Extraction of gold(III) from low-grade ores with amidines followed by its spectrophotometric determination with methylene blue

Analytica Chimica Acta, 231 (1990) 491-496 Elsevier Science Publishers B.V., Amsterdam

491

Extraction of gold( III) from low-grade ores with amidines followed by its spectrophotometric determination with methylene blue C. Agrawal, Department

M. Shrivastava,

R.K. Mishra

of Chemistry, Rauishankar (Received

and K.S. Pate1 *

University, Raipur-492

010, M. P. (India)

20th July 1989)

Abstract

A procedure is described in which gold(II1) is quantitatively extracted with an amidine into chloroform over the acidity range pH 3.0-11.0 M HCl, followed by its selective spectrophotometric determination by interaction of the extract with methylene blue in the pH range 3.0-9.0. The molar absorptivity of the coloured complex formed by extraction with ten different amidines and methylene blue reaction lie in the range 1.1 X 104-6.5 X lo4 1 mol-’ cm-’ The simplest compound, N, N’-diphenylbenzamidine, was chosen for detailed study. at A,,, (650 nm) in chloroform. The limit of detection is 5 pg Au 1-‘. The method is free from interferences from the metals that are generally associated with gold. The method is simple, reproducible and applicable to the accurate recovery of gold from low-grade ores containing the metal at levels of b 1.5 pg g-‘. Keywords:

Gold;

Extraction;

Ores; Amidines

Owing to the demand for gold and its low abundance in nature, it is desirable to develop an improved, accurate analytical procedure that is applicable to low-grade geological samples containing the metal at trace levels. Gold processing plants also need to analyse solutions containing the metal in the pg 1-l range. Analytical techniques such as atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, x-ray fluorescence spectrometry, neutron activation analysis and electrochemical methods have either insufficient sensitivity, are very time consuming or suffer from various matrix interferences. They usually need an accurate preconcentration method. Generally, ion-exchange and liquid-liquid extraction methods are used for this purpose. Solvents such as high molecular isobutyl methyl ketone, ethyl weight amines, acetate and tributyl phosphate are employed for 0003-2670/90/$03.50

0 1990 - Elsevier

Science Publishers

the preconcentration of Au [l-3], but the extraction is not selective. Many organic reagents such as dithizone, 4,4’bis(dimethylamino)thiobenzophenone, anisaldehyde4-phenyl-3-thiosemicarbazone, Rhodamine B, methyl violet and crystal violet have been reported for the spectrophotometric determination of traces of Au [4-lo]. The extraction of Au with dithizone and thiosemicarbazone suffer from inter-element interferences, and the sensitivities achieved are inadequate. Thiobenzophenone is a sensitive reagent for the determination of Au but the conditions for full colour development are critical. Methylene blue (MB) has been reported for the sensitive extraction spectrophotometric determination of the metal [ll]. In this method the determination of gold(II1) in complex materials requires three steps: prior extraction of the metal with ethyl methyl ketone to remove the inter-eleB.V.

C. AGRAWAL

ment interferences [e.g., Co(II), Fe(III), As(V), Sb(III/V)] and digestion of the extract by treatment with acid followed by oxidation of the metal by aqua regia, and then the metal is determined using MB. In the method described here, gold(II1) is extracted with N, N ‘-diphenylamidine (DPBA) from hydrochloric acid solution in the presence of MB, or with DPBA alone and subsequent addition of MB. The extraction of chloroaurate with DPBA is highly selective but the sensitivity is very low (6 = 650 1 mol-’ cm-‘). The extraction procedure for gold(II1) with MB and DPBA together is very sensitive (E = 1.2 x lo5 1 mol-’ cm-‘) but extraction of Co(II), Fe(III), As(V), Sb(V) and Tl(II1) also occurs. The procedure in which MB is added after extraction is fairly sensitive (c = 6.5 X lo4 1 mol-’ cm-’ ) and specific. Hence this sequential procedure was investigated in detail. The effect of the amidine substituent on the extraction and the spectral properties of the complexes are described. Amidine extraction simplifies the classical MB method [ll] and removes most of the interferences, as the amidine is a relatively weak base, thereby having less tendency to form a coloured extractable ion pair with various metal ion complexes. The selectivity and sensitivity of the method are also comparable to those of other established methods.

EXPERIMENTAL

Apparatus An ECIL Model GS-865 spectrophotometer

and a Carl-Zeiss Jena Spekol with matched l-cm quartz cuvettes was used for measuring absorbance. A Varian Techtron Model AA575 atomic absorption spectrometer with a variable tantalum nebulizer was also used. The conditions of operation of the instrument [12] were as follows: wavelength, 242.8 nm; slit width, 0.5 nm; lamp current, 5 mA; fuel, acetylene; and support, air. A gold hollow-cathode lamp was used.

Reagents All reagents were of analytical-reagent grade (Merck). A 0.2120-g amount of gold powder was digested with aqua regia, evaporated to dryness

ET AL.

and excess of nitric acid was removed by heating with concentrated HCl, then the residue was dissolved in 6 M HCl. The working solution was prepared in 1 M HCl. The ten amidines tested were synthesized according to the literature [13] and 0.2% (w/v) (ca. 0.007 M) solutions in chloroform were prepared. A 0.01% (w/v) (0.00033 M) solution of MB in doubly distilled water was used.

Sequential procedure For the extraction of gold(II1) with amidine, an aliquot of solution containing 5-30 pg of Au(II1) was placed in a 100-ml separating funnel followed by 2 ml of HCl (5 M) and distilled water to 10 ml. The aqueous solution was shaken with 5 ml of a chloroform solution of the amidine for 2 min. The aqueous solution was further washed with two l-ml portions of chloroform. For colour development with MB, the gold(II1) extract obtained as above was placed in a 100~ml separating funnel containing 5 ml of MB solution. The pH of the aqueous solution was adjusted with dilute ammonia so that its pH after extraction was 6 f 2. Both phases was shaken for 2 min and the extract was dried over anhydrous sodium sulphate (2 g) in a 25-ml beaker. The aqueous phase was further washed with two l-ml volumes of chloroform. All the extract after drying was transferred into a lo-ml volumetric flask and diluted to the mark with chloroform. The absorbance of the extract was measured at X,,, (650 nm) against the reagent blank.

Extraction of gold(III) with amidine in the presence of MB An aliquot of solution containing 3-15 pg of gold(II1) was transferred into a loo-ml separating funnel followed by 5 ml MB and the pH of the solution was adjusted to 6 f 2 in a lo-ml final dilution. The aqueous solution was shaken with 5 ml of a chloroform solution of the amidine for 2 min and washed with 2 ml of chloroform. All the extract after drying was transferred into a lo-ml volumetric flask and diluted to the mark with chloroform. The absorbance of the extract was measured at X,,, (650 nm) against a reagent blank.

EXTRACTION

OF GOLD

FROM

LOW-GRADE

ORES

WITH

Analysis of ore samples Low-grade gold ores obtained from Sonakhan Mines (Raipur, India) were tested for the recovery of gold. A known amount of the ore (5-8 g) was leached with hot dilute nitric acid to remove the matrix ions. The insoluble residue was digested twice with aqua regia and the excess of nitric acid was removed by heating with concentrated HCl. The dried mass was dissolved in 10 ml of HCl (1 M) and the filtrate was transferred into a loo-ml separating funnel followed by addition of an aliquot of standard Au(II1) solution. The final volume of the aqueous phase up to 20 ml was not critical for the recovery of Au. Gold was extracted with 5 ml of a chloroform solution of DPBA and the extract was reacted with MB as in the procedure. The absorbance of the extract was measured at the h,,, of the complex and the metal content was determined from a calibration graph prepared separately for each sample. The metal content of the ores was also confirmed by atomic absorption spectrometry. The metal was extracted with amidine in chloroform as above and stripped back into aqueous phase by shaking the extract with 5 ml of thiourea solution (1% w/v). The metal content was determined by atomic absorption spectrometry using the spike method, with a separate calibration graph for each sample.

RESULTS

AND

493

AMIDINES

DISCUSSION

Extraction of gold(III) with amidine Gold(II1) as AuCl, is quantitatively extractable (2 99.5% with 0.2% amidine solution in chloroform over the pH range 3.0-11 M HCl as an ion pair. Above pH 3, the extraction of the metal decreases and there is no extraction above pH 6.0. Quantitative extraction of the metal is also achieved in solvents such as isobutyl methyl ketone (IBMK), ethyl acetate, benzene and toluene. The percentage extraction of the metal with amidine in various organic solvents was evaluated by extracting 1.2 pg Au(II1) ml-’ with a 0.2% solution of the amidine in the respective solvent, keeping the organic to aqueous phase ratio at 1: 1 at the desired acidity, back-extraction with acid and measuring the gold by atomic absorption

spectrometry. The distribution ratios of DPBA between the organic solvent and aqueous solution (1 M HCl) were determined spectrophotometritally and were found to be 2.8, 1.7 and 1.4 for chloroform, benzene and toluene, respectively, at room temperature (22 rf: 2 o C). The selectivity of the reagent towards the extraction of gold into IBMK and ethyl acetate was tested, and in these solvent ions such as Fe(III), Cr(III), Se(W), Te(IV), Re(VI1) and Pt(IV) were also extracted, whereas Zn(II), Cd(II), Co(II), Ni(II), Pd(I1) and Mn(I1) were partially extracted. Therefore, chloroform was chosen as the solvent for further study. A 0.004 M DPBA solution is sufficient for complete extraction of the metal and the addition of more reagent has no adverse effect. Variation of the volume of the aqueous phase from 3 to 50 ml, variation of the temperature of the reacting solutions from 15’ to 40 o C, the colour stability of the extract with standing times up to 1 h and the order of addition of the reagents were not critical. The chloroaurate ion pair with DPBA in chloroform has a very low molar absorptivity (650 1 mol-’ cm-’ at A,,, = 390 nm). The stoichiometric ratio of AuCl, to amidine (AM) was determined by plotting log [A/( A,,, - A)] versus log (molar concentration of amidine taken in chloroform (A = absorbance). The slope obtained was found to be close to 2.0, which seems to incidate an equilibrium of the type AuCl,

+ H++

2AM + AuCl,H(AM);

The oxidation state (III) of gold in the extracted species is supported by the similar type of extraction of Au(II1) with high molecular weight amines [1,2]. Extraction of gold(III) with methylene blue Ganchev and Atanasova [ll] reported the extraction of gold(II1) with MB into chloroform. The maximum colour of the organic phase was seen above ca. 0.5 M HCl. Extraction of gold(III) with amidine in the presence of MB The extraction of gold(II1) into chloroform with MB and amidine (DPBA) together was tested. The

494

C. AGRAWAL

ET AL,

optimum acidity range was evaluated by extracting the metal from aqueous solution containing MB (0.00016 M) and was found to be pH 3.0-9.0. Below pH 3, maximum extraction of the metal is achieved but the blank absorbance is high and increases as the pH decreases. In this procedure the molar absorptivity is 1.20 x lo5 1 mol-’ cm-’ 650 nm, which may result from maxiat A,,= mum AuCl, MB+ ion-pair formation. The system follows Beer’s law in the range 0.3-1.5 pg Au ml-’ in the organic solution. Extraction of gold(III) with amidine with subsequent addition of MB The chloroform extract of Au(II1) obtained is mixed with an aqueous solution of MB as in the procedure. The amidine in the ion pair is replaced with MB in the chloroform layer, where a dense blue colour develops. The ratio of MB to Au is determined by again plotting log [A/(A,,, - A)] of the metal versus log (molar concentration of MB taken) and the results obtained show a 1: 1 species (Fig. 1).

/

Fig. 1. Determination of ratio of Au(II1) to the reagent in the extracted complex. (A) Log [ A/(A,,, - A)] versus lo&molar concentration of methylene blue taken). C,, =1.0X 10V5 M; M; C,,,A0=3.6x10-3 M; V,,,,=v,q,=6 ml. (B) c nc,=l of DPBA Log [A/(&l,, - A)] versus log (molar concentration taken). C,, =1.2X10-’ M; CHct =l M; C,, =1.6~10-~ M; v&s, = vaq. = 5 ml.

WAVELENGTH,

nm

Fig. 2. Absorption spectra of the complex and the reagent blank in chloroform. (A) Reagent blank; (B) complex; C,, = 6.0 x 10K6 M in the organic solution.

The extraction of Au(II1) with amidine in various solvents, followed by interaction of the extract with MB in the various solvents, and by extraction with amidine and MB together, were determined as described above. It was found to be 2 99.5% and >, 99.0%, respectively, for a single extraction with 1-pentanol, chloroform or benzene. The colour of the complex is stable in these solvents for at least 1 h, but there are spectral differences between solvents: 1-pentanol (e = 5.0 X lo4 1 mol-’ cm-‘; X,,, = 650 nm), chloroform (E = 6.5 X lo4 1 mol-’ cm-‘; h,,, = 650 nm) and benzene (E = 11.0 X lo4 1 mol-’ cm-‘; X,,, = 660 nm). Chloroform was adopted in subsequent studies. The absorption spectra of the complex and the reagent blank in chloroform are shown in Fig. 2. However, the absorbance of the reagent blank (0.04) in the sequential method (Fig. 2) at A,,, = 650 nm is significant and was subtracted from all measurements. MB of 0.00013-0.00028 M is adequate for maximum colour development of the complex in chloroform. An extraction time of 2 min was sufficient for maximum colour development. Variation in the volume ratio of the organic phase to the aqueous phase from 5 : 2 to 1: 3 and variation in temperature from 15 to 40” C were not critical. The optimum acidity range was evaluated measuring the pH of the aqueous solution after extraction and was found to be between pH 3.0 and 9.0.

EXTRACTION

OF GOLD

FROM

LOW-GRADE

ORES

WITH

Effect of amidine The effect of substituents in the amido N-phenyl ring of the amidine on the development of the final absorbance after extraction with amidine and subsequent MB addition was studied. The percentage extraction [determined by extracting 1.2 pg Au(II1) ml-’ with the various amidines, shaking the amidine extract with MB, back-extracting the MB extract with acid and measuring the gold by atomic absorption spectrometry] of the metal was found to be > 99.0% for a single extraction. The ease of the MB replacement is affected considerably by the nature of the amidine used. The introduction of groups causing a positive inductive effect, e.g., CH,, and breaking of the conjugation in the amidine molecule cause stronger electrostatic bond formation between the anion and cation, which in turn results in the incomplete displacement of AM by MB in the extract (or a decrease in the absorbance of the Au-MB extract). The role of amidine is assumed to be to enhance the extraction of the Au-MB ion pair at pH 3.0-9.0. With the ten different amidines tested the molar absorptivity of the complex in chloroform lies in the range 1.1 X 104-6.5 x lo4 1 mol-’ cm-’ at Lax 650 nm (Table 1). Of the ten, N, N’-diphenylbenzamidine (DPBA) gave the most sensitive colour reaction and was selected for detailed study.

TABLE 1 Spectral characteristics of the complexes C,H,-NH(R)C(C,H,)=NC,H, in chloroform with ten amidines and methylene blue R

6 (lo4 I mol-‘cm-‘) at X,,=650nm

GH,

6.50 5.42 5.50 5.61 6.30 6.42 6.45 5.70 11.00 30.40

2-CH,C,H, 3-CH,C,H, 4-CH&H, 2-C&H, 3-C&H, 4-ClC,H, 2,5-(CH,),C,H, C,H,CH, CH,(CH,),

495

AMIDINES

TABLE 2 Effect of diverse ions on the determination ml-’ with DPBA and MB Metal ion added

Maximum tolerable amount

Metal ion added

1.0 1.0

Se( IV)

1.2 1.0 3.0 3.0 3.0 3.0

Pt(Iv Sb(II1) Bi(II1) Ti(IV) Zr(IV) Ru(III), Rh(II1) Ir(Iv) Os(VII1) La(II1)

5.0 5.0 5.0 6.0

Maximum tolerable amount (mn)

(mn)

AdI) WW

of 1.2 g Au(II1)

Pb(I1) Ni(II),Co(II) Cr(V1) Fe(II1) Al(II1) Mn(I1) PO2 AsO,3 c*o4’ BO;EDTA Citrate, tartrate

6.0 10.0 10.0 15.0 10.0 10.0 25.0 25.0 30.0 30.0 30.0 35.0

Beer’s law, sensitivity and precision A calibration graph was prepared by extracting Au(II1) with DPBA and MB in sequence as described above. The system follows Beer’s law over the concentration range 0.5-3.0 pg Au ml-‘. The molar absorptivity of the complex formed with DPBA and MB in chloroform is 6.5 X lo4 1 mol-’ cm-’ at X max (650 nm). The limit of detection is 5 pg Au 1-i (twice the standard deviation of ten replicate measurements of the blank). The relative standard deviation for ten replicate measurements of 1.2 pg Au ml-’ in chloroform was 1.4%. Effect of diverse ions The effect of other ions on the determination of 1.2 pg Au ml-’ was examined by extracting Au(II1) with DPBA and MB in sequence; II, S2-, SOiand S,O,2- interfere. No colour is seen if thiourea is present as it masks the metal. The tolerated amounts of the diverse ions in the determination of Au(II1) causing error of less than f2% are given in Table 2. Application of the method The results obtained by application sequential method and atomic absorption

of

the spec-

C. AGRAWAL

TABLE

3

REFERENCES

Application of the method to the determination of gold in Sonakhan ores and comparison with results obtained by atomic absorption spectrometry (AAS) Ore

Gold obtained by this method a

Gold obtained by AAS (pg g-‘)

(pg g-‘) Sample I Sample II a Average

ET AL.

1.5 2.2

1.4 2.1

R.s.d. of this method

R.s.d. of AAS method

(48)

(9)

1.5 1.6

2.1 1.9

of five determinations.

trometry to ore samples are given in Table results agree closely with each other.

3. The

The authors are grateful to CSIR, New Delhi, for financial support of this work through research grant 1(1067)//87-EMR II.

1 Y. Marcus and A.S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, Wiley-Interscience, London, 1969. 2 V.S. Schmidt, Amine Extraction, Keter IPST, Jerusalem, 1971. 3 H.R. Das and S.N. Bhattacharya, Talanta, 23 (1976) 535. 4 H. Sanke Gowda and K.N. Thimmaiah, Indian J. Chem., Sect. A, 14 (1976) 632. 5 T. Sukhara, Talanta, 24 (1977) 633. 6 K.M.M.S. Prakash, L.D. Prabhakar and D. Venkata Reddy, Analyst, 111 (1986) 1301. 7 Z. Marczenko, Spectrophotometric Determination of Elements, Wiley, New York, 1976. 8 B.I. Nabirauets and E.M. Zadorozhnaya, Deposited Document, 1980, SPSTL 1041; KLP-D80, 9 pp (Russ.) Chem. Abstr., 97 (1982) 10350 x. 9 Z. Li, Fenxi Shiayanshi, 7 (1988) 28; Anal. Abstr., 51 (1989) 2. 10 F.E. Beamish and J.C. Van Loon, Analysis of Noble Metals, Academic, New York, 1977, Chap. 3. 11 N. Ganchev and B. Atanasova, Zh. Anal Khim., 22 (1967) 274. 12 Analytical Methods for Flame Spectroscopy, Varian Techtron, Australia, 1978, p. 4. 13 R.L. Shriner and F.W. Newman, Chem. Rev., 35 (1944) 351.