-dry disposal of iron ore slimes/residue. 

Production of Value-added Items from Iron Ore Slimes/Residue : Ecocements 

In order to accomplish the task of developing zero waste technology for Indian 

iron ores, it is important to find appropriate means of utilizing the ultra-fine 

iron-rich alumino-silicate residue obtained during the beneficiation of iron ore 

slimes. Amongst several industrially useful products being explored worldwide, 

made from waste materials, eco-cements are perhaps the most 

promising (90). 

Considering the large volumes of cement and concrete products consumed 

and the rates of growth anticipated in the buildings/construction industry in 

India, it is only natural that efforts are being made to incorporate industrial and 

mining wastes as substitutes for raw materials, admixtures, fillers, binders etc. 

in the construction industry. For example, the use of granulated blast furnace 

slag, volcanic ash, certain kinds of fly ashes and other materials having 

adequate lime reactivity in cement and concrete applications is now a 

standard industrial practice. Standard specifications are for instance, available 

in almost all the countries for blended cements. 

Since there are stringent specifications on the quality of raw materials 

permitted in the manufacture of Portland cements with respect of composition 

and the presence of certain impurities such as phosphate, chloride, sulfate, 

iron oxide, titania, magnesia, etc. the use of waste products is obviously 

limited. Recent work on special cements, in particular those based on novel 

alinite and sulpho-aluminate type solid solution cementitions phases, however 

indicates that good quality cement/concrete products could be manufactured 

almost exclusively from wastes such as the one produced during the 

beneficiation of iron ore slimes. These cements are thus called eco-cements 

(90-97). 

In addition to converting wastes into value added products of commercial 

significance both these classes of cements are also energy efficient as 

compared to portland cement. As an illustration, the compressive strengths of 

alinite cements made from gold ore tailings sands are compared with those of 

Portland cements in Fig. 6 (96). The cement was used as a binder (a 

substitute for Portland cement) during the pelletization of the same tailings 

sands. The pellets were required for subsequent heap leaching in order to 

recover the residual gold from the tailings. All the properties specifications 

such as pellet strength and permeability for this particular application were 

met or exceeded by alinite cements. It was established during trials that alinite 

16

cements were indistinguishable from Portland cement in all respects and can 

replace it as an inexpensive substitute binder. 

PORTLAND 

CEMENT 

LIGHT TAILINGS SANDS DARK TAILINGS SANDS 

Low Temperature Alinite Cements 

Fig.6 Low temperature alinite cements made from gold ore tailings sands – 

Compressive strengths are comparable to those of ordinary portland 

cement (After Ref 96) 

The above-mentioned case study illustrates the challenges and opportunities 

inherent in finding appropriate sinks for industrial and mining wastes such as 

the iron ore slimes and residue. Based on our work on cements, this approach 

of producing eco-cements from iron rich alumino-silicate residue produced 

during the beneficiation of iron ore slimes is certainly worth exploring. 

Need for an integrated approach 

A critical review of the available data and prior work presented in the 

preceding sections thus suggests that the serious problem of safe disposal 

and/or finding a commercially viable utilization strategy of Indian iron ore 

slimes remains unsolved not due to lack of technical expertise but because of 

the lack of proper problem definition. A comprehensive and integrated 

approach for Indian iron ore slimes calls for a mission mode R&D program 

aimed at coming up with a commercially viable solution, backed up by hard 

pilot plant/plant scale data for the proposed strategy and an independently 

verifiable techno-economic assessment of alternatives, to be completed in 

definite predefined time-frame. No one institution/organization or expert can 

do all that is required but one organization/individual must be given total 

responsibility/freedom and adequate funds to drive the project through to the 

end. Then only we can ensure that the end result does not suffer from the 

17

inadequacies, narrow expertise, vision or interest of the particular institution 

and investigators involved. The final outcome of such an effort can form a 

basis for arriving at business decisions subsequently. 

It is not only imperative but also a challenge to Indian professionals to 

demonstrate that they together as a team can solve the problem of iron ore 

slimes, a problem unique to India. 

Concluding Remarks 

A critical review of the work done thus far on the processing of Indian iron ore 

slimes indicates the urgent need to undertake a comprehensive, mission 

oriented, multi-institutional R&D program, in partnership with the industry, and 

directed towards an integrated solution to the problem of alumina rich iron ore 

slimes, as proposed in this paper. Development of selective reagents capable 

of accomplishing hematite-alumina/gibbsite separation has also been 

identified as a key research challenge facing Indian mineral processing 

community. 

Acknowledgements 

The author greatly benefited from many helpful discussions with Prof. P.C. 

Kapur during the preparation of this manuscript. The encouragement and 

support from Prof. Mathai Joseph is gratefully acknowledged. 

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78. S.B. Johnson, G.V. Franks, P.J. Scales, D.V. Boger and T.W. Healy, “Surface 

Chemistry – Rheology relationships in concentrated mineral suspensions”, Int’l J. 

Mineral Processing, 58 (2000) 267-304 

79. Q.D.Nguyen and D.V. Boger, “Thixotropic behavior of concentrated bauxite residue 

suspensions”, Rheol. Acta, (1985), pp 427-37 

80. D.V. Boger, “Rheology and the Minerals Industry”, Mineral Proc. and Extractive Met. 

Review, 20(1-3), (1999), 1-25 

81. V. Ramakrishnan, Pradip and S.G. Malghan, "The Stability of Alumina-Zirconia 

Suspensions", Colloids and Surfaces, 133, (1998), pp 135-142 

82. M. Subbanna, Pradip and S.G. Malghan, "Shear Yield Stress of Flocculated Alumina- 

Zirconia Mixed Suspensions : Effect of Solid Loading, Composition and Particle Size 

Distribution", Chemical Engineering Science, 53(17), (1998) pp 3073-79 

83. Manjunath Subbanna, Sandhya Kokil, P.C. Kapur, Pradip and S.G. Malghan, "An 

Aggregation Index for Monitoring the State of the Suspensions", Langmuir, 14, (1998) 

pp 7364-70 

84. V. Ramakrishnan, Pradip, S.G. Malghan, "Yield Stress of Alumina Zirconia 

Suspensions", J. American Ceramic Society, 79(10), (1996), 2567-76. 

85. Pradip, R.S. Premachandran and S.G. Malghan, “Electrokinetic Behavior and 

Dispersion Characteristics of Ceramic Powders with Cationic and Anionic 

Polyelectrolytes”, Bulletin of Materials Science, 17(6), (1994), 911-20 

86. Sasanka Raha, Kartic C. Khilar, P.C. Kapur and Pradip, Regularities in Pressure 

Filtration of Fine and Colloidal Suspensions, Accepted for publication in 

International Journal of Mineral Processing, 2006 

87. Sasanka Raha, Kartic C. Khilar, Pradip and P.C. Kapur, A Mean Phi Model for 

Pressure Filtration of Fine and Colloidal Suspensions, The Canadian Journal of 

Chemical Engineering, 84, 2006, pp 83-93 

23

88. Sasanka Raha, Kartic C. Khilar, Pradip and P.C. Kapur, Rapid Determination of 

Compressive Yield Stress of Dense Suspensions by a Mean Phi Model of High 

Pressure Filtration, Powder Technology, 155, 2005, pp 42-51 

89. Sasanka Raha, Kartic C. Khilar, Pradip and P.C. Kapur, Pareto Profile Benchmark for 

Kinetics of Filtration and Extent of Dewatering of Fine and Colloidal Suspensions, 

Industrial Engineering and Chemistry Research, 44, 2005, pp 9364-9368 

90. Pradip and E. Forssberg, Stabilization and Utilization of Solid Mining Waste in 

Effluent Treatment in the Mining Industry, (Ed.) S.H. Castro, F. Vergara, M.A. 

Sanchez, University of Concepcion, Chile, (1998), pp 1-55. 

91. Pradip, E. C. Subbarao, P. C. Kapur, N. R. Jagannathan and C.N.R. Rao, 

"Characterization of Alinite Cements through X-Ray Diffraction & MAS29 Si NMR 

Studies", Materials Research Bulletin, 22(8), (1987), pp 1055-62. 

92. Pradip, D. Vaidyanathan, P. C. Kapur and B. N. Singh, "The Production and 

Properties of Alinite Cements from Steel Plant Wastes", Cement and Concrete 

Research, 20, (1990), pp 15-24. 

93. P.K. Mehta, “Investigations on energy-saving cements”, World Cement Technology, 

21, (1980), 166-73 

94. M. Singh, S.N. Upadhyaya and P.M. Prasad, “Preparations of iron-rich cements using 

red mud”, Cement and Concrete Research, 27(7), (1997), pp 1037-46 

95. M. Singh, S.N. Upadhyaya and P.M. Prasad, “Preparations of special cements from 

red mud”, Waste Management, 16(8), (1996) 665-670 

96. Pradip, D. Vaidyanathan, Maneesh Singh, Ajay Panjwani and P. C. Kapur 

Manufacture of Energy Efficient Hydraulic Cements from Industrial and Mining Waste 

Materials Mineral Processing : Recent Advances and Future Trends, (Eds.) S.P. 

Mehrotra and R. Shekhar, Allied Pub., New Delhi, India (1995), pp 816-823 

97. Pradip and P.C. Kapur, Manufacture of Eco-friendly and Energy-efficient Alinite 

Cements from Flyashes and other Bulk Wastes, J. Resources Processing (Japan), 51 

(1), 2004, pp 8-13 

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