shifts of the ligand protons-e.g., the 6-proton chemical shifts. A set of curves of similar shape will be obtained . a given metal ion is plotted against the 7 when p K p . ~ for value that corresponds to the proton resonance of the 5substituted ligand (Table I). Further confirmation that the p K p. D . of the nickel(I1) chelate of 5-thiocyano-8-quinolinol is too low comes from an examination of the p K p . ~ values . of the nickel(I1) chelates of 2-methyl-8-quinolinols and the corresponding P K P . D . values of the 8-quinolinols. The steric effect of the 2-methyl group lowers the stability of the 2methyl-8-quinolinol chelates relative to the corresponding 8-quinolinol chelates ; hence the p K p . D . values of the 8-quinolinol chelates should be less than those of the 2-methyl-8quinolinol chelates, and p K p . ~ of , bis(5-thiocyano-8-quinolinolato)nickel(II) should be ((2.52 (Table 11). In this study of a series of chelating agents the effect of
halogen substitution on the following parameters was measured. Chemical shifts of the ring protons; the successive acid dissociation constants and metal chelate formation constants from which proton displacement constants were calculated. The relationships between any two of these parameters are nonlinear. This nonlinearity is especially marked when the parameters for the unsubstituted ligands as well as those for the pseudo halogen substituted compounds are examined. This conclusion agrees with previous work (11) that has shown that, in general, linear free energy relationships should not be expected, and that the relationships between the measured parameters can be quite complex. RECEIVED for review January 8, 1969. Accepted April 28, 1969. This work was supported by the U. S. Atomic Energy Commission.
Solvent Extraction in the Presence of Emulsion-Forming Residues Application to the Atomic Absorption Determinationof Gold in Low Grade Ores Stephen L. Law and Thomas E. Green U.S . Department of the Interior, College Park Metallurgy Research Center, College Park, Md. Extraction in the presence of insoluble residues caused a serious emulsion problem in the development of an aqua regia, methyl isobutyl ketone extraction, atomic absorption method for determining microgram quantities of gold in ore samples weighing up to 500 grams. A study of factors affecting the change in volume of ketone during the extraction showed that, under proper conditions, atomic absorption analysis of the small quantity of ketone which separated as a clean phase provided quantitative results. The use of solvent extraction in the presence of insoluble residues which cause the formation of large emulsions has not been previously reported. This technique should have general application in many other extraction methods of analysis.
SOLVENT extraction techniques provide an excellent means for separating small quantities of metals from interfering elements and for concentrating the desired metals in a small volume of extractant. Such techniques are especially useful in atomic absorption spectrophotometry if the extractant is suitable for aspiration into the flame. However, solvent extraction techniques are generally used under conditions which provide a clean separation between the aqueous phase and the organic extractant. A serious emulsion problem was encountered in our attempt to develop a simple aqua regia dissolution-methyl isobutyl ketone (MIBK) extraction-atomic absorption method for determining gold in large samples of low grade ores. Use of samples weighing several hundred grams was necessary to minimize the sampling errors caused by the occurrence of gold in discrete particles. Fire assay, which Beamish and Chow (1) considered to be the best method for determining gold in ores, is not generally applied to samples larger than 60 grams. Alternate methods employ such techniques as cyanide dissolution of the gold, treatment of silicate residues with hydrofluoric acid or filtration to remove insoluble -~
(1) A. Chow and F. E. Beamish, Talanta,14,219 (1967).
1008
0
ANALYTICAL CHEMISTRY
residues. These approaches are impractical for the routine rapid analysis of 500-gram samples. Two other methods have been reported for determining gold in large samples: an atomic absorption method using 100-gram samples by Van Sickle and Lakin ( 2 ) and a colorimetric method using 500-gram samples by Shima (3). Both methods employ mixtures of bromine and ethyl ether to dissolve the gold. This dissolution process is reported to be quite slow for large particles of gold (2). Shima reported results lower than those obtained by fire assay and Van Sickle and Lakin reported results with standard deviations of 16 to 39W. Neither method was considered to meet the Bureau of Mines requirements. EMULSION PROBLEM
When extraction of the aqua-regia-dissolved gold was attempted in the presence of the insoluble matter from 500gram samples, nearly all of the MIBK was held in an emulsion as shown in Figure 1. The insoluble residues were found to be the cause of the emulsion. No emulsions occurred when the residues were removed by filtration before the extraction. Emulsions did form when the well-washed residues were suspended in dilute hydrochloric acid and shaken with MIBK. Filtration of the sample solutions prior to extraction would therefore have eliminated the emulsion. However, filtering and thorough washing of the large residues were time consuming and negated the desired simplicity of the method. Attempts to break the emulsions or prevent their formation by chemical additives were unsuccessful. The same emulsion problem was encountered with other extractants reported to be suitable for aspiration into the atomic absorption flame. The authors therefore investigated the use of the small portion of MIBK which separated as the clean upper phase. (2) G. H. Van Sickle and H. W. Lakin, Geological Survey Circular 561 (1968). (3) M. Shima, Japan Analyst, 2,96 (1953); CA 47,7938~ (1953).
Atomic absorption results are obtained in units of weight per unit volume, generally micrograms per milliliter. In order to convert these results to total micrograms per original ore sample, it is necessary to know the volume of the phase which contains the gold. In solvent extraction procedures, the extracting phase is frequently transferred to a volumetric flask and diluted to volume. In the present situation this was not possible. The emulsion also prevented measurement of the final volume of MIBK. Groenewald (4) and Strelow et al. (5) suggested preparation of standards by the same extraction procedure used in the analysis to compensate for change in volume of the extractant. This approach was not applicable to our analysis of ore samples for the following reasons. There is an undetermined quantity of acid remaining in the residue after the aqua regia attack, and unknown quantities of other metals are dissolved; therefore, a representative solution can not be prepared for the extraction of standards. For the same reason, volume changes can not be calculated from known soluhility data. In the present method, the gold content of the ore sample is calculated on the basis of a known quantity of MIBK added before the extraction. The validity of the method depends on the following conditions: The composition of the MIBK in the small clear phase is the same as the composition of the MIBK droplets in the emulsion. The condition of the aqueous phase can he established so that the volume of MIBK does not change significantly during the extraction. Because the emulsion is of a mechanical nature, the first condition was expected to be valid and analysis of samples of known composition confirmed this assumption. The change in volume of MIBK during the extraction is subject to variation and was therefore investigated. STUDY OF MIBK VOLUME CHANGE
The change in volume of MIBK during equilibration with acid solutions is affected by two processes: dissolution of MIBK in the aqueous phase and extraction of acid or water into the MIBK. The change in volume of the MIBK caused by each of these processes is dependent to different degrees on the kind of acid and its concentration in the aqueous phase as well as on the ratio of the initial volumes of the two phases. Tests were therefore performed to measure the effect of these variables on the change in volume of MIBK. Reagent grade hydrochloric and nitric acid and MIBK were used. All measurements were made at ambient temperature using standard volumetric glassware. For tests involving large volumes of aqueous phase, a special flask was prepared by sealing the calibrated portion of a 50-ml buret to a 250-ml volumetric flask. In each test, measured volumes of MIBK were added to measured volumes of water or dilute acid. The two phases were shaken by hand for 2 minutes and allowed to separate and the volume of the MIBK phase was measured. Figure 2 shows the results obtained using aqueous phases consisting of distilled water, 3M HCI, and 3M HNOI when the ratio of aqueous volume to MIBK volume was varied from 0.2 to 20. The tests in which distilled water was used as the aqueous phase show that increase in water-to-MIBK ratio produces the
(4) T. Groenewald, ANAL.CHEM., 40,863 (1968). ( 5 ) F. W. E. Strelow, E. C. Feat, P. M. Mathews, C. J. C. Bothma, and C. R. van Zyl, ibid., 38, 115 (1966).
Figure 1. Typical emulsion A . ClearMfBK B. Emulsion C . Aqueous phase D. Insoluble residue
expected linear decrease in MIBK volume caused by the solubility of MIBK in water. The tests in which 3M HNOI was used as the aqueous phase show, at low aqueous-to-MIBK ratio, an increase in MIBK volume caused by the extraction of H N 0 3 into the MIBK phase. The volume change at the aqueous-to-MIBK ratio of 1.0 is in agreement with the value obtained by Powell and Newton (6). At higher aqueous-to-MIBK ratios the presence of nitric acid causes a large decrease in the volume of MIBK because HNOI increases the solubility of MIBK in the aqueous phase. Tests in which the 3M HCI was used as the aqueous phase show that HCI has less effect than HNOl on the solubility of MIBK. The absence of MIBK volume increase at low aqueous-to-MIBK ratio indicates little extraction of HCI as previously reported (7). The concentrating factor of the extraction depends on the ratio of the volume of the aqueous and MIBK phases. When the ratio between the volumes of the aqueous and MIBK phases is greater than 5, the change in volume caused by an increase in hydrochloric acid concentration from 0 to 3M will he limited to the shaded area in Figure 2. The portion of Figure 2 enclosed in dotted lines is enlarged in the insert to show this effect in more detail. Therefore, if the dilute hydrochloric acid aqueous phase was previously saturated with MIBK, any subsequent change in hydrochloric acid concentration would result in only a minor change in the volume of the MIBK phase. In the proposed procedure, the gold was to be dissolved by adding aqua regia and heating to remove most of the excess acid. The next series of tests was performed to determine (6) J. E. Powell and A. S. Newton, U. S. Atomic Energy Commission Report TID 5212, Paper No. 19 (1955). (7) E. D. Crittenden and A. N. Hixson, Ind. Eng. Chem., 46, 265 (1954). VOL. 41, NO. 8,JULY 1969
1009
TOI,
YI
n g ore Sowp
8
I
.io IO
20
30
L O L J M E R A T I O BEFORE ECUlLlBRATION,
7 0 "$."MI
5
ill 0 B E i O S i F O b , L , B P L I ON
what effect the acid remaining after such treatment has on the solubility of MIBK in dilute HCI. In these tests the aqueous phase was 5 % HCl which had previously been saturated with MIBK. Aqua regia was replaced by a mixture of four parts of reagent grade hydrochloric acid and one part of reagent grade nitric acid to provide an excess of hydrochloric over that required in the reaction
+ 3 HC1+
NOCl
+ Clz + 2H20
A quantity of this four-to-one mixture was boiled in an open beaker for 1 hour. Measured volumes of this boiled acid were mixed with measured volumes of MIBK-saturated dilute hydrochloric acid. The resulting solutions were then shaken with measured volumes of MIBK and the final volume of the MIBK was measured. The results are shown in Figure 3. In this figure, ratios of residual acid t o aqueous phase of 0.1, 0.05, and 0.02 represent evaporation t o volumes of 50, 25, and 10% of the aqua regia used in the proposed analytical procedure. When the ratio between the volumes of the aque-
Table I. Analysis of Bureau of Mines Reference Ore Proposed Fire assay method Number of samples 12 15 30 20-1 30 Sample weight, grams Average result, oz/ton 0.206 0.206 Standard deviation, oz/ton 0.003 0.005 Table 11. Analysis of U. S. Geological Survey Gold Quartz Standard Individual Average Methods results, ppm PPm Proposed method 2.70,2.67,2.60,2.65 2.66 Fire assay-atomic absorption 2.64, 2.65, 2.65, 2.64 2.65 (Denver) Fire assay-atomic absorption 2.61, 2 . 4 7 2.54 (Washington) Cyanide leach-atomic absorption 2.60, 2.63, 2.68, 2.65 2.64 Neutron activation-fire assay ... 2.45 _
1010
_
50
Figure 3. Effect of residual acid on the change in volume of MIBK with change in aqueous-to-MIBK volume ratio
Figure 2. Effect of distilled water, HCl, and "03 on the change in volume of MIBK with change in aqueousto-MIBK volume ratio
HN03
40
MIBU
~
ANALYTICAL CHEMISTRY
ous phase and MIBK was low, an increase in the volume of MIBK was observed. This is attributed to the transfer of water to the MIBK which had not been previously equilibrated with aqueous phase. The vertical lines in Figure 3 indicate the ratios between the volumes of aqueous and MIBK phases used in the proposed procedure, allowing 200 ml for the volume of the insoluble residue. In actual tests involving ore samples the quantity of acid remaining could not be conveniently determined by titration because of the formation of soluble salts of common metals, such as iron, during the aqua regia attack. Nor could the residual acid be determined by weighing because of the unknown extent of conversion of oxides and carbonates to chlorides. The residual acid will be only that quantity of dilute HC1 required to dampen the insoluble residue. Volume changes under actual conditions of the analytical procedure were determined as follows. A 500-gram ore sample was treated according to the proposed procedure through the point where the insoluble residue is taken up in equilibrated dilute HCl. Portions of clear solution were then decanted from the residue and shaken with measured volumes of MIBK. The change in MIBK volume is shown in Figure 3. In the recommended working range, the volume change is less than 2 %. This small volume decrease produces a small positive bias in the analytical results. This small positive bias tends to counterbalance the small negative bias inherent in any solvent extraction separation. PROCEDURE FOR THE ANALYSIS OF GOLD ORES
The above study indicated that the small quantity of MIBK available as a clear phase could be used in the analysis of large ore samples. Therefore, the following procedure was developed and evaluated. EXPERIMENTAL
Apparatus. A Perkin-Elmer Model 303 atomic absorption spectrophotometer equipped with a standard 10-cm premix burner head was used. Instrument settings were those recommended by the manufacturer except that flow rates of air and acetylene were reduced to produce a flame with a bright blue inner cone approximately 1 mm high when no sample was aspirated. Acetylene and air flow meter readings were 2.5 and 4.0, respectively, on our Perkin-Elmer burner regulator unit (approximately 1 and 10 liters per minute).
Laboratory Rand Minese East Geduldo Anglo Americana College Park* a
Numbered detns
Table 111. Analysis of Three South African Ores Av results and std dev, oz/ton Sample B Sample C
39 26 7 3
0.0216 =t0.00115 0.0195 f 0.00075 0.0192 f 0.00105 0.0230 f 0.0008
0.0056 f 0.0008 0.0049 f 0.0005 0.0052 0.00025 0.0048 =t0.00035
Sample H 0.0123 f 0.0010 0.0114 f 0.0006 0.0118 f 0.0008 0.0117 f 0.0006
Fire assay on five assay ton samples.
* Proposed method using two assay ton samples for B, five assay ton samples for C, and three assay ton samples for H.
Table IV. Sample No. Analyst No. 1 Analyst No. 2
28 0.010 0.009
Analysis of Low Grade Placer and Conglomerate Ores, 400- to 600-Gram Samples Individual results, oz/ton 26-s 0.006 0.007
104s 0.003 0.003
Reagents. Reagent grade hydrochloric acid, nitric acid, and methyl isobutyl ketone were used as purchased. Equilibrated dilute HC1 was prepared by adding 1 volume of concentrated HCl to 19 volumes of distilled water and shaking with an excess of MIBK. Equilibrated MIBK, used for preparing standards, was prepared by shaking MIBK with an equal volume of 10% vjv HCl. Stock solutions of standards were prepared by aqua regia dissolution of weighed amounts of 99.9% pure gold foil and extraction into MIBK. Further dilutions of this MIBK-gold stock solution were made with the MIBK pre-equilibrated with 10% HCl to minimize actinic reduction. Standards made in this manner are stable for up to one month if stored away from strong light. Procedure. Weigh approximately 500 grams of - 100 mesh ore sample into an 800-ml glass beaker. Place the beaker in a cold muffle furnace, raise the temperature to 550 "C and maintain that temperature for 1 hour. Allow the beaker to cool partially in the furnace before removing to avoid thermal shock. Add 160 ml of concentrated HC1 and 40 ml of concentrated H N 0 3 . Cover the beaker with a ribbed watch glass and evaporate the contents, with occasional stirring, to a damp condition with no visible excess liquid. Add equilibrated dilute HCl, stir to break-up any lumps formed during the evaporation, and transfer both solution and insoluble residue to a 1000-ml screw top Erlenmeyer flask using equilibrated dilute HC1 to complete the transfer. Add equilibrated dilute HCl until the total volume in the flask is approximately 900 ml. Pipet 50 or 100 ml of MIBK into the flask and shake for 2 minutes. An emulsion will form at this stage. Add sufficient equilibrated dilute HC1 to raise the upper level of the contents into the neck of the flask and mix thoroughly. Gently shake and swirl the contents of the flask until a clear portion of MIBK large enough for obtaining an atomic absorption reading (5 to 10 ml) separates above the emulsion. On rare occasions it may be necessary to centrifuge a portion of the emulsion to obtain the required quantity of clear MIBK. Take atomic absorption readings on the gold standards and MIBK extracts from the samples. Convert to micrograms of gold per milliliter and calculate the gold content of the sample on the basis of the volume of MIBK added. Discussion of Procedure. Samples are usually ground to - 100 mesh to expose the gold particles to the action of aqua regia. Placer type samples, in which the gold typically occurs as free particles, require less grinding. Satisfactory results were obtained on a series of such samples ground to -20 mesh. No evidence of failure to dissolve completely the gold from samples has been observed. The ore is roasted to destroy organic matter which might
104 0.005 0.004
1094 0.003 0.002
108 0.005 0.003
34s 0.009 0.010
145-S 0.004 0.002
171-S 0.001 0.001
cause a loss of gold by adsorption (8). If carbonaceous matter is known to be absent, roasting is not required. The temperature of 550 "C was adequate for destroying organic matter and was low enough to permit the roasting to be performed in glass beakers. Higher temperatures may cause sintering of clays in the sample resulting in the entrapment of gold (8). Samples of high carbonate content should be treated with sufficient concentrated HC1 to decompose the carbonates before the regular quantities of HCI and HNOs are added. The method provides a sensitivity of 1% absorption for ores containing 0.0003 ounce of gold per ton for 500-gram samples when 50 ml of MIBK is used in the extraction. RESULTS
In order to evaluate the accuracy of the method, it was necessary to use samples which minimize the problem of sampling error. An ore sample obtained from Carlin, Nev., and designated as Bureau of Mines Reference Ore was used for this purpose. Essentially, all the gold particles in this ore are under 1 micron in size. Table I gives the results obtained on this material by fire assay and by the proposed method. A comparison with other methods was performed using the U. S. Geological Survey gold quartz standard (9). Results are shown in Table 11. Limited quantitiesof three gold oresused in a precision study conducted by the Transvaal and Orange Free State Chamber of Mines (IO) were obtained. Insufficient material was available for replicate 500-gram samples. The proposed solvent extraction-atomic absorption method was therefore used on two, three, and five assay ton (29.17 grams) samples. Results obtained are shown in Table 111 along with results obtained at three South African laboratories. Table IV shows the results from the analysis of 400- to 600-gram samples containing microgram quantities of gold. The duplicate results were obtained by two different analysts using the proposed method. The samples analyzed were -20 mesh placer and conglomerate samples selected for this study (8) I. Nagy, P. Mrkusic, and H. W. McCulloch, National Institute for Metallurgy Research Report No. 38, Johannesburg, South Africa (1966). (9) H. L. Millard, Jr., J. Marienko, J. E. McLane, Geological Survey Circular 598 (1969). (10) C. H. Coxon, Corner House Laboratories, Rand Mines Ltd., Report Ref: R. 585/C217/67 (1967). VOL. 41, NO. 8,JULY 1969
1011
because they were of the type for which fire assay had previously produced widely divergent results.
This previously unreported technique should find general application in other analytical methods combining solvent extraction with an instrumental finish,
CONCLUSIONS The described method provides a means for accurately determining small quantities of gold in large samples of low grade ores. ~t can be performed in laboratories not equipped with specialized fire assay facilities. Of wider interest is the introduction of the technique of using solvent extraction separations in the presence of insoluble residues which cause the formation of large emulsions.
ACKNOWLEDGMENT The authors thank C. H. Coxon of the Corner House Laboratories, Johannesburg, South Africa ; the u. s. Geological Survey; and the Bureau of Mines Reno Metallurgy Research Center for reference RECEIVED for review November 29, 1968. Accepted April 8, 1969.
Quantitative Determination of Gold in Solution by Solvent Extraction and Atomic Absorption Spectrometry The0 Groenewald Chamber of Mines of South Africa, Research Organisation, Johannesburg, South Africa The solvent extraction of gold complex ions into diisobutyl ketone containing either trioctyl methyl ammonium chloride or trioctylamine was investigated. In the rapid determination of gold(ll1) in aqueous solution the step of extracting gold(ll1) from an aqueous chloride or cyanide medium into diisobutyl ketone containing the quaternary ammonium salt is recommended. The concentration of the gold in the organic phase may be determined subsequently by atomic absorption spectrometry. The extraction of the gold(ll1) from an aqueous chloride medium is quantitative at pH values of up to 4,and quantitative extraction of gold(1ll) from an aqueous cyanide medium i s possible at pH values up to at least 10, even when volume ratios of aqueous to organic phases as large as 1OO:l are employed. Gold concentrations of beto 5 x 1 0 - 8 ~(50 to 0.01 mg/liter) can tween 2.5 x be determined. This technique has been applied to the analysis of gold-bearing ores and has been found to be free of interference by other elements. Trioctylamine could also be used to extract quantitatively gold(l) cyanide and gold(ll1) complex ions into diisobutyl ketone, but only under more restricted experimental conditions than those required for the extraction by trioctyl methyl ammonium chloride.
THEgold content of a n aqueous solution has often been determined by the solvent extraction of gold as a gold(II1) complex ion, followed by analysis by atomic absorption spectrometry (1-6). In all of these instances the gold(II1) was extracted as either a chloro- or bromo- complex from an aqueous solution containing a high concentration of the hydrogen halide directly into the pure organic solvent. The rather rigid control of experimental parameters, especially that of pH, has been avoided by the presence of extractants in the (1) M. C. Greaves, Nature, 199, 552 (1963). (2) P. B. Zeeman, W. J. Naude, and 0. A. van der Westhuyzen, Tydskrif uir Natuurwetenskappe, 4, 193 (1964). (3) F. W. E. Strelow, E. C. Feast, P. M. Mathews, C. J. C. Bothma, and C. R. van Zyl, ANAL.CHEM., 38, 115 (1966). (4) L. R. P. Bulter, J. A. Brink, and S. A. Englebrecht, Inst. Mining Mer., Trans. Sect. C , 76, C188 (1967). ( 5 ) J. R. Beevers, Econ. Geol., 62,426 (1967). (6) E. N. Pollock and S . I. Andersen, Anal. Chim. Acta, 41, 441 (1968). 1012
ANALYTICAL CHEMISTRY
organic phase to ensure quantitative extraction of the gold complex ions from the aqueous phase. Thus the extraction of gold(II1) into an amyl acetate phase was facilitated by the presence of p-dimethylamino benzal rhodanine (5). This organic medium had to be kept under refrigeration when not in use. In a procedure for the determination of gold(1) in cyanide solutions, the gold(1) was extracted quantitatively over a very wide pH range using diisobutyl ketone containing a trioctyl methyl ammonium salt as extractant (7). Some aspects of the extraction of gold(1) cyanide into organic phases containing tertiary amines (8-10) and quaternary ammonium salts (10, 11) have been reported, but corresponding work on gold(II1) complexes has not been very comprehensive (12). Atomic absorption spectrometry is a very convenient technique for the determination of gold in solution. For this purpose the usefulness of tertiary and quaternary amines for the quantitative extraction of gold complex ions into diisobutyl ketone was investigated. EXPERIMENTAL Apparatus. The instrument has been described in a previous paper (7). Reagents. The extractants were obtained from General Mills, Chemical Division. The tertiary amine used, trioctylamine, marketed as Alamine 336, was specified to have an average molecular weight of 392, and the corresponding quaternary ammonium salt used was the trioctyl methyl ammonium salt, marketed as Aliquat 336, and supplied in
(7) T. Groenewald, ANAL.CHEM., 40, 863 (1968). (8) I. N. Plaksin and G. N. Shivrin, Dokl. Akad. Nauk SSSR,150, 86 (1963). (9) 0. E. Zvyagintsev, 0. I. Zakharov-Nartsissov, and A. V. Ochkin, Russ. J. Inorg. Chem., 6, 1012 (1961). (10) D. S . Flett and A. Faure, National Institute for Metallurgy, 1
Yale Rd., Milner Park, Johannesburg, Private Communication, 1964. (1 1) G. N. Shivrin, A. S. Basov, B. N. Laskorin, and E. M. Shivrina, Tsuer. Metal., 39, 15 (1966). (12) R. C. Mallett, J. D. Taylor, and T. W. Steele, National Institute for Metallurgy, 1 Yale Rd., Milner Park, Johannesburg, Res. Rept. No. 24, Project No. C92/65 (1966).
