Indian Journal of Chemistry Vol. 42A, September 2003, pp. 2447-2454

Some studies on recovery of chromium from chromite ore processing residues

K J Sreeram", M K Tiwarib & T Ramasami"

"Chemical L aboratory, Central Leather Research Institute, Adyar, Chennai 600 020, India "Centre for Advanced Technology, Indore, 452 013, India

E-mail : cheml ab@clrim.org

Received 7 JanuW)' 2003

Thi s work reports a methodology for near complete mobilization and recovery of chromium from chromite ore process ing res idues. It has been found that it is possible to mobilize chromium from the residues. Vari ous factors which influence the ex tent of extraction of chromium from COPR have now been reported. An extracti on using sodium peroxide enables leaching of more than 95% of chromium in COPR. The method is selecti ve to the lcachin 2. of chromium a ~

demonstrated by EDXRF measurements. Leaching of chromium from COPR using peroxides leads ~ l so to signifi c mt reduction in particle size. The reacti on between residue and oxidant has been found to be biphasic. By combinin 2. oxidati ve and chelati ve methods, a near complete recovery of chromium and iron from COPR has been possible. -

Int In '11strial applications of chromium compounds are

many. bout 12.5 million tons of chromium-based compoun s are used for various applications annually'. · ~ ': J "oread use of chromium in industrial

and real life applicatio about the environmental conseque ces of the metal ion2·3. Chromium occurs in its trivalent oxidation state in chromite ore and as chromium iron c xide (FeCr04). In this form , chromium is inert. It is not bio-avail able on account of poor solubility. Dressing ~of chromium involves the oxidation of chromiu (Ill) into chromium(VI{ Major chemi~al reaction.- asso_ciated with the processing of chrom1te ore are h. ted 111 Eq.

\ ( I )-Eq.(3).

'2FeCrc0 4 + 4NaiC03 + 3.50 2---+ 4Na2Cr04 + Fe20 3 + 4C02 ... (1)

2Ca0 + Si02 ---+ 2CaO.Si02 . . . (2)

The solid mater\al remaining after the leaching of sodium chromate is termed as the chromite ore processing residue (COPR). Dressing of chromite ore is known to lead to the generation of 1 ton of treated mineral was te per ton of rated sodium chromate processed. Such a waste contains5 typically 5- 10% Cr20 3. Landfill remains the most widely adopted disposal method employed worldwide. It has come to be recognized that disposal of such residues in

secured landfi lis causes serious long term environmental problems and injuries to human health6

·7

. Industrial methods of dressing of ore employ lime as the main source of alkali , although lime-free methods are reported5

.

Several common remediation strategies are available for the management of COPR disposed off as landfills8

-12

. These include a) reduction of particle size of COPR such that at least 20 per cent of the residue passes through a 200 mesh sieve to reduce the bleeding of water soluble chromium compounds from the residue; b) stabilization of the ore res idue with sludge dredged from salty or brackish water to obtain a hardened mass which presumably prevents th e leaching of chromium when exposed to weatheri ng conditions by virtue of a impermeable layer c) admixing with blast furnace coke for reducing chromium(VI) and d) treating with ferrous sulphate for converting Cr(VI) to Cr(lll ). In esse nce approaches employed for management of COPR are based on immobilization of the residue for landfill in ~ .

The efficacies of such methods are judged by the ability to a) reduce Cr(VI) to Cr(lll ) and at the same time prevent its reversal back to hexavalent chromium and b) prevent leaching of chromium on pro longed exposure to weathering conditions. There are reports

1.1

that the manganese oxides present in so il , under ambient conditions, can convert Cr(III ) to Cr(Vl ). It has been suggested that the single most significant factor dictating factory closures would be the inability to manage the landfill mineral wastes5

. Attempt s ha\·e been made to characterize various spec ies present in

2448 INDIAN J CHEM, SEC A, SEPTEMBER 2003

uch residues from chromite ore processing and develop chemical approaches to immobilize mobile species of chromium through vatious chemical means in solid state 12

. The present investigation, on the other hand, reports an approach to mobilize and extract almost all chromium species contained in COPR.

Materials and Methods COPR generated from two commercial chromate

manufacturing plants in India was collected and stored in plastic bags at room temperature. The mud collected was green in colour with distinct yellow aggregates. The mud obtained had particles of varying sizes and hence particles of residue with 2 mm, 0 .8 mm and 0.2 mm diameters were obtained through sieving for the study.

Chemical characterization of the mud The hexavalent chromium concentration was

determined by alkaline digestion (EPA 3060A) followed by the colorimetric method (EPA 7196A). For total chromium analysis, hydrofluoric and nitric ac id digestion (EPA SW-846) followed by inductively

14 couple plasma spectrometry (ICP) was used .

Determination of different phases of chromium in mud Known quantity (1 g) of mud was treated

sequentially with IM sodium acetate (NaOAc) at pH 8.2 to ex tract metals in the exchangeable phase, with I M NaOAc adjusted to pH 5.0 with acetic acid (HOAc) to extract metals bound to carbonates, with 0.04M hydroxylamine hydrochloride (NH20H.HCI) in 25% HOAc to extract the fraction bound to Fe-Mn ox ides, with 30% (w/v) H20 2 in nitric acid (HN03) to extract the fraction bound to organic matter and the final residue was digested with 40% (w/v) hydrogen peroxide (H20 2) and concentrated HN03 to extract the residual fraction. The selective extractions were carried out in 50 mL centrifuge tubes . After each extraction the separation was achieved by centrifuging and the supernatant was analysed for chromium content by atomic absorption spectroscopy. The residue was washed with 8 mL distilled water, cent rifuged and the supernatant discarded 14

•

Energy dispersive X -ray .fluorescence measurements on COPR

Energy dispersive X-ray tluorescence analysis (EDXRF) was employed for the non-destructive analysis of the elemental composition of the COPR before and after treatment. The fluorescence intensity of any element (say i) depends on its concentration W;

but not linearly. Other nonlinear effects such as absorption and enhancement also influence intens ity. To derive concentration values from nuorescence intensities a quantitative analysis program CA TX R F has been employed . This program is based on fundamental parameter (FP) method and corrects both the absorption and enhancement effects in EDXRF analysis.

Particle size measurement Particle size measurements were carried out either

by using optical microscope technique or a CILAS 1180 particle size analyzer. For optical microscopic measurements, the particles were spread on a grid marked appropriately for measuring the partic le dimensi'ons. The particles were moni tored through a CCD camera attached to a Hund Wetzlar microscope and the average particle size measured. The samples were dispersed in calgon and sonicated for 60 second · prior to measurement.

Extraction of chromium from COPR

,

Alkali extraction-COPR (I g. particle diameter 2 mm, 0.8 mm and 0.2 mm) was tre· .:c--1. ith NaOH (1-15M, 25 ml) for 60 min . 1he chromium leached out after the ex~r· ction process was quantified by spectrophotometr c techniques. The particle s ize or COPR after c romium extraction was determined through lase~ diffractometry and microscopi c techniques.

Chelative /extraction-A Soxhlet extraction ol chromium ,IJsing acetyl acetone (COPR: acetyl acetone = 11 :50) in a reducing zinc atmosphere w<~ s studied on .t COPR with particle size of 0.8 mm . A sequential ~xtraction employing acetyl acetone and tri ethylami .e (pH 9 , 4h) followed by ex traction with acetone was also carried out. The amount of chromium remaining in the residue after ext rac ti on was determined and the extracti o n effi c iency calculated.

Reductive treat111enr- COPR (parti cle size 0 .\:\ IT\lTi l

was treated with varying a mounts o r sod ium sulph ite (0-5 moles/mole(Cr+Fe)) in the presence I absence of sodium hydroxide (0-0.5 gig COPR ) in a reductive atmosphere at 600°C for 4h . The treated mixture was leached with water (I: 100) and the chromium in the extract determined.

In another study I g of COPR (pa rticle size O.X mm) was treated with 0.5 g of sod ium sulphit e and 0.5 g of NaOH for 4 h at 600°C, cooled and subj ec ted to treatment with hydrogen peroxide (30% soluti on.

SREERAM et a/.: RECOVERY OF Cr FROM CHROMITE ORE PROCESSING RESIDUES 2449

I :20 (COPR:H20 2)) at 60°C for 30 min. The concentration of chromium leached out after the treatment was determined.

Oxidative treatment-Oxidation of chromium(III) in the COPR (0.8 mm particle size) to chromium(VI) was can·ied out by employing various oxidants like sodium perox ide, sodium chlorate and sodium perborate at vary ing ratios (0- l 0 moles/mole(Cr+Fe)) in an oxidizing atmosphere at 500°C. The treated grog was cooled and ex tracted with water (I :200) and chromium in the liquid extract determined. The res idue remaining after ex traction was subjected to EDXRF measurements.

In another study COPR particles of 2 mm, 0.8 mm and 0.2 mm diameter were subjected to oxidation by sodium perox ide. sodium chlorate and sodium perborate (7 moles/mole (Cr+Fe)) and the extend of chromi um recovery and reduction in particle size determined .

In all the above studies the extraction efficiency or percentage removal of chromium has been calculated by employi ng the formu la

:i (01 ) [ Cr leached out from COPR \ to = x 1001

Cr extracte\ Total Cr in COPR J

Discussion Results and . .

Ch . , ore processmg residues from two

romite . d . 1 ns have been analyzed for their 111 ustna sourc _ ..

. "'ntal composition. The total chemical and e~e · VI) as er US EPA 3060 was extractable chr"'mlUm( . p ~s hi gh as s4oo and 16~00 PP.m on dry wei~ht bas.i s, respectively for the t ··o t mdependent mdus.tnal samples . The composition of he COPR as determm~d by EDXRF and other measuren;ents. are reported m Table 1. The rest Its reported i~s study are an average of three independent ~asurement.s. Although chromium in undressed chromjt~ore was m immobile form, dressing process d~verts a significant portion of chromium into mo1)Ile fori?s . The use of lime in chromite ore processmg encapsulates chromium with calci um s~~h.~t~nd thus could be expected to prevent its oxtdatJOn a.Qd subsequent extraction . The residue also contains som~ trivalent chromium compounds, which are poorly \ soluble. Chromium compounds present in COPR

1

include calcium chromate, CaCr04, calcium alumino­chromate, 3CaO.AI20 3.CaCrO •. l2H20 (which dissolves slowly in water) , tribasic calcium chromite, CaJ(Cr04h and basic ferric chromate, Fe(OH)Cr04 which decomposes slowly in the presence of water

Table 1- Typical composition of COPR collected fro m a chromite o re processing industry

Type of measurement Mean values Plant A Plant B

Total chromium(%) 9.8±0.2 9.7±0.2

Total extrac table 5,400 16.000 chromi um (VI) (ppm)

pH 9.0±0. 1 8.8±0.2

Calci um (%Ca) 2 1.0±0.8 22 (!±0.7

Iron (%Fe) 10.1±0.2 11 .2±0.2

producing water-soluble hexavalent chromium and insoluble tri valent' chromium hydroxide 15

. As the COPR from Plant A contained less hexavalenT chromium than Plant B, further studi es on mobilization of chromium was carried out on residue generated from Plant A.

Various forms of chromi um contained in COPR from Plant A has been categorized. While the unreacted chromite which, contributes mostly to the residual chromium (62.8% of the total) and reducible chromium (amounting to 16.2% of the total Cr) is not readily available for leaching under environmental conditions, 21 % is potentially leachable. Of the various chromium species found in COPR, the exchangeable chromium (5%), carbonate bound chromium (2.5 %) and oxidizable chromium (13.5 l7r ) are easily avai lab le for reaction with soil componen ts and can cause contami nation of so il and water. It has been reported that the soi ls contaminated with COPR have three distinct categories of mineral s - unreacted chromite, high temperature phases produced during ox idation of chromite (brownmillerite, periclase and larnite) and finally, minerals found under ambient weathering conditions on the di sposal sites (brucite, calcite, aragonite, ettringite, hydrocalumi te and hydrogarnet) 15

•

Compositions of these minerals have already been worked out 16 and are presented in Table 2. Transitions from one mineral phase to another are expected to he associated with heat changes as well as variations in crystal morphology. Leaching of chromium from COPR on ageing can therefore be expected to be associated with changes in heat of formation as we ll as lattice morphology. The residue under the present study has been obtained directly from the industry . prior to landfill. Therefore the presence of mineral phases such as brucite is not expected. Previous experience has shown that conversion of immobile into mobile forms under conditions of ageing and soiling could cause environmental hazards 17

. An

2450 INDIAN J CHEM, SEC A, SEPTEMBER 2003

Table 2-Composition of minerals present in COPR

SI. No Solid Phase Formula

I Magnesiochromite MgCr204 2 Cr(YI) containi ng hydrogarnet Ca3Al2((Cr/Si/H4)04h 3 Cr(Yl)-hydrocalumite Ca4Al2(0H)12Cr04.6H20 4 Calcium chromate CaCr04 5 Cr(Yl)-ettringite C<l(;Al 2(0H) 12(Cr04h.26H20 6 Periclase MgO 7 Brucite Mg(OHh 8 Calci te CaC03 9 Brownmillerite, C4AF Ca4Al2Fe20 w 10 Hydrogarnet, C3AH6 Ca3Alz(H404h II Grossu lar, C3AS3 Ca3Alz(S i04)3 12 CAH1o CaAI2(0H)s.6H20 13 CzAHs Ca2Al2(0H)w.3 Hz0 14 C4AHI9 Ca4Al2(0H) ~~. 1 2 H z0 15 C4AHn Ca4A l2(0H )I~ · 6H 20 16 Monosulphate, C4AsH1 2 Ca4Al2(0H)1 2S04.6H20 17 Ettringite, C6As3H32 Ca6Al2(0H)I 2(S04h26Hz0 18 Gypsum CaS04.2H20 19 Calcium si licate hydrate CaH2Si04 20 Gehlenite hydrate, C2AsH8 CazAI2(0H)6S iOsHs.H20 2 1 Wairakite Ca2Al2Si4012·2H20 22 Amorphous Si-gel Si02 (am) 23 Aluminium hydroxide Al(O l-1 ).1 (am) 24 Gibbsite y-A l(OH h 25 Dolomite CaMg(COJh 26 Portlandite Ca(OHh

Table 3-Percentage leaching of chromium and reduction in particle size of COPR treated with NaOH ~ [Total chromium present: 9.8 g/g of COPR]

Molarity of NaOH Chromium ex tracted (%) Parti cle size reduct ion (%) emp loyed 2mm 0.8 mm

I 3.3 3.9 2 4.3 4.2 3 4.7 5.2 5 5.8 6.3 10 5.9 6.4 15 6.7 7.0

Conditions employed: Ambient temperature, I h treatment

approach to leach the en tire chromi um contained in CO PR has now been made.

Based on the species wise distribution of chromium in the COPR, fou r different methodologies have been adopted to recover chromium present in various phases. They are a) alkaline aqueous extraction, b) a chelative extraction of detrital fractions (exchangeable, carbonate bound and oxidizable) of chromium, c) a reductive approach for recovery as soluble chromium(III) species, and d) an oxidati ve approach to convert all unreacted chromi um(III ) to chromium(VJ).

Alkali extraction method COPR has been subj ected to particle size se lec tion

prior to the leaching of chromi um and extent of

0.2 mm 2 mm 0.8 mm 0.2 mm 4. 1 2-5 1-3 0-2 4.0 2-5 1-3 0-2 5.4 3-5 2-3 1-i 6.5 3-5 2-3 I !: 12 6.6 3-5 2-3

~ 7.3 3-5 2-3 1-2

:ecovery of the ~~~rnent by aqueo us leaching methods tn .the presew e of a lkali is presented in Table 3. It is ev tden.t thj{' a significant portion ( -92%) of total chromwm is not leached. CheJari ve and redox­ass isted le"r.:hing methods for removal of chromium from COPR I--ta e th <" refore been e mployed .

,-- _./

:Jit.elative extraction of ~·hromh~nl . i Sol vent extraawn of chromtum with acety /accione , has been carried o ut. While trea tment using

acetylacetone (ac<.S) alone enables 43.4% extructi0n

of chromium, that Lsing acac and triethyl amine at !' H 9.0 enables extractio,1 of 67 .2% chromi um. It is .\CCn that a sequential e:v.raction of chrom iu m with an acetylacetone-aceto ne-acetylaceto ne cyc le enabl es the ex tractio n of 68 % of the chromi um in the mucl. The

SREERAM eta/.: RECOVERY OF Cr FROM CHROMITE ORE PROCESSING RES IDUES 2451

react ion of chromium(III) salts with acetylacetone does not seem to afford tris-complexes of chromium with a characteristic wine red colour. On the other hand the complex generated in this study is brown and extract may seem to contain ferrous salts in addition to those of chromium. In other words chelative extraction does not lead to selective or qualitative removal of chromium.

Reductive treatment of COPR In this work, the COPR (particle size 0.8 mm)

which contained 21% of leachable chromium compounds has been treated with sodium sulphite at 600°C and the percentage reduction in Cr(VI) to Cr(lll) determined. The results are presented in Fig. l. ln spite of a reductive treatment with 200% on weight basis of a reductant like sodium sulphite, 4 .2% of chromium is still extractable from the COPR. ln order to determine the influence of alkalinity on the reduction reaction, the treatment was carried out employing 0 .5 g of NaOH per gram of COPR and varyi ng amounts of sodium sulphite. It is found that under reductive atmosphere and in the presence of alkali , about 60% of chromium is extractable from the COPR (Fig. 1). The formation of chromium(III) hydroxide species as a result of reductive treatment is expected. However, under basic conditions, it is not unlikely that chromium(III) compounds formed in the lattice resi st extraction by alkali. When the resulting chromium(Ill) hydroxide was treated with hydrogen peroxide, it results in the generation of chromium(Vl). The extractable chromium determined as per EPA 3060A after this process is increased to 82%. The possible reactions involved in this process are

.. . (4)

2H20 2 + CaCr04+W ~Cr(0)(02)(0H)" + Ca2+ +H20

... (5)

Oxidative treatment ofCOPR Atmospheric oxidation of chromium from the

unreacted chromite in the COPR sieved to get particles of 0.2 mm diameter has been attempted in the presence of NaOH (1 g/g COPR) at varying temperatures and for varying time durations. The results are presented in the form of a bar chart in Fig. 2. At temperatures above 500°C and a time of oxidation of about 180 min, more than 90% of chromium in the COPR is oxidized to chromium(VI) by oxygen contained in atmospheric air. However,

100

90

80

l 70 TI Q)

60 t3 ~ 50 x Q)

E 40 :> E 30

e .<: 20 (.)

10

- ·--·- ·- · ------· ~---. ~-----. ----. ---·--. 0

0 3

Sodium sulphite [mol/mol (Cr+Fe)]

Fig. !-Extraction profile for chromi um from COPR (particle size 0.8 mm) with sodium sulphite in the presence/absence of NaOH at 600°C for 4 h [- • - Cr(Vl) extracted on treatment with sodium su lphite;- • - Cr (total) extracted with sodium sulphite; - - .A. - Cr (total) extracted using sodium sulphite in combination wi th 0.5 g NaOH/g COPR]

El15rrin

IJ3J rrin

li160 rrin

IJ90rrin

IJ120rrin

.1110 rrin

Fig . 2-Efficiency of extraction of chromium from COPR (particle size 0.2 mm) with atmospheric oxygen and sodium hydroxide at varying temperatures and ti me

when the COPR is present as aggregates of particle millimeter dimensions, chromium seems to remain encapsulated within the calcium sulphate matrix and leaching efficiency drops significantly to < 40% even when treated at 600°C for 180 min. Oxidation needs to be concurrent with the rupture of physical structure leading to the exposure of occluded chromium sites to the oxidant. Oxidation of chromium has been attempted in the presence of oxidants like sodium peroxide, sodium chlorate and sodium perborate and over a period of 60 min . Results obtained are presented in Fig. 3. It is found from Fig. 3 that reaction of COPR with sodium peroxide brings about significant extraction of chromium compared to treatment with other oxidants. Sodium peroxide reac ts with the moisture present in COPR and generates sodium hydroxide and hydrogen peroxide as per Eq . (6)

2452 INDIAN J CHEM, SEC A, SEPTEMBER 2003

0 0 0 (6)

Hydrogen and alkali metal peroxides are unstable. They are known to disproportionate in the presence of alkali, as in Eq. 7.

0 0 0 (7)

In the presence of water, sodium peroxide generates 142 kJ/mol of heat. The release of such energies is sufficient enough to cause micro­explosions within the reaction system of COPR. Under conditions of alkalinity, chromite is expected to be converted into sodium chromate. However, the use of sodium peroxide as an oxidant could pose safety problems in large scale industrial applications. A comparison has been made with efficacy of other oxidants like sodium perborate and chlorate for the recovery of chromium. The extraction efficiency of chromium from COPR is only around 45% for other oxidants under conditions similar to those employed for sodium peroxide. Further, the conversion of chromite to chromate is expected to be favoured by high pH conditions. When excess alkali (to the tune of 0.5glg COPR) is added to the oxidation reactions, the efficiency of chromium removal is better. Efficiencies of recovery of total chromium from COPR under various conditions have been listed in Table 4. In order to promote better contact of reagents with COPR, a microwave assisted extraction has been examined as seen from the data presented in Table 4. COPR prior to and after treatment with sodium peroxide (followed by leaching with water) was subjected to EDXRF measurements. It can be observed from data presented in Table 5 that leaching of chromium from COPR by the oxidative method employing sodium peroxide is selective to chromium.

Residual chromium contained in COPR at the end of oxidative extraction is less than 700 ppm. Disposal of such a leached residue free of chromium is made further easy by the extraction of iron from the chromium leached COPR using a combination of oxidative and chelative methods. When COPR was treated with a mixture of 0.5 g of NaOH and 0.3 g of Na20 2 at 500°C it enables the removal of 87.2% of the chromium, leaving behind the ferrous ions and some chromium compounds . When this residue was treated with acetyl acetone (50 parts for every part of COPR for 4h), it results in the near complete removal of chromium and iron from the mud, leaving behind only calcium and silica compounds. An analysis of acetyl acetone extract by atomic absorption indicates the

"0 Q)

u ~ x Q)

E _:;? E 0 _c 0

100

0 2 3 5 6

Oxidant [mol/mol (Cr+Fe)j

Fig. 3-Efficiency of extraction of chromium from COPR (particle size 0.8 mm) with oxidants at 500°C for I h [ - • -sodium perborate treatment;- • -sodium chlorate treatment: - • - sodium peroxide treatment)

Table 4-Extraction efficiency of chromium from COPR by oxidants treated at 500°C for I h

Extractant employed* 0.5 g NaOH 0.5 g NaOH + 0.1 g NaCI02

0.5 g Na202 + 0.5 g Na802 0.5 g NaOH + 0.1 g Na20 2

0.5 g NaOH + 0.5 g NaN02 0.5 g NaOH + 0.3 g Na20 2 0.5 g ~aOH + 0.5 g Na20 2 0.5 g NaOH [microwave treatment 3 min) 0.5 g NaOH [microwave treatment 3 min) + 0. 1 g Na20 2

* per gram of COPR

% Extraction of chromium 56.4 63.2 71.2 72.7 86.3 87.2 93.2

63.2

76.5

removal of 96.5% of iron compounds and 11.2% of chromium compounds (resulting in less than !50 ppm chromium in the residue). In order that mobilization of chromium from chromite ore is facilitated even at the time of ore dressing and COPR obtained is free of mobile chromium, a new method of dressing ore in the presence of oxidative agents in addition to oxygen from atmospheric air has been explored. Conventional methods of chromite ore processing afford an yield of 80% chromium present, while, the treatment of chromite ore with a combination of sodium perox ide and alkali (I g of NaOH and 0.4 g Na20 2 per gram of chromite) affords 88% recovery of chromium from the ore.

Implications of different leaching studies During chromite ore dressing process, sintering o f

chromium ore with attended changes in particle dimensions is observed. Such a sintering many seem to cause occlusion of some chromium containing

SREERAM et al.: RECOVERY OF Cr FROM CHROMITE ORE PROCESSING RESIDUES 2453

Table 5-Composition of COPR prior to and after treatment with 5 moles/mole of (Cr+Fe) of sodium peroxide at 500°C for I h

Sample Weight % calculated by EDXRF Calcium Chromium Iron Strontium

-4B%Cr

Not very Useful

Potentially Toxic

COPR before treatment

COPR after treatment

-38-39%Cr -~10%Cr

Immobilize As currently practiced

21.0±0.8

19.5±0.8

Proposed Mel hod

Fig. 4-Proposed options for better management of residues generated by chromite processing industries

phases. It can be expected that sintering processes retard the complete oxidation of chromium(III) and leaching of all chromium(VI) formed. Finally obtained COPR, seems to contain both chromium(III) and chromium(VI) containing phases. In order that leaching is complete and the COPR is free of chromium, approaches are required to gain access to chromium(III) and (VI) ions entrapped into the sintered mass for redox or chelative extractants.

The present work demonstrates the near complete recovery of chromium and iron from chromite ore processing residues. The oxidative recovery of chromium has been demonstrated through EDXRF measurements to result in the recovery of pure forms of chromate, which could be mixed with the chromate generated from the chromite ore processing. A schematic representation of the possibilities arising after employing the recovery methods is presented in Fig. 4.

Leaching efficiencies of various methods for different (initial) particle sizes of COPR have been presented in Fig. 5. It is now possible to mobilize more than 95% of chromium contained in COPR using an oxidative method . The efficiency of removal of chromium through oxidative methods employing sodium peroxide is not strongly influenced by the

9.8±0.2

1.3±0.1

b) 2.1

1.8

e 1.5 .§. ~ 1.2 rn "' 0.9 :!! t: .. 0.6 Q.

0.3

0

10. 1±0.2

10.6±0.2

8 b

b

986±40 ppm

943±37 ppm

c d

Treatment

d

Treatment

e g

e g

Fig. 5-a) Percentage extraction of chromium from COPR and b) particle size reduction of COPR treated with a. alkali ( I 0 M NaOH) for I h at ambient conditions; b. treated with 4 mol/mol (Cr+Fe) of sodium sulphite and NaOH (0.5 g/g COPR) at 600"C for 4 h; c. treated with acetylacetone for 4 h; d. treated with 7 mol/mol(Cr+Fe) sodium peroxide at 500°C; e. treated with 7 mol/mol(Cr+Fe) sodium chlorate at 500°C: f. treated with 7 mol/mol(Cr+Fe) sodium perborate at 500°C

initial sizes of the COPR particles. Diffusion of oxidants and oxidation of chromium(III) containing phases in chromite ore processing residues are required . The process of conversion of chromium(lll ) forms into leachable species in COPR involves solid phase reactions. It is necessary to consider factors that influence the diffusion and better contact of reactants and heats of reactions associated with phase and