Gold, silver, and other resources in the ash of incinerated sewage sludge at Palo Alto, California; a preliminary report

CECIL D. ANDRUS, Secretary

CONTENTS

Abstract -------------------------------------------------------- Introduction ---------------------------------------------------- Physical characteristics and mineral composition ---------------- Chemical composition -------------------------------------------- Mineral resources ----------------------------------------------- 4 Rare elements --------------------------------------------------- Phosphorus ------------------------------------------------------ Conclusion ------------------------------------------------------ References cited ------------------------------------------------

TABLES

Identified minerals, compounds, and native elements in Palo Alto ash---------------------------------- Chemical composition of Palo Alto ash ----------------------- 3.

Chemical composition of the ash of incinerated sewage from Palo Alto and other u.s. cities---------------- 4.

Amounts of elements dissolved in HCl ------------------------ The ash of incinerated sewage sludge in Palo Alto, Calif., contains large contents of gold (30 parts per million) and silver (660 ppm). As much as 9000 metric tons of ash is piled in the city dump and comprises a gold and silver deposit that has a market value of about $2.5 million. Annual production of ash is about 1800 metric tons with a value of nearly $.5 million. The contents of copper (8000 ppm) and tin (1000 ppm) increase the value of the ash appreciably. Phosphorus makes up about 6.6 percent of the ash and has a potential value for fertilizer manufacture.

The principal source of the metals appears to be the large electronics industry that is located in the area. Part of the silver likely comes from the photographic industry.

No resources are being recovered presently from the ash of incinerated sewage sludge in the United States. The ash from the Palo Alto sewage sludge appears to offer an attractive opportunity for the profitable recovery of a number of mineral resources.

Bizarre as it may seem, a gold and silver deposit is located in the city dump of Palo Alto, Calif. The deposit is composed of incinerated sewage sludge. The multiple hearth incinerator handles about 6500 metric tons (dry basis) of sludge annually and yields about 1800 metric tons of ash. As much as 9000 metric tons has been piled in the city dump. The teroperature of incineration is about 760°C, and very little unburned material remains in the

INTRODUCTION

ash.

The apparent value of the deposit, based on a gold price of $175 per troy ounce and a silver price of $5.00 per troy ounce, is nearly $2.5 million, and the apparent value of the ash produced annually is almost $.5 million. Significant'amounts of phosphate, copper, and tin increase the value of the ash appreciably.

The sewage system serves the cities of Mountain View, Los Altos, Los Altos Hills, Stanford, and East Palo Alto, in addition to Palo Alto itself. The principal source of the metals appears to be the large electronics industry that is located in the area served by the sewage system, although the photographic industry may contribute a significant part of the silver.

The remarkably large gold and silver contents of the ash were indicated first by semiquantitative spectrographic analyses made as part of a study of the ash as a potential source of phosphate for fertilizer. The spectrographic analyses were subsequently confirmed by neutron activation analysis for gold and atomic absorption analysis for silver.

Analytical data of two samples that were collected 5 months apart at the incinerator are presented here. The close similarity in composition of the samples with one another as well as with another sample that is reported by Gabler and Neylan (1977) indicates that the ash is remarkably homogeneous. Since the press release by the U.S. Geological Survey, in March, 1977, of the gold and silver contents of the ash, the Palo Alto City staff reports (oral commun., May 23, 1978) that subsequent determinations of the amounts of gold and silver in ash from the incinerator have not changed significantly.

CHEMICAL COMPOSITION

[Sample 1 is bulk sample of October, 1975; sample lA is the grain-size fraction of sample 1 that is less than .053 mm; sample 1B is the acid-insoluble (HCl:H20-1:3) of sample 1A; sample 2 is bulk sample of March 1976. Footnotes: 1, semiquantitative spectrographic analysis by N. Rait; 2, instrumental neutron-activation analysis (INAA) by P. A. Baedecker; 3, atomic absorption spectrophotometric· (AAS) analysis by Anne Childress; 4, average of INAA and AAS; 5, X-ray spectrographic analysis by R. A. Gulbrandsen; 6, spectrophotometric analysis by R. A. Gulbrandsen; 7, continental crust (Taylor, 1964); 8, Concentration factor (C. F.) = average ash/continental crust].

Nd2------------------

Zr1------------------

amounts. The only other city that is known to have a significant amount of gold in its sewage is Sunnyvale, which is located a few miles south of Palo Alto and which has a large electronics industry like that of Palo Alto. An analysis of this sewage by

Table 2.--Chemiaal composition of Palo Alto ash

.8 3700 Sample No. 1A

.6 4500 lB Percent by weight

K. E. McAbee (San Jose Mercury, 1977; oral commun., 1977) shows a gold content of 0.1- 0.2 troy ounce per ton of wet sludge. This content of gold in wet sludge would be concentrated a number of times if it were incinerated, and the ash would likely 4.8 4.1 1.8 7.0 1.1 .4

.7 Crustal Average of abundance 1 and 2

9.6 2.7 C.F •

1.8 contain about the same amount of gold as the ash of Palo Alto's incinerated sludge. Silver in the wet sludge was reported to be about 0.5 troy ounce per ton, an amount that would be much less than that of Palo Alto's ash even after incineration.

Sc ------- MINERAL RESOURCES

The concentration factors (C.F.) shown in table 2 provide one means of judging the economic potential of the elements in the Palo Alto ash. McKelvey (1960) shows that concentration factors of the elements in ore deposits range from about 3 to 10,000 and are, in general, inversely proportional to the respective abundances of the elements in the Earth's crust. Abundant elements, such as aluminum and iron have C.F.'s less than

10. Phosphorus, the eleventh most abundant

element, has C.F.'s in commercial deposits from about 20 to 150; the C.F. in the Palo Alto ash is 63. Silver is the most highly concentrated element in the ash, with a C.F. of 9400, and is present in an amount that is similar in magnitude to the rich deposits of the Coeur D'Alene district in Idaho (Heyl and others, 1973, p. 594). Gold is the next highest, with a C.F. of 7500, and the amount of gold is about double the average grade that is mined currently in the world (Simons and Prinz, 1973, p. 274). Tin is highly concentrated with a C.F. of 500 (using the value of Gabler and Neylan, 1977), but not especially high in actual grade. Palladium has a C.F. of 400 and would likely be recovered with gold. The C.F. of copper is only 145, but the amount present is of common ore grade. Cadmium is highly concentrated, with a C.F. of 420, but the amount present in the ash is not of significant value. Lead, with a C.F. of 150, is also of only small value. Seven more elements--cobalt, chromium, gallium, molybdenum, nickel, antimony, and zinc--have factors greater than 10 but are present in amounts that are of low value. All of the elements that have concentration factors greater than 10 make the Palo Alto ash an unusual material in comparison with the natural materials of the Earth's crust. Average shale (Turekian and Wedepohl, 1961), for example, has only one rare element with a C.F. as high as 10 from the group of those reported in the ash. Compared to natural materials, the ash may even be unique in containing a suite of elements that are significantly concentrated but that are of diverse geochemical affinities.

RARE ELEMENTS

Methods of recovery of some of the rare elements from the ashes of incinerated sewage sludges have been studied by Diosady (1975), Oliver and Carey (1976), and Gabler and Neylan (1977). All of the studies contain many data on the solubility of the elements in various kinds of solvents. Gabler and Neylan are the only ones to work on the Palo Alto ash and on the extraction of gold and silver in particular. They found in preliminary studies that a two-stage leach of the ash with boiling concentrated hydrochloric acid and a strong alkaline sodium cyanide solution was required to extract 77 percent of the gold and 40 percent of the silver. The only other extractive data on gold and silver in the Palo Alto ash are those that we obtained (sample lB, table 2). They comprise the determinations made on the acid-insoluble part of sample lA. Hydrochloric acid was used in a 1:3 ratio of concentrated HCl to H 0 at ambient temperature. The amount of gold, 64 ppm, indicates that essentially a~l of the gold is insoluble in the hydrochlor1c acid solution, and about half of the silver is not dissolved. We have observed gold in the metallic state as individual plates and variously shaped chunks and also incorporated in particles of fused or aggregated material. No other mode of occurrence has been recognized although Gabler and Neylan's data indicate that more than one exists. At least two modes of occurrence of silver are present, as our solubility data indicate as well as those of Gabler and Neylan's, but none has been identified. We found that the silver dissolved in the hydrochloric acid solution could be separated effectively by a small increase in the solution's pH. The addition of sodium hydroxide solution precipitated silver chloride, the mineral ceragyrite.

The recovery of rare elements in the Palo Alto ash is complicated by the number of elements that are present in significant amounts, as well as by the lack of knowledge about the elements' modes of occurrence. The ash is a new kind of ore for which efficient extractive methods for the greatest number of elements possible have yet to be developed.

PHOSPHORUS

The phosphorus content of worldwide commercial phosphate deposits ranges from about 2.2 to 16 percent. A proposed mine by U.S. Gypsum at Pine Mountain, Ventura Co., California, would have ore with an average grade of 3.7 percent phosphorus (Evans, 1976, p. 275) and would be the only phosphate mine in California that is mined for phosphate products (some phosphatic rock is mined for use as a soil conditioner).

Nearly all of the phosphorus mined in the United States occurs as carbonate fluorapatite, a phosphate mineral that is formed in the marine environment. About 83 percent of U.S. consumption is for fertilizer and animal feed supplement, about 9 percent for detergents, and the remainder is used in a large number of other products (Stowasser, 1977, p. 6).

Fertilizer manufacture employs two basic processes. In the first, the waterinsoluble apatite that is mined is converted into soluble monocalcium phosphate by sulfuric acid. Gypsum is also formed i~the reaction. The product is ordinary superphosphate, a fertilizer that is declining in popularity.

In the second process, sufficiently more sulfuric acid than that required in the superphosphate process is added in order to dissolve all of the phosphate compounds and yield phosphoric acid and gypsum. The gypsum and other insoluble residues are removed by filtration, leaving a phosphoric acid product. This method of processing is popular because many kinds of fertilizers can be made from phosphoric acid. There are three general types:

1. Triple superphosphate. A solid fertilizer made by the reaction of phosphoric acid with apatite ore. It contains up to three times as much available P o5 as ordinary superphosphate.
2. Mono- and diammonium phosphates. Solid fertilizers that can be mixed with potassium salts to make all grades of three-component (nitrogen-phosphorus-potassium) fertilizers.
3. Liquid fertilizers that contain phosphate and other element nutrients. Phosphorus compounds that have been

identified in the Palo Alto ash are the minerals carbonate hydroxyl apatite and whitlockite. Both are principally calcium phosphates like the apatite of most commercial ore deposits and are similarly soluble in acid solutions. The amount of calcium in the ash, however, is insufficient to account for all of the phosphorus in these compounds. The presence of a small amount of calcium carbonate (calcite) further accentuates the calcium deficiency. It seems most likely that iron, and possibly aluminum, are combined with the excess phosphate. A rough calculation indicates that about 35-40 percent of the ash is composed of phosphate compounds.

The amounts of constituents of the ash that are dissolved in the hydrochloric acid solution along with those of phosphate compounds are shown in table 4. Sample lB is the acid-insoluble part, 53 percent, of 1A. The amount of an element that is dissolved, therefore, equals the total (in 1A) minus the amount that remains in the acid-insoluble part (percent in lB multiplied by .53, the proportion of the total lA that is acid insoluble). The amount of phosphorus dissolved, for example, is: p (dissolved)=P (total)-P (acid insoluble~ 7.2 percent=7.4 percent-.2 percent (.3x.53l Nearly all the phosphorus, calcium, strontium, and uranium are dissolved (table 4)~

CONCLUSION

u v REFERENCES CITED

Diosady, L. L., 1975, Recycling of incinerator ash: Ontario Ministry of the Environment, Research Report 19, 82 p. Evans, J. R., 1976, Economic evaluation of Pine Mountain phosphate deposit, Ventura County, California: California Division of Mines and • Geology, California Geology, Dec. P• 275-279. Gabler, R. C., and Neylan, D. L., 1977, Incinerated municipal sewage sludge as a secondary resource for metals and phosphorus, in National Conference on Sludge-Management, Disposal and Utilization, 3rd, Miami Beach, 1976, Proceedings: Rockville, Md., Information Transfer, P• ·197-200. Heyl, A. V., Hall, W. E., Weissenborn, A. E., Stager, H. K., Puffett, W. P., and Reed, B. L., 1973, Silver, in Brobst, D. A., and Pratt, W. P., eds., U.S. mineral resources: u.s. Geological Professional Paper 820, P• 581-604. McKelvey, V. E., 1960, Relation of reserves of the elements to their crustal abundance: American Journal of Science, v. 258-A, p. 234-241. Oliver, B. G., and Carey, J. H., 1976, The removal and recovery of metals from sludge and sludge incinerator ash: Ontario Ministry of the Environment, Research Report 33, 58 p. San Jose Mercury, 1977, Gold found in sewage not worth recovery cost: San Jose Mercury, three stared., p. 25. Simons, F. S., and Prinz, W. C., 1973, Gold, in Brobst, D. A., and Pratt, W. P., eds., U.S. mineral resources: u.s. Geological Survey Professional Paper 820, p. 263-276. Stowasser, W. F., 1977, Phosphate: U.S.

Bureau of Mines Mineral Commodity Profiles MCP-2, 18 P• Taylor, S. R., 1964, The abundance of chemical elements in the continental crust--a new table: Geochimica et Cosmochimica Acta, v. 28, P• 1273-1285. Turekian, K. K., and Wedepohl, K. H., 1961, Distribution of the elements in some major units of the earth's crust: G~ological Society of America Bulletin, v. 72, p. 175-192.

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