Determination of gold in geologic materials by solvent extraction and atomic-absorption spectrometry

GEOLOGICAL SURVEY CIRCULAR United States Department of the Interior

William T. Pecora, Director

Abstract -------------------------------- Introduction ----------------------------- Reagents-------------------------------- Apparatus------------------------------- Cyanide atomic-absorption procedure ------ Sample preparation--------------------- Standardization ---------- --------------

2. Constant-temperature water bath with magnetic stirrer--------------------------------

conventional gravimetric fire-assay method ----------------------------------------

2. Comparison of gold values obtained by fire-assay atomic-absorption and cyanide atomic-absorption methods------------------------------------------------

TABLES

Fire-assay atomic-absorption procedure --- Results --------------------------------- Sample dissolution, cyanide procedure------ Extraction of gold with methyl isobutyl ketone-------------------------------- Conclusions - - - - - - - - - - - - - - - - - - - - - - -- - - - -- References------------------------------

INTRODUCTION

This paper describes two rapid andprecis,~atomic absorption methods for gold determination currently used in the U.S. Geological Survey's Denver laboratory: the cyanide atomic-absorption method and th~ fire-assay atomic-absorption method. In developing these methods, sensitivity was stressed to enable g?.ochemical prospecting for gold at theparts-per-billionlevel. Both methods can detect concentrations as low as 50 ppb (parts per billion) or 0.0015 ounce per ton of gold in a 15-gram sample.

In the cyanide method the sample is roasted and then treated with a sodium cyanide solution to dis·solve the gold. The cyanide solution is acidified with tydrobromic acid and bromine water to oxidize the m')novalent gold to the trivalent state. The gold is then con7.entrated by extracting into a measured volume of methyl isobutyl ketone (MIBK). The gold concentration is determined by an atomic -absorption spectrophotometer.

In the classical fire-assay method, the sil·rer ofthe gold-silver bead obtained by cupellation is dissolved and the gold residue is determined gravim<?.trically. The sensitivity of this method is approximately 0.02 troy ounce per ton (600 ppb) on a 15-g (half assay ton) sample. In the proposed fire-assay atomic-absorption method the gold-silver bead from fire assay is dissolved in nitric and hydrochloric acids anc then extracted into a small volume of organic solve·1t (MIBK) prior to estimation by atomic absorption. This procedure thus increases the sensitivity by about a factor of 10 when a 15-g sample is used. The sensitivity can be further increased by combining beads obtained from two fire-assay fusions of 30 g each of the same sample. With this technique about 15 ppb can be determined. This is about the limit imposed by reagent blanks.

REAGENTS

Methyl isobutyl ketone (MIBK). volume;volume in vrater, obtained fr01 Chemical Co., St. Louis, Mo.

APPARATUS

Elmer model 303 ato:-nic-absorption spet meter is used with the following instrum meters:

Wavelength: 2428 Angstroms.

Slit: 5 (3 millimeters, 20 Angstroms).

Source: Gold hollow cathode. 14 milliampc rent.

Water lev~~----- r

Acetylene: 9 pounds per square inch. Air: 25 pounds per square inch. Flame adjustments: With the burner lit, adjust the flow meters to approximately 2.8 on the acetylene and 3.0 on the air while aspirating water. Minor adjustments may be necessary to obtain a blue flame when the MIBK is aspirated. Water bath equipped for aeration and constant temperature (fig. 1). The aeration device constructed in this laboratory consists of a compressor-vacuum pump capable of delivering air at one-third cubic foot per minute under a pressure of 15 po·.mds per square inch. This amount of air is more than sufficient for the 20 outlets shown in figure 1. The compressor supplies a group of air regulator valves (available from aquarium supply houses) which are connected to 3/ 16-inch-diameter Pyrex tubes by Tygon tubing. The valves are used to individually adjust the air supply to -each sample. A steady air flow to the tubes is maintained by means of a small air tank and one or more bleed-off valves.

Figure 2.-Constant-temperature water bath with magnetic stirrer.

Constant-temperature water bath with magnetic stirrer (fig. 2). Teflon-coated ~-inch-diameter magnetic stirring bar. Pyrex culture tubes, 25 by 200 millimeters, with polyseal closure screw cap.

Crush sample in a jaw crusher to less than ¥!-inch size, then split and grind a portion on ceramic plates set for less than 80 mesh. The sample is no'" sieved; a large portion of the sample will be less than 200 mesh.

1. Weigh a 2- to 15-g sample and transfer to a porcelain dish or crucible. Place the crucible in a cold muffle furnace and heat for 1 hour after the temperature reaches 600°C. Do not exceed 650°C because samples of high clay content may tend to sinter and thus trap the goldandprevent solution during the cyanide step.
2. Cool. Add 0.25 g CaO powder and mix thcroughly; transfer sample to a Pyrex culture tube. Add 30 ml of the 0.25-percent NaCN solution to the tube, place cap on the tube, and shake by hand to wet the entire sample. Place tube in an automatic shaker and shake for 15 minutes; let stc.nd overnight.
3. Reshake to break up any cake formed in th~ bottom of the tube. Wash down sides of the tube with about 10 ml of the NaCN reagent. Add 4 dro'Js of the 0.25-percent Santomerse S wetting agent to the tube and heat for 30 minutes in the water bath at 80° -85°C while aerating the sample (fig. 1).
4. If a 2 -g sample is used, proceed to step 5. If a sample of more than 2 g is used, centrifuge the contents of the tube at 2, 000 revolutions per minute for 5 minutes, then decant the liquid phase into another culture tube containing a magnetic stirring bar. Wash the residue left in the first tube with 10 ml of the 0.25-percent NaCN solution containing 4 drops of saturated bromine water. Centrifuge again and combine wash solution with the original solution.
5. Add 5 drops bromine water and mix. Then, working under a well-ventilated hood, cautious 1 y add 5 ml of 48-percent HBr. (CAUTION: Hydrogen cyanide gas is evolved.) Heat in a water bath equipped for magnetic stirring (fig. 2) for 30 minutes at 90°C,
6. Cool. Remove the stir bar with another magnet held outside the tube, add 10 ml MIBK, and shake for 1 minute. Centrifuge if step 4 has been omitted.
7. Measure the absorption of the MIBK phase along with reference standards (see following section, "Standardization") on an atomic-absorption spectrophotometer at a wavelength of2428 Angstroms. Use a water -saturated so 1 uti on of MIBK as a reference blank. To obtain a standard calibration curve, plot the absorption readings against gold concentration expressed as parts per million in the organic phase. The gold content of the sample in parts per million is equal to: gold in ppm in organic phase X 10 ml weight of sample in grams If the sample has an absorbance greater than the lO ppm standard, a dilution of the organic layer can be made. The dilution method is illustrated in the following example: Transfer a 2-ml aliquot of the organic phase to a tube containing about 30 ml of water and 2 ml of 48-percent HBr. Add 8 ml of MIBK, shake, and measure the absorption. This dilution factor of 5 will handle a 10-g sample containing as much as 50 ppm Au.

Prepare reference standards containing 0.1, 0.2, 0.4, 0.8, 1.0, 2.0, 4.0, 8.0, and 10.0 ppm Au in the organic phase for working curves by adding appropriate aliquots of a gold standard solution to a series of culture tubes. Add 2 ml of 48-percent HBr to each and adjust the volume to about 40 ml with water. Add 10.0 ml of MIBK and shake for 1 minute to extract the gold into the organic phase. Measure the absorption of these standards on the 1X or lOX scales of the atomic-a b s or p t i o n spectrophotometer. P e r i o d i c a 11 y, standards can, and probably should, be checked by being subjected to the procedure starting with step 2, However, we have noted no difference in absorption readings between the standards taken through the entire procedure and those prepared as described.

FIRE-ASSAY ATOMIC-ABSORPTION PROCEDURE

The fire-assay method involves a fusion of ground ore with fluxes to form a slag phase and a metallic lead phase. The lead collects any gold, platinum metals., or silver and settles out at the bortmn when poured into a mold. After cooling., tl'le slag is broken away from the lead button which is then put into the furnace in a cupel. The lead melts and is slowly oxidized by the hot air. The lead oxide wets the semiporous cupel material, generally bone ash, and is absorbed. The cupel material is not wet by the molten lead nor by the noble metals, which remain as a bead after the gradual absorption of the lead oxide.

The procedure for the further treat11ent of the bead is as follows:

1. Transfer the silver-gold bead obtained by fire assay to a 50-ml beaker.
2. Add 5 ml of (1:1) HN03, cover, and digest on a steam bath for 45 minutes. Uncover and evaporate the solution to about 2.5 ml.
3. Add 6 ml of concentrated HCl, cover, and let stand 20 minutes. Place beaker on steam bath and digest for 30 minutes, uncover, and evaporate to hard dryness.
4. Add 7 ml of concentrated HCl, cover, and digest on the steam bath until all salts are in solution. Uncover, add about 15 ml of water, and digest an additional 30 minutes.
5. Cool. Quantitatively transfer the solution to a Pyrex culture tube by washing beaker with about 10 ml of water. Add 2 ml of 48-percerr: HBr, mix, add 10 ml of MIBK, and shake tube for 1 minute.
6. Determine the absorbance of the organic phase along with reference standards prepared in the same manner as described in the cyc.nide procedure.

We routinely add 2 milligramsofsilverinthe fusion step. Precut lengths of lead rod, eac'1 containing this amount of silver, are obtainable commercially under the trade name Inquarts. Silver is important in this procedure for three reasons: (1) It has a protective effect and thus reduces gold losses c'uring the cupellation step (Bugbee, 1940, p. 107), (2) it allows trace amounts of gold to be easily transferred from the cupel to a suitable container for analysis, and (3) the addition of silver provides a silver-gold bead that is easily dissolved, The ratio of silver to gold should be greater than 3:1 to allow dissolutioncfthebead; beads having lower ratios of silver to gold may be difficult to dissolve, even in aqua regia. For s~mples that contain sufficient silver, such addition is unnecessary; conversely, for samples than contain considerable gold, the addition of more than 2 rrilligrams silver may be necessary to attain this 3:1 ratio. The silver-gold bead is routinely weighed to allow an estimate or approximate check of the combined gold and silver content of the ore,

The accuracy of the two proposed atomic-absorption methods was first tested by analyzing a suite of six selected samples previously an a 1 y zed by the conventional gravimetric fire-assay method. The results (table l) show good agreement between the two atomic absorption methods and the gravimetric fire-assay method. These samples were selected because they represent a wide variety of rock and ore types as indicated by semiquantitative spectrographic analysis. For example, sample Dll9956 contains 1.5 percent As, 7 percent Fe, and 0.07 percent Zn; sample Dl19971 contains 710 percent As, lOpercent Fe, and 0.03 percent Sb; sample Dll9999 contains> lO percent Fe, 5 percent Cu, and 0.2 percent Mn. About 90 percent of sample Dl00874 is estimated to be pyrite.

The cyanide atomic-absorption method has been thoroughly tested by routinely analyzing several hundred samples of varying composition. All samples that contained at least 0. 5 ppm Au were reanalyzed by the fire-assay atomic-absorption method; in addition, many randomly selected samples containing less than 0.5 ppm Au were checked. Table 2 presents some of the data on 20 samples from Alaska and Nevada areas. The cyanide and fire-assay methods show concordant results.

SAMPLE DISSOLUTION, CYANIDE PROCEDURE

Simmons (1965) observed that low results were obtained on some samples if the ignition step was omitted prior to the cyanide treatment. We have confirmed his observation and routinely roast all samples.

The cyanide process has been used commercially for many years. The commercial processes are based on

the following chemical reaction:

4Au+8NaCN+2Hz0+02 4AuCN·NaCN+4NaOH.

The amount of gold which can be dissolved in sodium cyanide solution depends on the amount of oxyr-en present (Jacobson, 1949, p. 554). The aeraticndevice described in this paper supplies the oxygen for the preceding reaction and furnishes agitation to a large sample. It is desirable to keep the sample ir a single tube during the entire analysis. In the ir.itial stage of this investigation, 2 -g samples were aeratec, conveniently by means of magnetic stirring. However, when we attempted to increase our sample size to 15 g we found that the magnetic stirrer would not sufficiently agitate the larger sample. The aeration device provides sufficient agitation of a 15-g sample.

EXTRACTION OF GOLD WITH METHYL ISOBUTYL KETONE

We wish to emphasize that the gold is in the monovalent state after the cyanide treatment and will not be extracted quantitatively into the MIBK unlesE' oxidized to the trivalent state. In thi's method the dicyanogold (I) ion is oxidized to the bromaurate (III) ion by the addition of bromine water and hydrobromic acid. The reaction is as follows:

The bromaurate (III) ion is then extracted quan':itatively into MIBK prior to estimation by atomic ab"'orption.

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