Thiocyanate Removal Withactivated Carbon

Lakefield Oretest Job No: 9330 Client: Riddarhyttan Resources

Project: Thiocyanate Removal with Activated Carbon

Lakefield Oretest Pty Ltd 431 Victoria Rd, Malaga Western Australia 6090 A.B.N. 35 060 739 835 t +61 (0)8 9209 8700 f +61 (0)8 9209 8701 www.oretest.com.au

Member of the SGS Group (Société Générale de Surveillance)

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THIOCYANATE REMOVAL WITH ACTIVATED CARBON

JOB NO: 9330

CLIENT: Riddarhyttan Resources

DATE: 3 February 2004

Mike Adams John Angove

Author Technical Review






Table of Contents

EXECUTIVE SUMMARY .................................................................................................................. 1

1. INTRODUCTION .......................................................................................................................... 3

2. LITERATURE REVIEW.............................................................................................................. 3 2.1 Aqueous chemistry of thiocyanate........................................................................................................ 3

2.1.1 Formation............................................................................................................................... 3 2.1.2 Complexation.......................................................................................................................... 4 2.1.3 Oxidation ................................................................................................................................ 4 2.1.4 Leaching ................................................................................................................................. 7

2.2 Interaction of thiocyanate and iron with activated carbon .................................................................... 8 2.2.1 Catalysis ................................................................................................................................. 8 2.2.2 Adsorption .............................................................................................................................. 9

3. SAMPLE PREPARATION ......................................................................................................... 10

4. ANALYTICAL PROCEDURES................................................................................................. 10

5. RESULTS AND DISCUSSION................................................................................................... 11 5.1 Isotherms ............................................................................................................................................ 11

5.1.1 Acidic thiocyanate solution (A) ............................................................................................ 11 5.1.2 Ferric thiocyanate solution (B) ............................................................................................ 12

5.2 Kinetics............................................................................................................................................... 13 5.3 Carbon assays ..................................................................................................................................... 14

5.3.1 Chemical assays ................................................................................................................... 14 5.3.2 Mineralogical investigation.................................................................................................. 16 5.3.3 TGA/DTA analysis................................................................................................................ 17 5.3.4 FTIR spectroscopy................................................................................................................ 18

5.4 Stripping sighter tests ......................................................................................................................... 21 6. CONCLUSIONS........................................................................................................................... 21

7. RECOMMENDATIONS ............................................................................................................. 22

8. REFERENCES ............................................................................................................................. 22

APPENDICES

Appendix A: Testwork Flowsheet

Appendix B: Detailed Test Data

Appendix C: Mineralogy Report

Appendix D: TG-DTA Report



- 1 - Lakefield Oretest Pty Ltd 431 Victoria Rd, Malaga Western Australia 6090 A.B.N. 35 060 739 835 t +61 (0)8 9209 8700 f +61 (0)8 9209 8701 www.oretest.com.au



EXECUTIVE SUMMARY

Riddarhyttan Resources, in conjunction with Allan Brown & Associates, have demonstrated at small scale the efficacy of activated carbon in the removal of thiocyanate from their bioleached carbon-in-leach (CIL) tailings, which may contain in excess of 5 g/L of thiocyanate as a result of the reactions of cyanide with partially oxidized sulphur species. The recycled return dam water to the bioleach process can typically tolerate <10 mg/L of cyanide or thiocyanate. This process route was considered a possible option to reduce the thiocyanate levels in the tailings at Riddarhyttan.

This study was commissioned at SGS Lakefield Oretest to start elucidating the mechanism and adsorbed species involved, so that the process can be optimised and demonstrated at pilot scale. The study comprises two parts:

● literature review; and

● batch testwork to determine reaction mechanisms and adsorbed species.

The literature review (35 references) has uncovered some useful prior knowledge that pertains to the process chemistry.

The following conclusions have been arrived at on the basis of the batch testwork results to date, incorporating adsorption isotherm tests, chemical analysis, visible spectroscopy, FTIR spectroscopy, mineralogical (SEM, XRD) and thermogravimetric (TG-DTA) work:

● In the absence of ferric iron, relatively low adsorption of thiocyanate occurred (~50 mg/g at 25 g/L carbon). Carbon molar loadings based on solution assays are consistent with the adsorption of HSCN on to carbon from acidic thiocyanate solution.

● In the presence of ferric iron, much higher adsorptions of thiocyanate occurred (~300 mg/g at 25 g/L carbon). Carbon molar loadings and visible spectroscopy results are consistent with the adsorption of H(SCN)3 on to carbon from ferric thiocyanate solution (B) at lower loadings with some conversion to polymeric (SCN)x species at higher loadings, as well as oxidation products including sulphates. Further work is required to identify the most likely reaction path (cyanides, cyanate or ammonia/carbonates).

● The indicated presence by FTIR spectroscopy of sulphates and absence of thiocyanates on the ferric thiocyanate loaded carbon suggests that oxidation of thiocyanate occurs on the carbon. The absence of precipitated material in the SEM micrographs confirms that the adsorbates and reaction products are adsorbed on to the carbon surface.

● Removal of thiocyanate during cold water washing of the loaded carbon can be significant under some conditions (~80% SCN removal from acidic thiocyanate loaded carbon, and ~10% SCN removal from ferric thiocyanate loaded carbon). The practicalities of washwater disposal should be considered in the design of a full-scale plant.

● Carbon assays for S were generally high for the acidic thiocyanate solutions, which may be owing to the co-adsorption of some sulphuric acid. Carbon assays were low for the ferric thiocyanate solution, and this may be a result of some thiocyanate oxidation occurring in solution.

● After dehydration at 110oC, the main mass loss from the carbon occurred at about 550oC and 460oC for the carbons contacted with acidic thiocyanate and ferric thiocyanate, respectively. This indicates that relatively mild carbon regeneration conditions are sufficient to remove the loaded thiocyanate. In practice, higher temperatures would be periodically necessary to remove any






adsorbed organics. The potential toxicity of the adsorbed gases (eg, HCN, SO2 or NH3) was not evaluated in the current work.

More complete elucidation of the mechanism by which thiocyanate is removed from solution by activated carbon is desirable before operating a pilot plant. It is recommended that the following tests be undertaken:

1. Thermal regeneration of unwashed, non-dried freshly loaded carbon and stored below 20oC.

a. Chemical analysis of off-gases.

b. Chemical analysis of regenerated carbon.

2. TG/DTA/MS analysis of off-gases as a function of temperature.

3. Column elution of unwashed loaded carbon using hot (i) water and (ii) caustic solution.

4. FTIR analysis of unwashed, freshly loaded carbon with no drying and kept at below 20oC.

5. Analysis of freshly contacted solutions and eluates for NH4+, CNO–, HCO3

–, cyanides.






1. INTRODUCTION

Riddarhyttan Resources, in conjunction with Allan Brown & Associates, have demonstrated at small scale the efficacy of activated carbon in the removal of thiocyanate from their bioleached carbon-in-leach (CIL) tailings. Such process routes are becoming increasingly used, as the tailings may contain in excess of 5 g/L of thiocyanate as a result of the reactions of cyanide with partially oxidized sulphur species. The recycled return dam water to the bioleach process can typically tolerate <10 mg/L of cyanide or thiocyanate. While not commonly regulated in discharge to the environment, thiocyanate may be toxic to aquatic species, though to a much lesser extent than cyanide (Smith & Mudder, 1991). For these reasons, this process route was considered a possible option to reduce the thiocyanate levels in the tailings at Riddarhyttan.

This study was commissioned at SGS Lakefield Oretest to start elucidating the mechanism and adsorbed species involved, so that the process can be optimised and demonstrated at pilot scale. Development of a means of eluting the adsorbed species would also be potentially beneficial. The study comprises two parts:

● literature review; and

● batch testwork to determine reaction mechanisms and adsorbed species.

2. LITERATURE REVIEW

A review of the appropriate literature was undertaken, covering the following main areas:

● Aqueous chemistry of thiocyanate.

● Interaction of thiocyanate with activated carbon.

While this review is not intended to be exhaustive, the main aspects are covered.

2.1 Aqueous chemistry of thiocyanate

2.1.1 Formation

The free thiocyanate ion is regarded as the combination of two resonance forms:

–S–C–N ↔ S=C=N–

Thiocyanate is formed in gold plant cyanidation pulps by the reaction of cyanide with labile sulphur in the ore, which are released by the attack of lime or cyanide on sulphide minerals.

CN– + S = SCN– (1)

Thiosulphate and polythionate ions are produced during the oxidation of many sulphide minerals. While they are unstable to oxidation by dissolved oxygen, they can be present for extended periods, allowing the reaction with cyanide, eg:

2S2O32– + O2 + 2CN– = 2SCN– +2SO4

2– (2)

Although thiocyanate does not take part directly in the leaching of gold under alkaline cyanide conditions, it does result in increased cyanide, lime and oxygen consumption. Moreover, it may be






necessary to remove thiocyanate from the tailings, depending on local legislation, and this is a factor for the Riddarhyttan Project.

2.1.2 Complexation

Thiocyanic acid is a strong acid, with a pKa of 1.6 (Flynn & Haslem, 1995).

Stability constants have been measured (Smith & Martell, 1976) for complexes of thiocyanate with a large number of elements, making it a versatile ligand.

Ferric ions are well known (Bjerrum, 1985) to form strong complexes with up to six thiocyanate ligands, forming characteristic dark red solutions, according to the following reactions:

Fe3+ + nSCN– = Fe(SCN)n(3-n) (3)

Only one complex with iron in the ferrous state is known, FeNCS+.

Distribution curves for thiocyanate complexes with iron are given in Figure 2.1. Low concentrations of free thiocyanate (high pSCN) result in cationic complexes, whereas anionic complexes predominate at high free thiocyanate concentrations (low pSCN).

Figure 2.1: Distribution curves for thiocyanate complexes with (a) Fe(III) and (b) Fe(II) (after Barbosa-Filho & Monhemius, 1994)

These complexes are particularly applicable to bioleach residues, as well as the acidic thiocyanate leaching of gold.

Thiocyanate can form insoluble salts with gold, silver, mercury, lead, copper and zinc (Flynn & Haslam, 1995).

2.1.3 Oxidation

The pe-pH stability diagram for the S-CN-H2O system is given in Figure 2.2. The stability region of thiocyanate is restricted to low potentials, mainly the regions of stability of So and HS– in the absence of cyanide.






Figure 2.2: pe-pH stability diagram for the S-CN-H2O system (after Wang & Forssberg, 1990)

There are three well known redox reactions leading to the oxidation of thiocyanate to sulphate (Barbosa-Filho & Monhemius, 1994):

SO42– + CN– + 8H+ + 6e– = SCN– + 4H2O (4)

SO42– + CNO– + 10H+ + 8e– = SCN– + 7H2O (5)

SO42– + CO3

2– + NH3 + 11H+ + 8e– = SCN– + 7H2O (6)

HCNO has a pKa of 3.91, so below pH 4, it tends to decompose rapidly to ammonia and carbon dioxide.

The option of stopping the oxidation at the cyanide formation stage (Equation 4) to facilitate the recycle of cyanide to the leach has been considered (Botz et al, 2000), using ozone (Soto et al, 1995; Nava et al, 1999) and electrochemically (Byerley & Enns, 1984). In both cases, the cyanide is protected as HCN under acidic conditions.

Copper in solution has been indicated as a catalyst for the oxidation of thiocyanate (Flynn & Haslem, 1995); various intermediate species such as S(CN)2 and OSCN– have been indicated as being present.

Early work on the oxidation of thiocyanate suggested the presence of intermediate species such as thiocyanogen (Figlar & Stanbury, 1999):

(SCN)2 + 2e– = 2SCN– Eo = 0.77 (vs. SHE) (7)

Thiocyanogen then undergoes rapid hydrolysis:

3(SCN)2 + 4H2O = 5SCN– + SO42– + HCN + 7H+ (8)






Thiocyanogen in the free state polymerizes, forming brick-red polythiocyanogen (Cotton and Wilkinson, 1972), but it is stable in acetic acid solution. When chlorine is passed under less than excess conditions through a moderately strong solution of thiocyanate at ambient temperatures, the yellow amorphous pseudothiocyanogen or pseudosulphocyanogen (H6C8N8S7O) is formed, also a polymer of thiocyanogen (Williams, 1948). Known as canarine, it has been used as a permanent yellow dye for wool. Nitric acid and presumably other oxidants can have the same effect.

A moderately strong solution of thiocyanate in the presence of strong acid will turn red, then form yellow crystals of the pentagonal xanthane hydride, or perthiocyanic acid, H2C2N2S3:

3SCN– + 3H2SO4 = H2C2N2S3 + HCN + 3HSO4– (9)

Other products may also be formed, including carbon oxysulphide, ammonia, hydrogen sulphide, formic acid and thiolcarbamic acid.

The trithiocyanate ion was also implicated from more recent spectroelectrochemical measurements (Barnett & Stanbury, 2002):

(SCN)3– + 2e– = (SCN)3

– Eo = 0.68 (vs. SHE) (10)

(SCN)2 + SCN– = (SCN)3– (11)

This ion is moderately stable in acidic thiocyanate solutions containing Fe(III) and excess protons, due to the formation of H(SCN)3 and/or ferric trithiocyanate complexes. A yellow colloidal precipitate, identified as H(SCN)3, has been isolated (Nicholson, 1959).

Thiocyanate thus behaves as a pseudohalide, with redox chemistry between that of bromine and iodine.

The redox chemistry of thiocyanate has become of considerable interest recently after the discovery that thiocyanate is a key component of an antibacterial system in human saliva. Barnett and Stanbury (2001) have confirmed the presence of equilibrium mixtures of (SCN)2 and (SCN)3

– in the reactions of Cl2/HOCl and H2O2 with SCN–:

Cl2 + 2SCN– = 2Cl– + (SCN)2 (12)

H+ + HOCl + 2SCN– = Cl– + (SCN)2 + H2O (13)

(SCN)2 + SCN– = (SCN)3– (14)

The autocatalytic oxidation of ClO2 with SCN– has elicted a large amount of interest since 1985 when it was found to display oscillations in a continuously stirred tank reactor (CSTR) (Figlar & Stanbury, 1999). A 14-step mechanism was put forward, summarized by the following 2-stage scheme:

2ClO2 + 10SCN– + 8H+ = 2Cl– + 5(SCN)2 + 4H2O (15)

3(SCN)2 + 4H2O = 5SCN– + H2SO4 + HCN + 5H+ (16)

The blood-red colour of acidic thiocyanate solutions in the presence of ferric iron corresponds to a strong absorption band in the visible range with a maximum at about 480 nm. These solutions have been observed to gradually fade in colour on standing, an effect attributable to the auto-reduction of






Fe(III) to Fe(II) with simultaneous oxidation of thiocyanate (Barbosa-Filho and Monhemius, 1994). Bands at 320 nm and 285 nm correspond to (SCN)3

– and (SCN)2, respectively.

Ferric ion increases the stability of thiocyanate to oxidation at pH 1.5, presumably owing to complex formation. The results in Table 2.1 show that at similar solution potentials (700 mV vs SHE), most of the thiocyanate was oxidized by hydrogen peroxide, whereas <10% was oxidized by ferric iron (Fleming, 1986).

Table 2.1: Stability of thiocyanate to oxidation by hydrogen peroxide or ferric sulphate (after Fleming, 1986)

Thiocyanate decomposition, % H2O2 g/L

Fe3+

G/L ORP at t = 0mV (vs SHE) t = 6 h t = 24 h t = 168 h

0 0 520 0 0 0

2.5 0 620 9 18 18

7.5 0 660 18 18 54

30.0 0 700 73 78 91

0 2.5 705 0 0 9

0 12.5 740 0 9 9

0 50.5 765 0 9 9

The overall thiocyanate oxidation process in the presence of ferric iron is postulated to involve the production of the radical SCN• through the breakdown of the ferric thiocyanate complexes, eg:

Fe(NCS)n3-n = Fe2+ +(n-1)SCN– + SCN• (17)

The thiocyanogen and trithiocyanate species form in subsequent reactions with thiocyanate, with HCN, HCNO and SO4

2– eventuating. The actual hydrolysis mechanism of thiocyanogen and trithiocyanate remains controversial. Itabashi (1984) showed that the intermediate products are sufficiently stable for molar absorptivity measurements to be made and trithiocyanate is particularly stable in acidic solutions, with formation of the H(SCN)3 acid molecule being postulated:

H+ + (SCN)3– = H(SCN)3 (18)

The demonstrated stability of trithiocyanic acid in the presence of excess acid and the long-term stability of thiocyanogen in anhydrous organic solvents such as acetonitrile and acetic acid suggests that the H(SCN)3 species could be further stabilized by adsorption on to activated carbon.

2.1.4 Leaching

Ferric thiocyanate solutions have been used successfully for the leaching of gold (Barbosa-Filho and Monhemius, 1994b; Broadhurst and du Preez, 1993). The thiocyanogen and trithiocyanate species appear to play a role in the leaching reaction. Fleming (1986) considered the use of ion-exchange resins for the recovery of the gold from ferric thiocyanate leachates. It should also be noted that thiocyanate itself adsorbs strongly onto strong-base ion-exchange resins and requires conversion to the cationic complexes FeSCN2+ and Fe(SCN)2

+ for effective elution (Fleming, 1984). Ion-exchange






therefore represents a potential means of recovering thiocyanate from solution without undergoing oxidation.

2.2 Interaction of thiocyanate and iron with activated carbon

2.2.1 Catalysis

Activated carbons have been found to behave as oxidation-reduction catalysts for a variety of reactions (Austin et al, 1980) of the general type shown in Equation 19:

Ox2 + Red1 = Red2 + Ox1 (19)

The respective electrochemical half-reactions result in a mixed potential for the system:

Ox2 + ne– = Red2 (20)

Red1 = Ox1 + ne– (21)

The reduced and oxidized species can be either in solution, adsorbed on the carbon surface or oxygen-containing functionalities on the surface.

Two reactions that have been found to be positively catalysed by activated carbon are of particular relevance, given the pseudohalide nature of thiocyanate:

2FeIII + 3I– = 2FeII + I3– (22)

2Fe(CN)63– + 3I– = 2Fe(CN)6

4– + I3– (23)

The iodine was found to adsorb on to the carbon.

Activated carbon has been found (Huang, 1978) to catalyse the oxidation of Fe(II) to Fe(III) at pH values greater than 3 and the reduction of Fe(III) to Fe(II) at pH values of less than 3. High-temperature activated carbons (>650oC) have been found (Garten, V.A and Weiss, D.E., 1957) to be the most effective reductants for ferric iron.

Monhemius and Ball (1995) have demonstrated the dramatic effect that activated carbon has on the rate of reduction of ferric ion in thiocyanate solution, shown in Figure 2.3.






Figure 2.3: Rate of reduction of ferric ion in thiocyanate solution in (a) presence and (b) absence of activated carbon. Data from (a) Monhemius & Ball, 1995; SCN– 0.05 M; init. Fe3+

0.05 M; carbon, 20 g/L (b) Barbosa-Filho & Monhemius, 1994; SCN– 0.05 M; init. Fe3+ 0.055 M.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 500 1000 1500

Time, min

Fe(II

I) co

nc, m

ol/L

Carbon 20 g/L

Carbon 0 g/L

2.2.2 Adsorption

Activated carbon is well known to favour large, low-charge anions. Mattson and Mark (1971) report the order of the anion effect on the adsorption capacity of protons on activated carbon to be:

SCN– > I– > NO3– > Br– > Cl– > SO4

2–

If the (SCN)3– is indeed stabilised by activated carbon under conditions of excess acid, then it would

be expected to appear at the top of this list, being a large, singly-charged anion. Further reaction under the conditions within the pores of activated carbon, to the various polymeric thiocyanogen species, is also a distinct possibility.

There are several reports in the literature on the detrimental effect of thiocyanate towards gold loading on to activated carbon. Boehme and Potter (1983) report a 16 and 39% reductions in gold loading in four hours from 4 mg/L gold solutions containing 1 g/L and 3 g/L thiocyanate, respectively. Davidson et al (1979) showed a reduction in capacity constant for gold on carbon from 58 to 40 mg/g on addition of 100 mg/L thiocyanate at pH 5. This effect would be caused by the adsorbed thiocyanate species taking up adsorption sites on the carbon.

The use of activated carbon in a CIP configuration has been proposed (Adams, 1994) as a process for the removal and recovery of cyanide from gold plant tailings. Oxidation of cyanide to ammonia and carbonate can take place under certain condition. The process comprised a multi-stage CIP train for cyanide after the gold CIP train, with a separate elution circuit. The potential exists for the removal of cyanide along with thiocyanate. The levels of cyanide in certain gold plants, particularly involving the






treatment of bioleach residues, typically results in high levels of weak-acid dissociable cyanide and in some cases copper, making a combined approach potentially attractive.

3. SAMPLE PREPARATION

Solutions were made up fresh immediately prior to each test, using AR grade H2SO4, NaSCN and Fe2(SO4)3 to the following compositions:

Solution A : 10.6 g/L SCN–; pH 1.8.

Solution B : 10.6 g/L SCN–; 10.2 g/L Fe3+; pH 1.8.

Activated carbon (Haycarb YAD 65) was pre-abraded, washed free of fines and solubles with de-ionised (DI) water, and stored at 100oC prior to use.

4. ANALYTICAL PROCEDURES

The following analyses were conducted at Lakefield Oretest as required:

Metals in solution: Atomic absorption spectroscopy (AAS).

Free Acid (FA) : Titration with standardised NaOH in an oxalate matrix.

Titratable Cyanide: Titration with silver nitrate with rhodanine indicator.

Free Cyanide: Ion-selective electrode (ISE) measurement on solution diluted to remove matrix, pH and Eh effects.

SCN: Standard APHA method.

CNO: Standard APHA methods at SGS Environmental Laboratories.

Total and WAD CN: (a) Standard APHA methods at SGS Environmental Laboratories; (b) derived by cyanide speciation using base metal and titratable cyanide concentrations in solutions.

Oxidation-reduction potential (ORP) and pH values were measured with a combination redox probe (Pt vs Ag/AgCl) and a combination glass electrode, respectively.

Visible spectrophotometry was carried out using a LKB Biochrom Novaspec II 4040 Spectrophotometer.

Carbon samples were either air-dried, or dried overnight at 60 or 110oC, as required. They were assayed for metals and total S and N by ICP and LECO at SGS Analabs.

Carbons were titrated for acid content by back-titrating standardised NaOH solutions of different initial concentrations, after contact with known dry masses of activated carbon for 24 hours.

Fourier transform infrared spectroscopy (FTIR) was carried out using a Brucker FTIR spectrophotometer at Curtin University’s Molecular Spectroscopy Laboratory, Western Australia on dried carbon samples after pulverizing with a mortar and pestle. Thermogravimetric analysis






(TG/DTA) was carried out using a TA Instruments SDT 2960 thermal analyser at Curtin University’s Thermal Characterisation Laboratory, Western Australia. Mineralogical investigation was carried out by Roger Townend of Roger Townend Associates, using optical microscopy, scanning electron microscopy (SEM), electron-dispersion spectroscopy (EDS) and x-ray diffractometry (XRD).

5. RESULTS AND DISCUSSION

A schematic flowsheet for the testwork programme is given in Appendix A.

5.1 Isotherms

Detailed results are given in Appendix A1 and summarised in Table 5.1.

Table 5.1: Isotherm data summary

Solution A : Aqueous Assays, g/L Solution B : Aqueous Assays, g/L Solution mL

Carbon g SCN Fe Fe(II) pH FA ORP SCN Fe Fe(II) pH FA ORP

- 0.0 10.60 0 0 1.78 1.12 10.90 10.20 0.00 1.75 2.43 526

200 0.5 - - - - - - 8.70 9.66 9.72 1.47 4.30 450

200 2.0 - - - - - - 6.06 9.93 13.52 1.40 5.60 482

200 5.0 9.49 0 0 5.12 0.19 361 4.04 9.50 13.96 1.59 4.19 433

200 10.0 9.52 0 0 6.28 0.16 303 1.64 9.40 10.33 1.77 2.40 448

200 15.0 8.90 0 0 7.50 0.13 277 0.79 9.20 8.10 1.89 1.47 452

200 20.0 8.41 0 0 8.13 0.08 253 0.46 9.10 7.12 1.98 1.12 453

200 25.0 8.02 0 0 8.60 0.00 226 0.30 8.80 6.28 2.03 0.85 454

5.1.1 Acidic thiocyanate solution (A)

In the absence of ferric iron, relatively low adsorption of thiocyanate occurred. Free acid and thiocyanate both decreased; calculated carbon molar loadings based on solution assays are consistent with the adsorption of HSCN on to carbon from acidic thiocyanate solution (A), as shown in Table 5.2. If the 25 g/L carbon point is considered an outlier, an adsorption capacity of the carbon for HSCN of about 23 mg/g is implied.






Table 5.2: Molar loadings of H+ and SCN on to activated carbon from acidic thiocyanate solution

Carbon conc., g/L

SCN loading, mg/g

SCN/H, mol/mol

25 44 0.70

50 22 0.66

75 23 1.01

100 22 1.24

125 21 1.36

5.1.2 Ferric thiocyanate solution (B)

In the presence of ferric iron, a much higher degree of thiocyanate adsorption occurred. Free acid and thiocyanate both decreased at higher carbon concentrations; lower carbon concentrations showed an effect of increased free acid, which is most likely an interference artefact. Calculated carbon molar loadings based on solution assays are subject to interference at lower carbon concentrations; at higher carbon concentrations a greater than 1:1 stoichiometry for H:SCN is apparent. This is consistent with the adsorption of H(SCN)3 on to carbon from ferric thiocyanate solution (B) at lower loadings with some conversion to polymeric (SCN)x species at higher loadings, as shown in Table 5.3. This would account for the much higher thiocyanate loadings and the lack of maximum loading evident in the isotherm in Figure 5.1, even at high loadings. Reduction of Fe(III) to Fe(II) was evident.

Table 5.3: Molar loadings of H+ and SCN on to activated carbon from ferric thiocyanate solution

Carbon conc., g/L

SCN loading, mg/g

SCN/H, mol/mol

2.5 880 -0.69

10 484 -0.90

25 274 -2.31

50 185 182.68

75 135 6.23

100 104 4.72

125 85 3.97






Figure 5.1: Adsorption isotherm for thiocyanate on to activated carbon (Initial SCN 10.9 g/L, Fe(III) 10.2 g/L)

0

200

400

600

800

1000

0 2 4 6 8 10S, g/L

C, m

g/g

5.2 Kinetics

Detailed results are given in Appendix A2. Negligible reduction in solution concentration was observed for the acidic thiocyanate solutions, at two acidities. A reduction in solution thiocyanate concentration was observed for the ferric thiocyanate solution, with less effect on iron levels, as shown in Figure 5.2.

Figure 5.2: Rate of adsorption of thiocyanate and iron from solution on to activated carbon

0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500 600

Time, min

SCN

, Fe

Con

c, m

g/L

SCN Fe






Figure 5.3: Visible absorbance of thiocyanate solutions contacted with carbon at several wavelengths (nm); (a) Fe/SCN; (b) H/SCN

(a) (b)

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0 100 200 300 400 500 600

Time, min

Abs

orba

nce 325

350400480

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 100 200 300 400 500 600

Time, min

Abs

orba

nce 325

350400480

Visible absorbance of thiocyanate solutions contacted with carbon are given in

Figure 5.3. Previous work (Barbosa-Filho and Monhemius, 1994b; Figlar & Stanbury, 1999; Barnett and Stanbury, 2001) has assigned peaks in the absorbance spectrum of ferric thiocyanate solutions at 320 nm and 285 nm to (SCN)3

– and (SCN)2, respectively, while the ferric thiocyanate species absorb at about 475 nm. Figure 5.3a shows a drop over time in relative absorbance at 325 nm compared with the 480 nm peak, for the ferric thiocyanate solution. This effect is absent for the acidic thiocyanate solution (Figure 5.3b), and is consistent with the formation and removal of thiocyanogen and trithiocyanate species.

Eh measurements decreased in all three cases; pH values increased. Free acid titration in the ferric thiocyanate test increased, and this is likely an assay interference as noted earlier.

5.3 Carbon assays

Samples of carbons after contact with both Solutions A and B were dried at 60 and 110oC and pulverised for chemical assay and FTIR spectroscopy, along with dried virgin carbon. A portion of the carbon was also retained as air-dried product for mineralogical and TGA/DTA analysis as well as possible future investigation.

5.3.1 Chemical assays

Chemical analysis was carried out on the carbons from the kinetics tests, after drying at the two temperatures.






Table 5.4: Elemental analysis of carbons after contact with various thiocyanate solutions

Solution Drying temp., oC S, g/t N, g/t Fe, g/t Na, g/t K, g/t Ca, g/t Mg,

g/t

None 110 1,200 1,800 730 1,570 7,250 655 360

A, pH 5 60 6,300 4,400 580 7,170 <500 620 335

A, pH 1.8 60 15,000 9,000 630 3,620 <500 340 135

B, pH 1.8 60 61,600 33,000 21,000 795 <500 65 32

A, pH 5 110 6,000 4,400 380 7,100 <500 600 330

A, pH 1.8 110 13,900 9,000 820 3,420 <500 305 125

B, pH 1.8 110 63,000 32,500 21,000 795 <500 80 50

Table 5.5: Molar analysis of major adsorbed elements on carbon after contact with thiocyanate

Solution Drying temp., oC

S, mol/t

N, mol/t

Fe, mol/t

Na, mol/t

H, mol/t

A, pH 5 60 159 186 0 244 322

A, pH 1.8 60 431 514 0 89 826

B, pH 1.8 60 1,888 2,229 363 0 2,125

A, pH 5 110 150 186 0 241 322

A, pH 1.8 110 397 514 0 80 826

B, pH 1.8 110 1,931 2,193 363 0 2,125

Molar analysis for the major adsorbed elements after baseline correction is given in Table 5.5. Nitrogen, acid and sulphur are adsorbed in an approximately equimolar ratio, consistent with HSCN, although excess acid is present in the absence of iron. Other mineral acids, such as H2SO4, are adsorbed to some extent on to activated carbon. Iron is adsorbed at about a 5:1 ratio with sulphur, suggesting the co-adsorption of some Fe(SCN)3 from ferric thiocyanate solutions. Sodium ions replace some protons when NaSCN solutions at pH 5 are contacted with carbon.

A comparison of inferred SCN loadings from carbon S and solution SCN assays is shown in Table 5.6. Carbon assays are high for the acidic thiocyanate solutions, which may be owing to the co-adsorption of some sulphuric acid and the removal of thiocyanate during cold water washing of the loaded carbon (thiocyanate levels of 2,150 mg/L; 1,990 mg/L and 841 mg/L were assayed in the 1 L wash waters from the carbons listed in Table 5.6, respectively). This translates to ~80% SCN removal from acidic thiocyanate loaded carbon, and ~10% SCN removal from ferric thiocyanate loaded carbon. The practicalities of washwater disposal should be considered in the design of a full-scale plant. Carbon assays are low for the ferric thiocyanate solution, and this may be a result of some thiocyanate oxidation in solution.






Table 5.6: Inferred SCN loadings on carbons contacted with various solutions

Solution Drying temp.,

oC

Carbon assays, SCN g/t

Solution assays, SCN g/t

A, pH 5 60 9,244 5,000

A, pH 1.8 60 25,013 13,500

B, pH 1.8 60 109,475 161,500

A, pH 5 110 8,700 5,000

A, pH 1.8 110 23,019 13,500

B, pH 1.8 110 112,013 161,500

5.3.2 Mineralogical investigation

Carbons from the kinetics tests were air-dried and submitted for investigation. The mineralogical report is included in Appendix B. Optical microscopy and SEM of broken surfaces within the carbon granules showed no evidence of precipitated compounds or coloured deposits within the carbon pores. The XRD patterns showed only the amorphous activated carbon peaks.

EDS analysis of selected areas in carbon contacted with acidic thiocyanate solution showed about 0.85 – 1.3% S and ~0.3% Fe. Carbon contacted with ferric thiocyanate solution showed 5.1 – 6.2% S and ~3% Fe. These assays are in the range of the bulk chemical analyses.

These results are consistent with adsorbed species rather than insoluble precipitates.






Figure 5.4: SEM micrographs of activated carbon internal surfaces after contact with (a) ferric thiosulphate and (b) acidic thiosulphate solutions

5.3.3 TGA/DTA analysis

Carbons from the kinetics tests were air-dried and submitted for investigation. The thermal characterization report is included in Appendix C. After dehydration at 110oC, a single major thermal event (73% mass loss) in the carbon contacted with acidic thiocyanate (Solution A) occurred at about 550oC. In the case of the carbon contacted with ferric thiocyanate, (Solution B) several smaller events occured at about 300oC, accounting for ~10% mass loss, while a major event (67% mass loss) occurred at about 460oC. With both carbons, there was no further reaction beyond 600oC. Decomposition temperatures for some relevant compounds are listed in Table 5.7.

(a)

(b)






Table 5.7: Decomposition temperatures for some relevant compounds (after Weast, 1980)

Compound Decomposition temp., oC

Fe2(SO4)3 480 (d)

H2SO4 338 (b)

(NH4)2SO4 235 (d)

NaSCN 287 (m)

NH4SCN 170 (d)

HCN 26 (b)

(NH4)2CO3.H2O 58 (d)

CO2 -79

NH3 -31

d-decomposes, b-boils, m-melts

5.3.4 FTIR spectroscopy

Carbons from the kinetics tests were dried at 60oC for FTIR investigation using a technique similar to that of Adams (1992). The spectra are typically featureless for activated carbons, with a broad O–H stretch at about 3500 cm–1, weak C–H stretches at about 2900-2800 cm–1 and broad, weak carbonyl C=O and ester C–O–C bands at ~1600 cm–1 and 1100 cm–1, respectively. The doublet at 2350 cm–1 is due to atmospheric CO2.

Significantly, the typically strong C=N stretch at around 2200-2000 cm–1 is absent in both carbons, although a very weak band at 2060 cm–1 is possibly present in the ferric thiocyanate loaded carbon.






Table 5.8: Infrared absorption bands for activated carbons contacted with thiocyanate solutions

Carbon contacted with solution

Band, cm–1 Assignment Comments Ferric thiocyanate

(B)

Acidic thiocyanate

(A) None

3500 O–H Act. carbon s, b s, b s, b

2900 C–H Act. carbon w w w

2850 C–H Act. carbon w w w

1600 C=O Act. carbon w, b w, b w, b

1100 C–O–C Act. carbon w, b w, b w, b

2060 C=N SCN vw – –

1190 S–O HSO4– vs w vw

1124 S–O SO42– vs w vw

1080 S–O HSO4–; SO4

2– vs w vw

1000 S–O SO42– w, sh w

970 S–O HSO4– w

639 O–S–O SO42– m

610 O–S–O SO42– m

475 O–S–O SO4 complexes w

s-strong, b-broad, m-medium, w-weak, sh-shoulder, v-very






Figure 5.5: FTIR spectra of activated carbons after contact with (a) acidic thiosulphate and (b) ferric thiosulphate solutions, compared with (bl) virgin carbon






The weak band at 2060 cm–1 corresponds to SCN–, which typically occurs at 2041-2066 cm–1 (Foley et al, 1985). There is no evidence of thiocyanogen, which typically occurs at 2158-2175 cm–1. There is also no evidence for cyanide related species such as Prussian Blue, Fe4[Fe(CN)6]3, which would occur at about 2084 cm–1 (Adams, 1992), HCN (2093-2097 cm–1), CN– (2080 cm–1), NCO– (2169 cm–

1), HNCO (2246-2274 cm–1) (Mucalo et al, 1990, Nakamoto, 1970).

Very strong bands at 1450-1400 and 1370-1290 cm–1 typical of CO32– and HCO3

–, 3335-3030 and 1485-1390 cm–1 typical of NH4

+ and 1410-1340 cm–1 typical of NO3– (Socrates, 1994) are not

significant over the baseline blank carbon.

The indicated presence of sulphates and absence of thiocyanates on the carbons, particularly in the ferric thiocyanate loaded carbon, suggests that oxidation of thiocyanate via Reactions (4-8) occurs on the carbon. The non-detection of ammonium may indicate that HCN is a by-product (Reaction 4); the boiling point of HCN of 26oC (Table 5.7) suggests that any HCN that was present on the carbon would have volatilised in the 60oC drying process or earlier, or reacted with iron.

FTIR analysis of carbon soon after contact with thiocyanate solution and with only external drying should be undertaken to test this hypothesis.

5.4 Stripping sighter tests

Sighter tests were carried out for the elution of thiocyanate-loaded carbon from the kinetics tests. Only a small amount of thiocyanate was eluted with caustic solution from the carbon loaded in ferric thiocyanate solution (B), even in the presence of a reducing agent, sodium metabisulphate (SMBS). In the case of the acidic thiocyanate-loaded carbons, a significant degree of elution was achieved, and this, coupled with the significant levels of thiocyanate in the wash waters from these carbons, is again consistent with the adsorption as HSCN from acidic thiocyanate solutions.

6. CONCLUSIONS

The following conclusions have been arrived at on the basis of the testwork results to date, incorporating adsorption isotherm tests, chemical analysis, visible spectroscopy, FTIR spectroscopy, mineralogical (SEM, XRD) and thermogravimetric (TG-DTA) work:

● In the absence of ferric iron, relatively low adsorption of thiocyanate occurred (~50 mg/g at 25 g/L carbon). Carbon molar loadings based on solution assays are consistent with the adsorption of HSCN on to carbon from acidic thiocyanate solution.

● In the presence of ferric iron, much higher adsorptions of thiocyanate occurred (~300 mg/g at 25 g/L carbon). Carbon molar loadings and visible spectroscopy results are consistent with the adsorption of H(SCN)3 on to carbon from ferric thiocyanate solution (B) at lower loadings with some conversion to polymeric (SCN)x species at higher loadings, as well as oxidation products including sulphates. Further work is required to identify the most likely reaction path (cyanides, cyanate or ammonia/carbonates).

● The indicated presence by FTIR spectroscopy of sulphates and absence of thiocyanates on the ferric thiocyanate loaded carbon suggests that oxidation of thiocyanate occurs on the carbon. The absence of precipitated material in the SEM micrographs confirms that the adsorbates and reaction products are adsorbed on to the carbon surface.

● Removal of thiocyanate during cold water washing of the loaded carbon can be significant under some conditions (~80% SCN removal from acidic thiocyanate loaded carbon, and ~10% SCN






removal from ferric thiocyanate loaded carbon). The practicalities of washwater disposal should be considered in the design of a full-scale plant.

● Carbon assays for S were generally high for the acidic thiocyanate solutions, which may be owing to the co-adsorption of some sulphuric acid. Carbon assays were low for the ferric thiocyanate solution, and this may be a result of some thiocyanate oxidation occurring in solution.

● After dehydration at 110oC, the main mass loss from the carbon occurred at about 550oC and 460oC for the carbons contacted with acidic thiocyanate and ferric thiocyanate, respectively. This indicates that relatively mild carbon regeneration conditions are sufficient to remove the loaded thiocyanate. In practice, higher temperatures would be periodically necessary to remove any adsorbed organics. The potential toxicity of the adsorbed gases (eg, HCN, SO2 or NH3) was not evaluated in the current work.

7. RECOMMENDATIONS

More complete elucidation of the mechanism by which thiocyanate is removed from solution by activated carbon is desirable before operating a pilot plant. It is recommended that the following tests be undertaken:

1. Thermal regeneration of unwashed, non-dried freshly loaded carbon and stored below 20oC.

a. Chemical analysis of off-gases.

b. Chemical analysis of regenerated carbon.

2. TG/DTA/MS analysis of off-gases as a function of temperature.

3. Column elution of unwashed loaded carbon using hot (i) water and (ii) caustic solution.

4. FTIR analysis of unwashed, freshly loaded carbon with no drying and kept at below 20oC.

5. Analysis of freshly contacted solutions and eluates for NH4+, CNO–, HCO3

–, cyanides.

8. REFERENCES

Adams, M.D., 1992. The removal of cyanide from aqueous solution by the use of ferrous sulphate, J. S. Afr. Inst. Min. Metall., 92:17-25.

Adams, M.D., 1992a. Fourier-transform infrared spectrophotometric study of adsorbed aurocyanide species on activated carbon, Hydrometallurgy, 31:111-120.

Adams, M.D., 1994. Removal of cyanide from solution using activated carbon, Minerals Engineering, 7:1165-1177.

Austin, J.M., Groenewald, T. and Spiro, M., 1980. Heterogeneous catalysis in solution. Part 18. The catalysis by carbons of oxidation-reduction reactions, J. Chem. Soc., Dalton Trans., 6:854-859.

Barbosa-Filho, O. and Monhemius, A.J., 1994. Leaching of gold in thiocyanate solutions – Part 1: chemistry and thermodynamics, Trans. Instn. Min. Metall (Sect. C: Mineral Process. Extr. Metall.), 103:C105-C110.

Barbosa-Filho, O. and Monhemius, A.J., 1994a. Leaching of gold in thiocyanate solutions – Part 2: redox processes in iron(III)-thiocyanate solutions, Trans. Instn. Min. Metall (Sect. C: Mineral Process. Extr. Metall.), 103:C111-C116.






Barbosa-Filho, O. and Monhemius, A.J., 1994b. Leaching of gold in thiocyanate solutions – Part 3: rates and mechanism of gold dissolution, Trans. Instn. Min. Metall (Sect. C: Mineral Process. Ectr. Metall.), 103:C117-C124.

Bjerrum, J., 1985. The iron(III)-thiocyanate system. The stepwise equilibria studied by measurements of the distribution of tris(thiocyanato)iron(III) between octan-2-ol and aqueous thiocyanate solutions, Acta Chemica Scandinavica A, 39:327-340.

Boehme, W.R. and Potter, G.M., 1983. Carbon adsorption of gold. Ultimate loading and ionic contaminant effect on loading rates, Prepr. Soc. Min. Engrs., 83-422. 7 pp.

Botz, M., Dimitriadis, D., Polglase, T., Phillips, W. and Jenny, R., 2000. Technologies for the regeneration of cyanide from thiocyanate, 2000 SME Annual Meeting, Salt Lake City, Utah, Preprint 00-123.10 pp.

Broadhurst, J.L. and du Preez, J.G.H., 1993. A thermodynamic study of the dissolution of gold in an acidic aqueous thiocyanate medium using iron(III) sulphate as an oxidant, Hydrometallurgy, 32:317-344.

Byerley, J.J. and Enns, K., 1984. Electrochemical regeneration of cyanide from waste thiocyanate solutions, CIM Bulletin, Jan 1984:87-93.

Cotton, F.A. and Wilkinson, G., 1972. Advanced Inorganic Chemistry, 3rd Edn, p. 301(Interscience, New York).

Davidson, R.J., Veronese, V. and Nkosi, M.V., 1979. The use of activated carbon for the recovery of gold and silver from gold-plant solutions, J. S. Afr. Inst. Min. Metall., May 1979:281-297.

Figlar, J.N. and Stanbury, D.M., 1999. Kinetics and a revised mechanism for the autocatalytic oxidation of SCN– by ClO2, J. Phys. Chem., 103:5732-5741.

Fleming, C.A., 1984. Regeneration of thiocyanate-loaded resins, S. Afr. Pat. 84/9620.

Fleming, C.A., 1986. A process for the simultaneous recovery of gold and uranium from South African ores, in Gold 100, Vol. 2: Extractive Metallurgy of Gold (ed: C.E. Fivaz) (South African Institute of Mining and Metallurgy, Johannesburg).

Flynn, C.M. and Haslem, S.M., 1995. Cyanide Chemistry – Precious Metals Processing and Waste Treatment, Information Circular 9429. 282 pp. (US Bureau of Mines).

Foley, J.K., Pons, S. and Smith, J.J., 1985. Fourier transform infrared spectroelectrochemical studies of anodic processes in thiocyanate solutions, Langmuir, 1:697-701.

Garten, V.A and Weiss, D.E., 1957. The ion- and electron-exchange properties of activated carbon in relation to its behaviour as a catalyst and adsorbent, Rev. Pure & Appl. Chem., 7:69-122.

Huang, C.P., 1978. Chemical interactions between inorganics and activated carbon, in Carbon Adsorption Handbook (Ed: P.N.Cheremisinoff and F. Ellerbusch), pp. 281-329 (Ann Arbor, Michigan).

Itabashi, E., 1984. Identification of electrooxidation products of thiocyanate ion in acidic solutions by thin-layer spectroelectrochemistry, J. Electroanal. Chem., 177:311-315.






Mattson, J.S. and Mark, H.B. 1971. Activated Carbon, pp. 134-136 (Marcel Dekker, New York).

Monhemius, A.J. and Ball, S.P., 1995. Leaching of Dominican gold ores in iodide-catalysed thiocyanate solutions, Trans. Instn. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.), 104:C117-C124.

Mucalo, M.R., Cooney, R.P. and Wright, G.A., 1990. Fourier-transform infrared studies of the corrosion of nickel in aqueous cyanide media, J. Chem. Soc. Faraday trans., 86:1083-1086.

Nakamoto, K., 1970. Infrared Spectra of Inorganic and Coordination Compounds, 2nd Edn., pp. 176-191 (Wiley, New York).

Nava, F., Carrillo, F.R., Uribe, A., Perez, R., Mendez, J., Mendez, M. 1999. Use of ozone to treat cyanidation effluents, in: REWAS '99: Global Symposium on Recycling, Waste Treatment and Clean Technology, Volume III, pp. 2113-2122 (Minerals, Metals and Materials Society/AIME, Warrendale).

Nicholson, M.M., 1959. Anal. Chem., 31:128.

Smith, A. and Mudder, T., 1991. The Chemistry and Treatment of Cyanidation Wastes, 345 pp. (Mining Journal Books, London).

Smith, R.M. and Martell, A.E., 1976. Critical Stability Constants, pp. 29-34 (Plenum, New York).

Socrates, G., 1994. Infrared Characteristic Group Frequencies, 2nd Edn., pp. 209-212 (Wiley, New York).

Soto, H., Nava, F., Leal, J. and Jara, J, 1995. Regeneration of cyanide by ozone oxidation of thiocyanate in cyanidation tailings, Miner. Eng., 8:273-281.

Wang, X. and Forssberg, K.S.E., 1990. The chemistry of cyanide-metal complexes in relation to hydrometallurgical processes of precious metals, Mineral Processing & Extractive Metallurgy Review, 6:81-125.

Weast, R.C., 1980. CRC Handbook of Chemistry and Physics, 60th Edn. (CRC Press, Boca Raton, FL).

Williams, H.E., 1948. Cyanogen Compounds. Their Chemistry, Detection and Estimation, 2nd Edn. pp. 257-326. (Edward Arnold, London).

V:\Clients A-F\Allan Brown & Assoc\9330\rpt001rid [SCN carbon].doc






Appendix A: Testwork Flowsheet






Testwork Flowsheet: Thiocyanate Removal with Activated Carbon Client: Riddarhyttan Contact: Allan Brown

Solution Preparation

A. H2SO4 + NaSCN B. Fe2(SO4)3 + H2SO4 + NaSCN

Solutions to be made up fresh for each test. Conditions to be advised - as per optimal leach assay.

Isotherms (x2)

Ambient temp. Coconut shell carbon

24 hours 5 points

Assay solution for Fe(t), Fe(II), SCN, Eh, pH, FA.

Kinetics (x2)

Ambient temp.

Sample solution over 24 hours. Assay solutions for Fe(t), Fe(II), SCN, Eh, pH, FA;

UV-VIS. Collect & wash final carbons (x2) (known volume of

DI water; check assay).

Collect precipitates if formed.

Preliminary CIP configuration calculations

Stripping sighters

Carbons A & B, moist. Contact with

(i) NaOH solution at 60 C; (ii) NaOH/Na2SO3 at 60 C;

immediately, and after 7 days ambient storage. Assay solutions for for Fe(t), Fe(II), SCN, FCN,TCN, Eh.

Carbon Analyses

(i) Blank carbon (ii) Carbon A, dried at 60 C (iii) Carbon B, dried at 60 C (iv) Carbon A, dried at 110 C (v) Carbon B, dried at 110 C

Assay for Fe, S(t), N(t), Na, Ca, K,Mg. Titrate carbon for acid content. FTIR spectroscopy.






Appendix B: Detailed Test Data






B.1: Isotherms

Client Allan Brown/RiddarhyttanTest Description Carbon Adsorption IsothermsTest Sample Solutions A & BTest No.Job No. 9330Date

Test Objective Test ParametersSolution Volume (mL) 200

To determine adsorption isotherm Temperature ambientfor SCN with Haycarb YAD 65 carbon Pulp Density (kg/m3) na

Pulp Density (%S;w/w) naCarbon Type Haycarb YAD 65Carbon Size (mm) CrushedContact Time (days) 4

Test DataNo. Solution Carbon Solution A : Aqueous Assays g/L Solution B : Aqueous Assays g/L

mL g SCN Fe Fe(II) pH FA ORP SCN Fe Fe(II) pH FA ORP0 10.60 0.00 0.00 1.78 1.12 10.90 10.20 0.00 1.75 2.43 5261 200 0.5 8.70 9.66 9.72 1.47 4.30 4502 200 2.0 6.06 9.93 13.52 1.40 5.60 4823 200 5.0 9.49 0.00 0.00 5.12 0.19 361 4.04 9.50 13.96 1.59 4.19 4334 200 10.0 9.52 0.00 0.00 6.28 0.16 303 1.64 9.40 10.33 1.77 2.40 4485 200 15.0 8.90 0.00 0.00 7.50 0.13 277 0.79 9.20 8.10 1.89 1.47 4526 200 20.0 8.41 0.00 0.00 8.13 0.08 253 0.46 9.10 7.12 1.98 1.12 4537 200 25.0 8.02 0.00 0.00 8.60 0.00 226 0.30 8.80 6.28 2.03 0.85 454

Thiocyanate extraction by activated carbon

Isotherm Results/Summary

Client Allan Brown/RiddarhyttanTest Description Carbon Adsorption IsothermsTest Sample Solution ATest No.Job No. 9330Date




Test DataNo. Solution Carbon Aqueous Assays g/L Carbon* mg/g

mL g SCN FA pH ORP SCN FA SCN FA0 10.60 1.12 1.78 0 0 0 01 200 5.0 9.49 0.19 5.12 361 44 37 10.47 83.072 200 10.0 9.52 0.16 6.28 303 22 19 10.19 85.743 200 15.0 8.90 0.13 7.50 277 23 13 16.04 88.414 200 20.0 8.41 0.08 8.13 253 22 10 20.66 92.875 200 25.0 8.02 0.00 8.60 226 21 9 24.34 100.00

* Calculated assays

Isotherm for thiocyanate extraction by activated carbon

% Extraction

Adsorption Isotherm - SCN

0

10

20

30

40

50

7.5 8.0 8.5 9.0 9.5 10.0S, g/L

C, m

g/g

Adsorption Isotherm - FA

05

10152025303540

0.00 0.05 0.10 0.15 0.20S, g/L

C, m

g/g

Isotherm Results/Report Soln A

Client Allan Brown/RiddarhyttanTest Description Carbon Adsorption IsothermsTest Sample Solution BTest No.Job No. 9330Date




Test DataNo. Solution Carbon Aqueous Assays g/L Carbon Assays* mg/g

mL g SCN Fe Fe(II) pH FA ORP SCN Fe FA SCN Fe FA0 10.90 10.20 0.00 1.75 2.43 526 0 0 0 0 0 01 200 0.5 8.70 9.66 9.72 1.47 4.30 450 880 216 -748 20.18 5.29 -76.952 200 2.0 6.06 9.93 13.52 1.40 5.60 482 484 27 -317 44.40 2.65 -130.453 200 5.0 4.04 9.50 13.96 1.59 4.19 433 274 28 -70 62.94 6.86 -72.434 200 10.0 1.64 9.40 10.33 1.77 2.40 448 185 16 1 84.95 7.84 1.235 200 15.0 0.79 9.20 8.10 1.89 1.47 452 135 13 13 92.75 9.80 39.516 200 20.0 0.46 9.10 7.12 1.98 1.12 453 104 11 13 95.80 10.78 53.917 200 25.0 0.30 8.80 6.28 2.03 0.85 454 85 11 13 97.28 13.73 65.02

* Calculated

Isotherm for thiocyanate extraction by activated carbon

% Extraction

Adsorption Isotherm - SCN

0

200

400

600

800

1000

0 2 4 6 8 10S, g/L

C, m

g/g

Adsorption Isotherm - Fe

0

50

100

150

200

250

0 10 20 30 40 50 60 70S, g/L

C, m

g/g

Adsorption Isotherm - FA

-800

-600

-400

-200

0

200

0 1 2 3 4 5 6

S, g/L

C, m

g/g

Isotherm Results/Report Soln B






B.2: Kinetics

Client Name: Allan Brown/RiddarhyttanTest Description: Kinetic isothermTest Sample: Solution ATest Number: PW964Job Number: 9330Date:


To determine adsorption kinetics Pulp Density (kg/m3) na of SCN on to Haycarb YAD carbon Pulp Density (%S;w/w) na

Solution Volume (mL) 5000Carbon Type Haycarb YAD 65Carbon Size (mm)Carbon Addition (g); 100.0 Temperature; ambDuration (hr) 24

Test DataTime pH Eh Fe 2+ F/Acid Aqueous Assays mg/L Carbon Assays* g/tmins g/L SCN Fe SCN Fe SCN Fe

0 5.2 485 0 0 10700 0 0 0 0 010 9.1 491 0 0 10500 0 10000 0 1.87 020 9.5 381 0 0 10600 0 5000 0 0.93 030 9.7 378 0 0 10600 0 5000 0 0.93 060 9.8 356 0 0 10600 0 5000 0 0.93 0120 10.0 339 0 0 10300 0 20000 0 3.74 0240 10.1 240 0 0 10300 0 20000 0 3.74 0480 10.0 179 0 0 10600 0 5000 0 0.93 0

* Calculated0 ######

08/10/03

% Extraction

PW964

Kinetic Test for Thiocyanate Extraction by Activated Carbon

Concentration with Time

0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500 600

Time, min

SCN

, Fe

Con

c, m

g/L

SCN Fe

Kinetic Results/Results Soln A high pH

UV-VIS Diln 1000Timemins 325 350 400 480

0 0.108 0.045 0.04310 0.149 0.083 0.06520 0.157 0.088 0.07030 0.156 0.091 0.06860 0.164 0.102 0.083120 0.169 0.102 0.073240 0.188 0.123 0.094480 0.180 0.119 0.081

Wavelength, nm

Visible Absorbance Data

Visible absorbance of Fe/SCN solution with carbon at several wavelengths

0.0000.0200.0400.0600.0800.1000.1200.1400.1600.1800.200

0 100 200 300 400 500 600

Time, minA

bsor

banc

e 325350400480

Visible absorbance of Fe/SCN solution with carbon with time (min)

0.0000.0200.0400.0600.0800.1000.1200.1400.1600.1800.200

300 350 400 450 500

Wavelength, nm

Abs

orba

nce

010203060120240480

Kinetic Results/Results Soln A high pH

Client Name: Allan Brown/RiddarhyttanTest Description: Kinetic isothermTest Sample: Solution BTest Number: PW965Job Number: 9330Date:




Test DataTime pH Eh Fe 2+ F/Acid Aqueous Assays mg/L Carbon Assays* g/tmins g/L SCN Fe FA SCN Fe FA SCN Fe FA

0 1.7 554 0 1.7 10400 10890 1700 0 0 0 0 010 1.7 531 0 2.0 9490 10960 2000 45500 -3500 -15000 8.75 -0.64 020 1.7 523 0 2.1 9290 10850 2100 55500 2000 -20000 10.67 0.37 030 1.7 521 0 2.2 9220 10880 2200 59000 500 -25000 11.35 0.09 060 1.7 513 0 2.5 8840 10840 2500 78000 2500 -40000 15.00 0.46 0120 1.6 506 0 3.0 8210 10750 3000 109500 7000 -65000 21.06 1.29 0240 1.6 498 0 3.3 7670 10810 3300 136500 4000 -80000 26.25 0.73 0480 1.5 489 0 3.9 7170 10710 3900 161500 9000 -110000 31.06 1.65 0

* Calculated544500 100.00

08/10/03

% Extraction

PW965



0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500 600

Time, min

SCN

, Fe

Con

c, m

g/L

SCN Fe

Kinetic Results/Results Soln B


0 0.369 0.176 0.05310 0.340 0.157 0.052 0.04320 0.320 0.152 0.052 0.04330 0.319 0.149 0.054 0.04560 0.309 0.146 0.058 0.046120 0.279 0.127 0.047 0.038240 0.258 0.117 0.045 0.037480 0.271 0.143 0.057 0.040

Wavelength, nm



0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 100 200 300 400 500 600

Time, minA

bsor

banc

e 325350400480


0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

300 350 400 450 500

Wavelength, nm

Abs

orba

nce

010203060120240480

Kinetic Results/Results Soln B

Client Name: Allan Brown/RiddarhyttanTest Description: Kinetic isothermTest Sample: Solution ATest Number: PW966Job Number: 9330Date:




Test DataTime pH Eh Fe 2+ F/Acid Aqueous Assays mg/L Carbon Assays* g/tmins g/L SCN Fe FA SCN Fe FA SCN Fe FA

0 2.2 430 0 0.40 10200 0 401 0 0 0 0 010 2.3 413 0 0.35 10300 0 347 -5000 0 2700 -0.98 0 020 2.4 402 0 0.32 10200 0 320 0 0 4050 0.00 0 030 2.5 395 0 0.24 10100 0 240 5000 0 8050 0.98 0 060 2.8 380 0 0.16 9770 0 160 21500 0 12050 4.22 0 0120 3.1 378 0 0.13 9830 0 134 18500 0 13350 3.63 0 0240 3.5 369 0 0.13 9810 0 134 19500 0 13350 3.82 0 0480 3.9 348 0 0.13 9930 0 134 13500 0 13350 2.65 0 0

* Calculated0 ######

09/10/03

% Extraction

PW966



0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500 600

Time, min

SCN

, Fe

Con

c, m

g/L

SCN Fe

Kinetic Results/Results Soln A low pH


0 0.144 0.078 0.08610 0.138 0.069 0.08220 0.138 0.068 0.07030 0.138 0.066 0.06660 0.126 0.058 0.054120 0.134 0.063 0.055240 0.140 0.070 0.059480 0.144 0.074 0.063

Wavelength, nm



0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0 100 200 300 400 500 600

Time, minA

bsor

banc

e 325350400480


0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

300 350 400 450 500

Wavelength, nm

Abs

orba

nce

010203060120240480

Kinetic Results/Results Soln A low pH






B.3: Carbon Assays

Client Name: Allan Brown/RiddarhyttanTest Description: Kinetic carbon assaysTest Sample: Solutions A, BTest Number: PW964-6Job Number: 9330Date:

Test No. Solution Drying temp., oC

S, g/t N, g/t Fe, g/t Na, g/t K, g/t Ca, g/t Mg, g/t

None 110 1,200 1,800 730 1,570 7,250 655 360PW964 A, pH 5 60 6,300 4,400 580 7,170 <500 620 335PW966 A, pH 1.8 60 15,000 9,000 630 3,620 <500 340 135PW965 B, pH 1.8 60 61,600 33,000 21,000 795 <500 65 32PW964 A, pH 5 110 6,000 4,400 380 7,100 <500 600 330PW966 A, pH 1.8 110 13,900 9,000 820 3,420 <500 305 125PW965 B, pH 1.8 110 63,000 32,500 21,000 795 <500 80 50

Inferred SCN on carbon, g/t

Test No. Solution Drying temp., oC

S, mol/t N, mol/t Fe, mol/t Na, mol/t

H, mol/t Carbon assays

Soln assays

PW964 A, pH 5 60 159 186 0 244 322 9,244 5,000PW966 A, pH 1.8 60 431 514 0 89 826 25,013 13,500PW965 B, pH 1.8 60 1,888 2,229 363 0 2,125 109,475 161,500PW964 A, pH 5 110 150 186 0 241 322 8,700 5,000PW966 A, pH 1.8 110 397 514 0 80 826 23,019 13,500PW965 B, pH 1.8 110 1,931 2,193 363 0 2,125 112,013 161,500

08/10/03

PW964-6

Analyses of Activated Carbon after Thiocyanate Extraction

Molar Analyses after Baseline Subtraction

Kinetic Results/Analyses

Client Name: Allan Brown/RiddarhyttanTest Description: Kinetic carbon acid content assaysTest Sample: Solutions A, BTest Number: PW964-6Job Number: 9330Date:

Test No. Solution Wet carbon, g

Dry carbon, g

NaOH vol., mL

Titre with 0.102 M HCl, mL

NaOH fin., M

NaOH init., M

Acid on carbon, mol/t

PW964 A, pH 5 2.0200 1.1933 50 2.90 0.0059 0.0094 1462.0140 1.1517 50 9.10 0.0186 0.0236 2192.0216 1.2041 50 20.50 0.0418 0.0479 2522.0052 1.1836 50 44.45 0.0907 0.0983 322

PW966 A, pH 1.8 2.0079 1.2147 50 0.20 0.0004 0.0094 3702.0362 1.2507 50 3.60 0.0073 0.0236 6502.0196 1.2468 50 14.55 0.0297 0.0479 7312.0188 1.2643 50 37.95 0.0774 0.0983 826

PW965 B, pH 1.8 2.0307 1.2282 50 0.00 0.0000 0.0094 3832.0366 1.2513 50 0.00 0.0000 0.0236 9432.0166 1.2309 50 2.50 0.0051 0.0479 17392.0230 1.2237 50 22.69 0.0463 0.0983 2125

PW964-6

Acid Content Analyses of Activated Carbon after Thiocyanate Extraction

08/10/03

Kinetic Results/Acid content






B.4: FTIR Spectra






B.5: Stripping

Client Name: Allan Brown/RiddarhyttanTest Description: Carbon stripping sightersTest Sample: Carbons contacted with Solutions A, BTest Number: PW964-6Job Number: 9330Date:

Stripping after 1 hour

Test No. Loading Solution

Stripping Solution S on carbon, g/t

Wet carbon, g

Dry carbon,

g

Strip vol., mL

Fe, mg/L

Eh, mV

CN total, mg/L

SCN, mg/L

S stripped as SCN, %

PW964 A, pH 5 1% NaOH 6,300 10.0705 6.1190 179 0.12 -120 1.20 440 1131% NaOH/0.1%

Na2SO3

6,300 10.3750 6.3731 177 0.12 -120 0.14 400 97

PW966 A, pH 1.8 1% NaOH 15,000 10.1458 6.3206 177 0.40 -121 1.20 1130 1161% NaOH/0.1%

Na2SO3

15,000 10.1071 6.3190 177 0.18 -118 0.09 3140 324

PW965 B, pH 1.8 1% NaOH 61,600 10.2013 5.3241 166 0.14 -123 1.00 3190 891% NaOH/0.1%

Na2SO3

61,600 10.2411 5.4608 178 0.41 -121 0.25 1250 36

Stripping after 1 week

Test No. Loading Solution

Stripping Solution S on carbon, g/t

Wet carbon, g

Dry carbon,

g

Strip vol., mL

Fe, mg/L

Eh, mV

CN total, mg/L

SCN, mg/L

S stripped as SCN*,

%

SCN stripped**,

%

PW964 A, pH 5 1% NaOH 1,200 10.0705 6.1190 179 0.12 -120 1.20 440 593 161% NaOH/0.1%

Na2SO3

1,200 10.3750 6.3731 177 0.12 -120 0.14 400 511 14

PW966 A, pH 1.8 1% NaOH 159 10.1458 6.3206 177 0.40 -121 1.20 1130 10946 151% NaOH/0.1%

Na2SO3

159 10.1071 6.3190 177 0.18 -118 0.09 3140 30463 41

PW965 B, pH 1.8 1% NaOH 431 10.2013 5.3241 166 0.14 -123 1.00 3190 12743 31% NaOH/0.1%

Na2SO3

431 10.2411 5.4608 178 0.41 -121 0.25 1250 5199 1

* Based on carbon S assays.** Assumes reduction in thiocyanate levels during extraction was all loaded on to carbon.

PW964-6

Stripping of Activated Carbon after Thiocyanate Extraction

08/10/03

Kinetic Results/Stripping






Appendix C: Mineralogy Report

Correspondence to Box 3129, Malaga D.C. WA 6945 ACN 069 920 476 ABN 92 076 109 663

MIKE ADAMS 1-11-2003 LAKEFIELD ORETEST PTY LTD, 431 VICTORIA RD, MALAGA WA OUR REF. 20839 YOUR REF. OPTICAL/SEM AND XRD EXAMINATION OF TWO ACTIVATED LOADED CARBONS. R TOWNEND

eÉzxÜ gÉãÇxÇw tÇw TááÉv|tàxá VÉÇáâÄà|Çz `|ÇxÜtÄÉz|áàá

Unit 4, 40 Irvine drive, Malaga Western Australia 6062 Phone: (08) 9248 1674 Fax: (08) 9248 1502 email: [email protected]

<LAKEFIELD > 2 Ref No <20839>

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INTRODUCTION Two loaded activated carbons were submitted for examination, with respect to the presence of various sulphur- bearing organic compounds. The two carbons , labeled A. and B. contained 1.5 and 6% sulphur, with iron at 0.3% in A and 3% in B. A blank carbon was also available for comparison. RESULTS. 1. OPTICAL. Optical examination of original and fresh broken faces did not detect any coloured deposits. In addition, there appeared to be low levels of non carbon material. 2. XRD XRD traces were run of both samples and the blank. This found that the loaded carbons gave a similar result to the blank , with low flat amorphous “peaks”. 3. SEM An SEM/EDS examination was made of numerous broken surfaces SAMPLE A. Analyses of spots and areas gave similar results, with sulphur from 0.85 to 1.3% and low peaks (<0.5% ) for Na, Mg,Al , Si and Ca. Surprisingly Fe was not detected. SAMPLE B The result was similar, except that the sulphur value for spots or areas, ranged from 5.1 –6.2%. BSE IMAGES. See the attached BSE images of typical broken faces. CONCLUSIONS It is concluded that the sulphur bearing compound(s) is adsorbed on the carbon surfaces.

(a) ferric thiosulphate

(b) acidic thiosulphate






Appendix D: TG-DTA Report






This report has been prepared by Lakefield Oretest Pty Ltd (Oretest) at the request of Allan Brown of Riddarhyttan Resources (Client). This Report presents the results of the metallurgical testwork conducted by Oretest on the samples (the Samples) provided by the Client.

This Report is provided to the Client on the basis that the Client expressly acknowledges that:

(a) no representations have been made to Oretest as to the purpose for which the tests are required to be conducted; and

(b) the Testwork was carried out by Oretest on the Samples provided by the Client.

(c) Oretest was in not involved in:

• the drilling, collection or transportation of the Samples; and

• the handling of the Samples prior to their delivery to Oretest.

By this Report, Oretest makes no representation or warranty (express or implied) as to the nature, source, completeness or handling of the Samples and Oretest and its directors, employees, agents and consultants denies and disclaims all liability (including for negligence) for any loss, cost, expense or damage arising from the opinions or conclusions contained in this report to the extent that loss, cost, expense or damage arises from the nature, source, completeness or handling of the Samples prior to their delivery to Oretest.

Oretest expressly denies liability for all damages for loss of opportunity, loss of revenue, loss of actual or anticipated profit or other consequential loss arising either directly or indirectly from reliance by the Client or any other person on the content and conclusions of this report.

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Thiocyanate Removal Withactivated Carbon

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