Loading...
Quick Order

Already know what items you want to order? Enter the SKUs and quantities of the items you wish to order in the form below to add them to your cart.

    Item SKU # Quantity
    The item with this SKU was not found.
    Invalid quantity.
    Processing...

    Large Scale Gold Refining By the Aqua Regia Acid Method

    0. Contents

    1. Introduction

    It is known that the aqua-regia technique herein described is not the only method. It may not be the best, and for the sake of experience electrolytic cells for refining precious metals have been examined. These are probably very similar to the units used by many large professional refiners. These electrolytic methods eliminate some of the problems of the acid method described here, but they too have their own set of disadvantages and are not as suitable for the small lots and needs of individual jewelry manufacturers.

    The amounts which have been refined range from small lots of about 45 g (1-1/2 oz Troy) of fine gold recovered from about l00 g (3 on Troy) of scrap, to more than 3 Kg (100 oz Troy) from about 6 Kg (200 on Troy) of scrap. The latter is about the maximum that is reasonably handled in the equipment herein described.

    The usual refining lot in practice is 300-1000 g (10 to 30 oz Troy) of fine gold recovered from 600-3000 g (20 to 100 oz Troy) of scrap.

     

    2. Outline of the Process

    The gold refining technique described here is the rather ancient wet chemical method whereby the gold-bearing scrap is dissolved in aqua-regia. m is gold solution is then filtered and the jewelers bench dirt, sandpaper grit, grinding wheel grains and similar material remains on the filter as a solid sludge, together with any silver present which will be in the form of silver chloride.

    The filter and sludge are washed with repeated small amounts of water to wash all gold chloride solution down through the filter. Other metals that were in the alloy or in the scrap (nickel, zinc, copper, iron, etc.) are also in this solution which is usually green in color. However, if no nickel or copper is present it will most likely be the characteristic yellow of gold chloride.

    The nitric acid from the excess of aqua-regia used in the digestion is removed either by boiling or chemical reaction.

    To recover the gold as metal a reducing chemical is added to selectively change the gold chloride into solid gold particles and leave the other metal chlorides unchanged and in solution. When tests show this to be complete the solution is filtered and the gold in the filter thoroughly washed. The clean gold is then melted and poured into molds or made into shot.

    The acids used in the process are very corrosive and highly toxic fumes are produced. Please see safety precautions in the operation of the process and should be carefully studied before any part of this work is undertaken.

    3.1. Mixing Aqua Regia

    Type of Scrap Considered and Preliminary Treatment

    The gold scrap that is considered in this report is old jewelry and the material from jewelry bench work, filings, clippings, scrap jewelry pieces, grinding wheel dust, casting spills, sprues, strip pot sludge, etc. Such material has been found to contain from 20% gold in fairly dirty bench scrap to more than 70% gold in pure strip pot sludge. Experience indicates that most shops produce a bench scrap (lemel) that contains 30% to 40% gold.

    The dust from floor sweepings or from polishing wheel vacuum collectors and similar low-grade scrap requires extensive preliminary treatment which is not described here.

    So-called "green gold" and some low carat white golds contain considerable silver and are very difficult or impossible to dissolve in aqua-regia as an insoluble silver chloride film is formed which prevents further action by the aqua-regia. Such golds or any high silver alloy must be melted with several times their weight of copper or brass and shotted to permit dissolution. (See later section on Gold Shot.)

    If the scrap contains shellac, rubber wheel particles, rouge or similar material it is best simmer it in lye (sodium hydroxide and water (a saturated solution) in a ratio of 10 volumes of lye/water to 1 volume of bench sweeps.

    Massive pieces of metal take a very long time to digest in aqua-regia. Any such large pieces should be shotted as described later. Strip pot sludge should be well washed with water to remove cyanide residues before acid is added to it.

    3.2. Mixing Aqua Regia

    This and many of the operations described here should be carried out under an efficient fume extraction hood. Details of the hood used in this work are given in 4. Fume Hoods.

    Aqua regia is a combination of nitric acid and hydrochloric acid (the industrial grade of hydrochloric is sometimes called muriatic acid), it is made by mixing 1 volume of concentrated nitric acid with 4 volumes of concentrated hydrochloric acid.

    If muriatic acid is used (it is usually less costly) the proportions are calculated to be: 1 nitric acid to 4.5 muriatic acid by volume. There are reasons to err on the side of using more hydrochloric acid than theoretically necessary rather than too much nitric acid.

    The precautions for mixing the acids are simple. Avoid splashes, protect eyes and work in the open or under a fume hood. These acids mix quietly.

    Both acids and especially hydrochloric emit acrid fumes. No heat is evolved when mixing but the aqua-regia at once starts to emit chlorine gas, which evolves slowly for several days. DO NOT PLACE STOPPERS aqua-regia bottles. A closed aqua-regia vessel can develop enough chlorine pressure to burst. Store in the open or in a fume hood.

    The aqua-regia can be used immediately, days or weeks and probably months after preparation.

    3.3. Digesting the Scrap

    The scrap gold is placed in the digesting vessel. Glass may be used for small batches. Teflon plastic is also suitable for the strong oxidizing conditions of the aqua-regia and Teflon will tolerate heating if done with care.

    For many batches 6 liter Ehrlenmeyer flasks are used, or the glass jars illustrated are suitable if heated with much care.

    1 to 2 kilos of scrap material in a 6 liter Ehrlenmeyer flask is typical though up to as much as 35 kilos have beentreated in these flasks. This quantity requires considerable agitation and stirring to keep the reaction going and is better done in a jar under a fully ventilated fume hood.

    The amount of aqua-regia required for a given batch varies and depends on the proportion that is acid soluble and the quality of the metal present. It has been found that from 3.5. to 5 liters of aqua-regia are required per kilo of scrap, most batches fall in the range of 4 to 4.5 liters of aqua-regia per kilo of scrap, in smaller amounts this is equivalent to 4 to 4.5 ml (cc) per-gram of scrap.

    If the scrap is the usual filings and dust from jewelers benches, the aqua-regia will react very rapidly and may boil over, so the acid must be added slowly and with care.

    A slow flow or dropwise addition can be made from a bottle (with a bottom outlet) set on a shelf above the flask. If the aqua-regia is several days old and is no longer producing chlorine gas a siphon from a high container is also a convenient way of adding aqua-regia slowly.

    If the scrap is in the form of old jewelry or metal shot or other large pieces, the reaction will be slower and a considerable amount of aqua-regia can just be poured onto the scrap. Care is advised as the reaction is often quite slow to start and then after some minutes becomes very, very active. The jar or flask may get quite hot which increases the reaction speed.

    When there is jewelry with diamonds, rubies, sapphires and similar acid-resistant gems these can be left in place and recovered from the filter.

    The reaction of the aqua-regia with the metals in the scrap produces nitrogen oxides. Some of these are red-brown in color, others are colorless but take up oxygen as soon as they reach air and then turn red-brown .these fumes are acrid, choking and extremely toxic; they dissolve quite easily in water and in caustic solutions; they are heavier than air and the aqua-regia digestion should be done under a good fume hood.

    The preferred practice is to add the aqua-regia to the batch in two or three separate additions. Up to about half or three-quarters of the expected amount of aqua-regia is added and the mixture is allowed to stand for some time. Occasional agitation is good, especially with finely divided material.

    When brown fumes are no longer evolved and the bubbling of the solution is quiet a little hydrochloric acid is usually added. Sometimes a further spurt of activity is seen, the original hydrochloric acid having been depleted leaving some unused nitric acid available, excess hydrochloric acid is not harmful. If no more brown fumes appear the liquid is carefully poured off into a glass or plastic container, being careful not to disturb the solid material in the reaction vessel.

    More aqua-regia is then added as before and the cycle repeated until the addition of fresh aqua-regia produces no reaction, i.e. brown fumes and bubbling.

    Enough aqua-regia must be added to dissolve all of the gold, however the excess aqua-regia that is required to accomplish this will later have to be removed so large excesses should be avoided.

    Toward the last the reaction is much slower and it is desirable to warm the solution and to agitate it regularly, but the aqua-regia should not be heated to boiling. If heated too much it will produce brown fumes merely because it is too hot, this wastes acid and obscures the end of the solvent action.

    The reaction also slows down near the end because of the amount of fine, sludge present which tends to restrict the contact between aqua-regia and undissolved gold, so frequent agitation is helpful.

    When pieces of jewelry or larger pieces of metal are being dissolved it often seems that the jewelry is not being attacked because it is still there in its original shape, however such pieces usually crumble if crushed with a stirring rod. Most jewelry alloys contain silver and the aqua-regia dissolves the gold and other alloying metals leaving insoluble silver chloride as a residue in the original size and form. It is good to break these as there may be a yet undissolved core that will dissolve more quickly if exposed.

    When it is apparent that the reaction is complete the solution should be cooled to room temperature.

    3.4. Filtering

    The aqua-regia now contains various metal chlorides (and perhaps nitrates) in solution and insoluble silver chloride as well as a lot of unwanted material in the sludge, and this mixture (when cooled) must be filtered. me reason for tooling is that silver chloride, though quite insoluble in water, is slightly soluble in strong acids and this volubility is lower in cold acids. Silver is probably the major non-gold constituent of gold refined by this procedure; though few assays have been made, these have consistently shown 996/1000 gold.

    The aqua-regia solutions are filtered with a Buchner filtering funnel and a 4 liter vacuum filtering flask. Two sizes of funnel have been employed, a small one about 125 mm diameter, and a larger one about 250 mm diameter.

    Experience has shown that the paper discs usually used in these filters by chemists tend to float away when the filter is filled with liquid, but coffee urn filters obtainable from hotel and restaurant supply shops have proved very satisfactory. These should be large enough to line the bottom and sides of the filter funnel, inserted dry, wetted thoroughly with water and firmly seated and pressed into the corners to avoid wrinkles and vacuum leaks. Two ply's of filter paper are used to help filtration and avoid breakthrough.

    Vacuum is produced by means of a water pump (aspirator) available from chemical laboratory suppliers. Plastic, not metal pumps should be used, as the acid fumes from aqua-regia filtering rapidly reduce the pumping ability of a metal pump. For the same reason mechanical vacuum pumps are not recommended unless provided with efficient acid vapor traps.

    me filtrate is usually clean and clear. If, however, some solids come through at first the filter should be stopped after a little while and the liquid poured back for refiltering. Usually the liquid will then be clear and clean.

    Filtering proceeds more rapidly if the clearest part of the aqua-regia is decanted onto the filter first. When the sludge and solids get into the filter the process usually slows up. All of the solids should be washed into the filter with a small stream of water, a wash bottle is useful for this operation.

    When the filtering is complete the paper and the sludge should be washer with repeated small streams of water. This is to get all gold chloride solution out of the filter and sludge into the liquid.

    The filter paper now contains the unwanted material and also the silver chloride. It is recommended that this should be dried and saved until at least a 30 gallon drumfull is accumulated. The silver and any residual gold can be recovered separately or the material sent to a refiner.

    The filtered liquid is usually a rather handsome clear green color, due to nickel and copper etc. If only gold chloride were present it would be yellowish.

    The filtered solution is poured into a plastic container (plastic buckets or 5 gallon pickle pails are suitable) for the next steps of eliminating excess nitric acid and precipitating the gold.

    3.5. Nitric Acid Elimination

    The excess aqua-regia that was added to insure complete solution of all gold is, of course, still in the solution at this stage and must be eliminated to allow the gold to be precipitated.

    The classic procedure for nitric acid elimination is repeated boiling to near dryness with the addition of hydrochloric acid with some sulphuric acid near the end. This is a lengthy and patience-trying process.

    The best way is with urea. Add urea to the solution until there is no more fizzing.

    3.6. Precipitating the Gold

    The classic method of reducing gold chloride in solution to solid gold particles is to add "copperas" to the solution. "Copperas" is an ancient name for ferrous sulphate, a rather cheap chemical. A number of other chemicals will also 'reduce' the gold chloride but Storm Precipitant (available from Shor) is better. In hot water, dissolve a weight of Storm Precipitant equal to the weight of dissolved metal.

    The precipitation of gold can be seen as a 'cloud' of particles in the solution. The end point of the precipitation is difficult to see, some clues may be noted in the density of the 'cloud' of gold particles. The solution will be clearer and noticeably less yellowish especially if a drop is viewed one white chinaware surface. m is is because the yellow gold chloride is gone and the green of the other chlorides remains. Deliberate care during this gold precipitation work is advised. Observe the signs and test the solution frequently to avoid large excesses of Storm Precipitant.

    A rapid and satisfactory test for gold in solution is described in 5. Gold Test Procedure. sulphur. Careful addition of Storm Precipitant and a slow approach to the end point can avoid this.

    The sulphur dioxide odor, however, can be used as one of the signs that gold precipitation is complete.

    A problem that occurs when too much Storm Precipitant is added is that copper chloride, which is very soluble in the cupric chloride (CuC12) form, is reduced by the excess Storm Precipitant to the cuprous chloride (CuCl) state, which forms a white precipitate. Limited experience with this contaminant has shown that it will reduce gold quality a little and it will affect the color of the gold.

    If cuprous chloride is present it will make the melting of the gold a memorable experience. Dense clouds of choking white fumes will clear all persons out of the furnace room quite quickly.

    Some excess of Storm Precipitant is required to cause this undesirable side reaction, and it is felt that the advantages of no boiling and little or no emission of brown fumes make it worthwhile to use Storm Precipitant even though larger volumes of liquids are handled and some care at the end point is needed.

    If, through error, some cuprous chloride crystals are formed, they can be removed as described later.

    3.7. Filtration

    Then the solution has been cleared of gold it should be allowed to stand for several hours. Although gold is heavy and most settles quickly, some particles are very small and require time to settle to the bottom.

    Standing for a period, if possible overnight, facilitates the subsequent filtering operation.

    In the interest of reducing the time and aggravation of filtering work the clear upper portion of the barren solution can be decanted and only the bottom few inches near the gold filtered. A simple siphon will remove the upper portion of the liquid quietly without stirring up gold particles. The bottom few inches of liquid are then poured through the filter keeping the gold largely in the pail.

    The same Buchner filters and the same kind of filter and paper are used for gold as previously used to filter aqua-regia. me gold is washed with repeated small amounts of water until the water coming through the filter is quite clear and colorless. The gold in the pail is then just covered with concentrated hydrochloric acid and thoroughly stirred. The acid is added to the filter and the treatment repeated several times, followed by repeated washing with water. This treatment will remove small amounts of contaminants including cuprous chloride.

    When the gold has been treated with hydrochloric acid and thoroughly washed it is then ready to transfer into the filter.

    A soft kitchen scraper helps move the gold into the filter and a small water jet is very useful in clearing the last particles into the filter.  Often there is gold adhering to the walls of the pail, this can be scrubbed down with a stiff brush and washed into the filter with the water jet.  When the gold is all in the filter the vacuum should be run for a while to get the gold as dry as possible.

    3.8. Melting the Gold

    Any gas or electric furnace normally used for gold melting or alloy production may be used for melting the precipitated gold.

    Crucible material is not critical and may be selected to suit the melting furnace. Crucibles, however, should be clean and kept solely for melting pure gold.

    An ordinary spoon is convenient to transfer the gold into the crucible and working over a smooth clean surface so any spilled particles are not lost. When the filter is quite empty of gold the paper and the last of the gold are transferred into the crucible. The paper should be pressed down firmly.

    When the gold is being melted the filter paper will burn and leave any adhering gold in the crucible. It usually burns quite slowly because the furnace flame at gold melting temperature does not have excess oxygen. With time it will burn away.

    Fluxes are not usually needed, but if a surface film appears on the melt a very little borax may be added. However, the filter paper can be burned out very quickly by adding a small amount (say ~ teaspoonful or less) of sodium nitrate. When this is added to a hot crucible the paper burns with an eye-dazzling flame.

    Slag from such fluxes are often very liquid and cannot be easily skimmed off. If it is necessary to skim, a graphite rod is used to dip out the slag as adhering lumps on the end of the rod.

    When the gold is well molten and 'quick' it can be poured into the mold. The mold should be smoked with a sooty flame,` or sprayed with a silicone parting fluid. Mineral oil coatings are sometimes recommence but they may discolor the gold.

    The mold must be warm and dry. A cold would may have traces of moisture| and molten metal poured onto traces of moisture can create the most amazing, costly and potentially painful eruptions.

    3.9. Making Gold Shot

    • If the gold is to be used for alloying, making it is much more convenient in the form of shot rather than bars.

    • Various methods have been employed for making shot or grain. In this work the molten gold is poured slowly into a container of water which has a fan-shaped jet of water passing across the vessel about 1 inch below the surface. Excess of water flows out over the top of the vessel.

    • To help slow the fall of the metal and prevent agglomeration of the shot a sloping metal baffle is fitted. The whole assembly is illustrated to the right.

    The materials involved in these processes are not what those experienced in chemical and metallurgical work would consider especially dangerous. They are, however, strong acids and produce acrid fumes and solutions that can badly burn and stain the skin. These deserve respect. Very valuable materials are also involved and it makes sense to have the equipment and procedures set up to avoid loss.

    The acids used, nitric and hydrochloric, are both somewhat volatile and emit fumes and should be well stoppered to keep fumes out of the work areas. The acids (especially hydrochloric) emit some fumes when poured for measuring and mixing. These corrode equipment and make breathing difficult, and this work should be done in a chemical hood or in clear open spaces.

    The acids, if spilled on the clothes, will eat holes and produce stains but not instantly. Acid on bare skin will soon sting and burn, but also not instantly, and spills should be promptly washed away with water. Reasonable precautions are to wear glasses or goggles as a splash in the eye can be serious. Emergency showers and eye washes are good sense in areas where this type of work is carried out. Such equipment may be an official requirement in most places.

    A box or two of ordinary baking soda (sodium bicarbonate) should be kept at hand because it effectively neutralizes acid that spills onto floors and clothing. It is in itself harmless. It is, however, too strong for and NOT to be used in the eyes.

    The hydrochloric and nitric acids mix quietly without spatter no matter which acid is poured into which.

    The aqua regia, as soon as mixed, begins to emit chlorine gas. It does this slowly over many days. Chlorine is a most toxic gas and should no be inhaled (there is so much discomfort in breathing the emission from the aqua regia that an area would normally be vacated long before there is any danger to life).

    The aqua regia container must be glass or plastic. No metal will hold it except very costly titanium. The container must be open or vented to allow the escape of the chlorine. A convenient way to vent the chlorine is by means of a cork equipped with a tube and small rubber hose to the ventilating stack.

    Although cold aqua regia does not rapidly attack plastics, Teflon is the only plastic truly resistant to the oxidizing attack of hot aqua regia.

    The brown fumes generated during aqua regia digestion are toxic but are so acrid that their presence is soon detected. A food fume hood with a moving current of air to a stack or washer is required for this operation. Essentially all the fumes produced in the process are heavier than air but will be swept out by a properly designed hood.

    Gold chloride is formed when aqua regia digests gold scrap. Gold chloride has photochemical properties similar to the properties of the silver salts used in photography. Very minute amounts of this solution will stain the skin. Any work with this gold solution will almost surely result in the hands being stained a dark purple. The stain only appears hours after contact. It is not dangerous, just very visible. It is prevented by consistently wearing rubber gloves.

    Safety

    The materials involved in these processes are not what those experienced in chemical and metallurgical work would consider especially dangerous. They are, however, strong acids and produce acrid fumes and solutions that can badly burn and stain the skin. These deserve respect. Very valuable materials are also involved and it makes sense to have the equipment and procedures set up to avoid loss.

    The acids used, nitric and hydrochloric, are both somewhat volatile and emit fumes and should be well stoppered to keep fumes out of the work areas. The acids (especially hydrochloric) emit some fumes when poured for measuring and mixing. These corrode equipment and make breathing difficult, and this work should be done in a chemical hood or in clear open spaces.

    The acids, if spilled on the clothes, will eat holes and produce stains but not instantly. Acid on bare skin will soon sting and burn, but also not instantly, and spills should be promptly washed away with water. Reasonable precautions are to wear glasses or goggles as a splash in the eye can be serious. Emergency showers and eye washes are good sense in areas where this type of work is carried out. Such equipment may be an official requirement in most places.

    A box or two of ordinary baking soda (sodium bicarbonate) should be kept at hand because it effectively neutralizes acid that spills onto floors and clothing. It is in itself harmless. It is, however, too strong for and NOT to be used in the eyes.

    The hydrochloric and nitric acids mix quietly without spatter no matter which acid is poured into which.

    The aqua regia, as soon as mixed, begins to emit chlorine gas. It does this slowly over many days. Chlorine is a most toxic gas and should no be inhaled (there is so much discomfort in breathing the emission from the aqua regia that an area would normally be vacated long before there is any danger to life).

    The aqua regia container must be glass or plastic. No metal will hold it except very costly titanium. The container must be open or vented to allow the escape of the chlorine. A convenient way to vent the chlorine is by means of a cork equipped with a tube and small rubber hose to the ventilating stack.

    Although cold aqua regia does not rapidly attack plastics, Teflon is the only plastic truly resistant to the oxidizing attack of hot aqua regia.

    The brown fumes generated during aqua regia digestion are toxic but are so acrid that their presence is soon detected. A food fume hood with a moving current of air to a stack or washer is required for this operation. Essentially all the fumes produced in the process are heavier than air but will be swept out by a properly designed hood.

    Gold chloride is formed when aqua regia digests gold scrap. Gold chloride has photochemical properties similar to the properties of the silver salts used in photography. Very minute amounts of this solution will stain the skin. Any work with this gold solution will almost surely result in the hands being stained a dark purple. The stain only appears hours after contact. It is not dangerous, just very visible. It is prevented by consistently wearing rubber gloves.

    4. Fume Hoods

    The aqua-regia process has the disadvantage of producing several kinds :| of chemical fumes and all of them are unpleasant and several decidedly toxic. the problem of dealing with considerable volumes of these fumes when refining large batches of gold scrap makes the slower electrolyte methods attractive to larger refiners.

    The small lots dealt with in this report can be handled in an ordinary chemical laboratory type of fume hood. The hood should be deep enough to carry away the fumes but shallow enough for hand working. If possible there should be enough room for a one or two disk hot plates and an area for mixing aqua-regia. A corner to be used for precipitating gold is very desirable because of the sulphur dioxide fumes that may liberated in the process described.

    The hood used in this work has a main area 2-1/2 feet deep x 5 feet long with a 2-1/2 feet deep x 1-1/2 feet 'L' section at one end for aqua-regia I mixing and storage. The normally active working area is 2-1/2 x 2-1/2 feet and is used largely for aqua-regia - gold digestion.

    The maximum possible front opening is 2 feet high x 4 feet long. For good fume removal this is usually reduced working area. In cases of extensive heave fuming a partial door is lowered to reduce the opening to 1 foot high 2 feet long. I

    The duct through which the air and fumes are extracted is 10 x 12 in The vertical height is only 8 feet, which is too low to create a natural draft and an effective draft is produced by the use of an inexpert external blower. This arrangement and the essential dimensions are in Figure No. ll
    .

    At the scale of working used, no fume or odor has been detected excess when reaction rates have been allowed to become excessive.

    It has been found that banana plants are most sensitive to these act fumes and a small grove in the immediate area of this stack provides useful monitor of the effectiveness of fume control, good crops being produced.

    If a larger hood is required the following may be of interest.

    A chemical laboratory hood with a 25 x 6 feet front opening and an ~ 18 inch diameter, 12 feet high, stack works well on dense fumes. The blower fan has a 15 horsepower motor and a 3 inch x 4 inch pipe in the stack.

    The fan operates at quite high pressure compared to the one in the previous set-up, but is used only occasionally, the natural draft usually sufficient. The fact that a large hot plate in the hood is reactant use probably helps this natural flow. This hood has been in regular use for many years.

    A spray of water in the duct system would be helpful in reducing fume emission and a spray of caustic soda solution with a small recirculating pump even more so. Still better would be a tower packed with saddles or other shapes and a caustic soda solution system. One refinery in Texas has been using such a home built system for some years. It is recommended that in all cases qualified professional or official advice should be sought before making a major commitment to these process.

    5. Gold Test Procedure

    You will need:

    Precious Metal Detection Liquid (available from Shor)

    5.1 Testing For The Presence of Dissolved Gold

    To insure that when you pour off your waste water, you're not pouring off dissolved gold, you must test for dissolved gold. This test will detect the presence of as little as 4 parts of dissolved precious metal in 1,000,000 parts of water (less than 1/1,000 gram per batch). It will not detect the presence of particles of gold.

    To Test:

    Place a drop of Precious Metal Detection Liquid on a paper towel.

    Using your pipette (the big eye dropper) take a sample of the water in which you precipitated your gold. Put a drop or two on the paper towel.

    If dissolved gold is present, the color will turn purple-brown or purple-brown-black. If it turns any of these colors, add some more precipitant or give it more time or heat the water.

    The colors yellow, orange and red indicate the presence of an insignificant amount of platinum group metals, 

    Discarding gold solution that has been tested "no gold" by old and used up test solution can be very costly.

    6. Bibliography

    • Refining Precious Metal Wastes by C.M. Hoke. Metallurgical Publishing Co., 123 William Street, New York, U.S.A. 1940

      University Microfilms International, 300 N. Zeeb Road, Ann Arbor, Michigan 48106, U.S.A. and 18 Bedford Row, London WC1R 4EJ, England.

      Notes: This is probably the best of available literature on this subject.

    • Methods for the Recovery of Platinum, Iridium, Palladium, Gold and Silver from Jewelers Waste by C.W. Davis. Technical Paper No. 342, U.S. Bureau of Mines, U.S. Government Printing Office, Washington D.C., U.S.A. 1924.

      Notes: Now out of print.

    • Recovering Precious Metals from Waste Liquid Residues by Geo. M. Gee. E. & F.N. Spon Ltd., 57 Haymarket, London SW1. 1920

    • "Gold Refining" by Geo. Gajda. CA 90406, U.S.A. 1976.

    6.1. Further Reading

    For electrolytic methods general descriptions of silver and gold cells may be found in various texts on electrochemistry, electrorefining, etc. These methods are not new, having been devised at the turn of the Centur or earlier.

    Geo. Gajda, P.O. Box 1846, Santa Monica, Appendix No. 1.