GEOLOGICAL SURVEY CIRCULAR 784 

Gold, Silver, and Other Resources in the 

Ash of Incinerated Sewage Sludge at 

Palo Alto, California --

A Preliminary Report 

Gold, Silver, and Other Resources in the 

Ash of Incinerated Sewage Sludge at 

Palo Alto, California--A Preliminary Report 

By R. A. Gulbrandsen, Norma Rait, Donathan J. Krier, 

Philip A. Baedecker, and Anne Childress 

GEOLOGICAL SURVEY CIRCULAR 784 

1978 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 
CONTENTS 

Page 

Abstract -------------------------------------------------------- 1 

Introduction ---------------------------------------------------- 1 

Physical characteristics and mineral composition ---------------- 2 

Chemical composition -------------------------------------------- 2 

Mineral resources ----------------------------------------------- 4 

Rare elements --------------------------------------------------- 4 

Phosphorus ------------------------------------------------------ 5 

Conclusion ------------------------------------------------------ 6 

References cited ------------------------------------------------ 7 

1. 

2. 

3. 

4. 

TABLES 

Identified minerals, compounds, and native 

elements in Palo Alto ash----------------------------------

Chemical composition of Palo Alto ash ----------------------- Chemical composition of the ash of incinerated 

sewage from Palo Alto and other u.s. cities----------------

Amounts of elements dissolved in HCl ------------------------

III 

Page 

1 

3 

4 

6 

Gold, Silver, and Other Resources 

in the Ash of Incinerated Sewage Sludge at 

Palo Alto, California--A Preliminary Report 

By R. A. Gulbrandsen, Norma Rait, Donathan J. Krier, 

Philip A. Baedecker, and Anne Childress 

ABSTRACT 

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. 

INTRODUCTION 

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 

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. 
In addition to the analytical data, 

this report treats some features of the ash 

that bear on recovery methods for some of 

the elements, particularly phosphorus. This 

study is not intended to be thorough or 

comprehensive in scope. 

Acknowledgments.--We are grateful to the 

city of Palo Alto for allowing us to make 

this study. We especially thank Raymond H. 

Remmel, Chief Engineer, Water Quality 

Control, for providing the ash samples and 

describing the incineration process. We 

also thank Mark Harris, Acting Chief 

Engineer of the Water, Gas, and Sewer 

Division for information about the city's 

plans for sale of the ash. 

PHYSICAL CHARACTERISTICS AND 

MINERAL COMPOSITION 

The reddish-brown fine-grained ash_is 

similar in general appearance to natural 

material. More than 80 percent of the ash 

is smaller than .061 mm in grain size (silt 

and clay). Size fractions are presented in 

the following table (sample taken in October 

1975). 

Grain size 

(mm) 

>0.833 

.495-.833 

.149-.495 

.061-.149 

<.061 

Percent of 

total sample 

------------- 1.3 

------------- 1.5 

------------- 6.2 

------------- 9.4 

------------ 81.6 

Microscopic examination shows that the ash 

differs markedly from natural materials in 

containing bits of aluminum foil, wire, 

common glass, and animal bone. X-ray 

diffraction reveals the presence of another 

anomalous component, silicon, which is an 

important semiconductor used by the electronics industry. 

The minerals, compounds, and native 

elements in the ash that have been 

identified are listed in table 1. Most are 

minerals, that is, naturally occurring 

chemical compounds and native elements, and 

many of them, particularly quartz, plagioclase, potassium feldspar, amphibole, and 

pyroxene, are likely derived from the sand, 

soil, and dust of the area. Other minerals, 

such as corundum and garnet, have been used 

by man as abrasives and then discarded. 

Possibly some part of all of the minerals 

have been used in some way. The only 

identified minerals in the ash that do not 

occur naturally are aluminum, silicon, and 

silicon carbide. Some of the minerals are 

synthetic, at least in part; cristobalite, 

for example, is commonly formed in ceramics. 

Much of the hematite and probably all of the 

whitlockite are developed during the 

incineration of the sludge. 

The number of identified constituents 

in the ash is unusually large in comparison 

2 

with natural sediment, and, because of the 

heterogeneous origin of sewage, many more 

are undoubtedly present. Quartz, cristobalite, whitlockite, and apatite are the 

major components, and many others occur in 

significant amounts. 

Table !.--Identified minerals, compounds, and 

native elements in Palo Alto ash 

Elements: 

Iron 

Aluminum 

Gold 

Silicon 

Silicates: 

Plagioclase 

Potassium feldspar 

Amphibole 

Pyroxene 

Muscovite 

Talc 

Garnet 

Oxides: 

Hematite (ferric oxide) 

Corundum (aluminum oxide) 

Rutile (titanium oxide) 

Quartz (silicon oxide) 

Cristobalite (silicon oxide) 

(sodium calcium aluminum silicate) 

(potassium aluminum silicate) 

(many compositional variables) 

(many compositional variables 

(potassium aluminum hydroxyl silicate) 

(magnesium hydroxyl silicate) 

(many compositional variables) 

Zircon (zirconium silicate) 

Phosphates: 

Whitlockite (calcium magnesium carbonate(?) phosphate) 

Carbonate hydroxyl apatite (calcium hydroxyl carbonate phosphate) 

Carbonate: 

Calcite 

Carbide: 

(calcium carbonate) 

silicon carbide 

CHEMICAL COMPOSITION 

Data that we have obtained on the 

chemical composition of ash from Palo Alto 

are presented in table 2. Other constituents that have not been determined but that 

are known qualitatively to be present are 

carbon as inorganic carbonate, carbon of 

unburned organic material, and sulfur as 

sulfate. Also included in the table is a 

measure of concentration of the elements 

relative to their respective abundances in 

the continental crust of the earth, called 

the concentration factor (C.F.). Table 3, 

compares the average composition of the ash 

that we determined with an analysis of the 

ash made by Gabler and Neylan (1977) of the 

U.S. Bureau of Mines as well as with a 

summary of their analyses of incinerated 

sludge ashes of other United States cities. 

The analyses of the Palo Alto ash compare 

well for most constituents. The large 

difference between the tin determinations 

deserves attention, however, because of the 

appreciable apparent value of the tin in the 

ash. Since our determination of tin was 

only semiquantitative, we ac~ept the 

quantitative determination, 1000 ppm, of 

Gabler and Neylan as the better one. 

Comparison of the 14 rare elements common to 

both the average composition of Palo Alto 

ash and the average composition of ash from 

13 plants in other U.S. cities shows that 

the concentrations of 10 of the elements are 

greater in the Palo Alto ash and that 8 of 

these 10 occur in amounts more than twice as 

large as those in the ash of other cities. 

Gold was not detected in the ash of 

other cities, and compared to the Palo Alto 

ash, silver is present in much smaller 
Table 2.--Chemiaal composition of Palo Alto ash 

[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]. 

Element 

~-----------------­ Al1------------------

Fe

2

------------------ Mgl------------------

Ca 5------------------ Na1------------------

Kl ------------------ Ti1------------------

p6 ------------------

Ag4------------------ Au2------------------

Bl ------------------ Ba2------------------

Be 1------------------

Cd~------------------

Ce 2------------------ Co2------------------

Cr2------------------ Cs2------------------

Cu3------------------ Ga1------------------

Hf2------------------ La2------------------

Mn 1------------------ Mo1------------------

Nb 1------------------ Nd2------------------

Ni 3------------------ Pb3------------------

Pd 1------------------ Sb2------------------

Sc 2------------------ Sm2------------------

Sn1------------------ Sr1------------------

Ta2------------------ Th2------------------

o2 ------------------ vi ------------------ yl ------------------ Yb1------------------

Zn~-----------------­

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

1 

21.0 

5.4 

4.8 

1.9 

8.6 

1.1 

.5 

.3 

6.2 

680 

28 

89 

2600 

5 

68 

170 

360 

1450 

18 

7400 

98 

5 

67 

680 

23 

17 

50 

1700 

1800 

5 

14 

4 

4 

3100 

430 

8 

6 

11 

41 

8 

.8 

3700 

270 

Sample No. 

1A lB 2 

17.0 

4.4 

5.4 

1.7 

8.7 

1.3 

.5 

.3 

7.4 

760 

34 

59 

2750 

5 

80 

200 

425 

1700 

22 

8200 

100 

6 

81 

650 

14 

17 

55 

2200 

2300 

5 

18 

4 

4 

3600 

400 

9 

7 

12 

33 

7 

.6 

4500 

240 

Percent by weight 

23.0 

6.4 4.8 

3.2 4.1 

• 9 1.8 

.4 7.0 

.9 1.1 

.6 .4 

.6 • 3 

.3 7.1 

Parts per million 

710 635 

64 32 

51 74 

1850 2500 

8 13 

62 

170 

2150 

22 

6400 

84 

11 

35 

270 

7 

18 

1400 

770 

11 

14 

4 

2 

3500 

61 

17 

7 

2 

33 

6 

3100 

580 

.7 

99 

180 

230 

1200 

12 

8600 

290 

7 

65 

580 

27 

23 

44 

1100 

2000 

4 

24 

3 

3 

3000 

480 

6 

4 

14 

43 

8 

.7 

4000 

340 

Average of 

1 and 2 

22.0 

5.1 

4.4 

1.8 

7.8 

1.1 

.4 

.3 

6.6 

660 

30 

82 

2550 

9 

84 

175 

295 

1300 

15 

8000 

190 

6 

66 

630 

25 

20 

47 

1400 

1900 

4 

19 

4 

4 

3050 

455 

7 

5 

12 

42 

8 

.8 

3850 

305 

Crustal 

abundance7 

28.15 

8.23 

5.63 

2.33 

4.15 

2.36 

2.09 

.57 

.105 

0.07 

.004 

10 

425 

2.8 

.2 

60 

25 

100 

3 

55 

15 

3 

30 

950 

1.5 

20 

28 

75 

12.5 

.01 

.2 

22 

6 

2 

375 

2 

9.6 

2.7 

135 

33 

3 

70 

165 

C.F •

8 

0.8 

.6 

.8 

.8 

1.9 

.5 

.2 

.5 

63 

9400 

7500 

8.2 

6 

3.2 

420 

2.9 

12 

13 

5 

145 

13 

2 

2.2 

.7 

16 

1 

1.7 

19 

150 

400 

95 

.2 

.7 

1500 

1.2 

3.5 

.5 

4.4 

.3 

.2 

.3 

55 

1.8 

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 

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 

3 
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. 

Phosphorus is a major constituent of 

the ash, averaging 6.6 percent by our 

analysis, and is about double the average 

amount for other u.s. cities (table 3). 

Analyses of Toronto, Ontario, ash have an 

average of 5.1 percent phosphorus 

( Diosady, 197 5, p. 11). ~ The _reason for the 

high amount in the Palo Alto ash is not 

known. 

Table 3.--ChemicaZ. composition of ash of incinerated sewage 

sZ.udge from PaZ.o Az.to and other U.S. cities 

Palo Alto Other U.S. cities 1 

This Gabler and (Gabler and Neylan, 1977) 

Element Paper Neylan, 1977 Average Range 

81 -------

Al -------

Fe -------

Kg -------

Ca -------

Na -------

K -------

Ti -------

P -------

LOI 2 -----

Ag -------

Au -------

B ------- Ba -------

Be -------

Bi -------

Cd -------

Ce -------

Co -------

Cr -------

Cs ------- Cu -------

Ga -------

Hf -------

La -------

Li -------

Mn -------

Mo -------

Nb -------

Nd -------

Ni -------

Pb -------

Pd -------

Sb -------

Sc ------- Sm -------

Sn -------

Sr -------

Ta -------

Th -------

22.0 

5.1 

4.4 

1.8 

7.8 

1.1 

.4 

.3 

6.6 

660 

30 

82 

2550 

9 

84 

175 

295 

1300 

15 

8000 

190 

6 

66 

630 

25 

20 

47 

1400 

1900 

4 

19 

4 

4 

3050 

455 

U ------- 12 

v ------- 42 

y ------- 8 

Yb ------- • 8 

Zn ------- 3850 

Zr ------- 305 

Percent by we1ght 

21.0 

4.8 

4.3 

1.8 

9.3 

1.5 

1.2 

.5 

5.6 

3.0 

Parts 

575 

30 

8400 

120 

59 

290 

1500 

7300 

10 

600 

1500 

1500 

9 

1000 

170 

3800 

per million 

20.0 

4.0 

6.1 

2.0 

14.0 

1.4 

.8 

.6 

3.1 

3.8 

110 

5700 

83 

25 

31 

1200 

1200 

24 

1100 

360 

760 

330 

170 

3100 

9.3-42 

1.2- 7.0 

1.6-15 

.3-10 

.8-30 

.06-2.3 

.13-1.4 

.2-1.4 

1.2-5. 7 

.2-16 

24-280 

1200-8700 

39-150 

6-96 

11-57 

52-3600 

210-2200 

7-66 

260-2400 

19-1300 

160-1800 

110-1100 

12-950 

570-800 

1

Albany, N.Y,: Atlanta, Ga.; Cleveland, Ohio; Detroit, Mich.; 

Laure 1, Md; Minneapolis - St. Paul, Minn. (3 plants); Monterey, 

Calif; St. Louis, Mo. (2 plants); San Mateo, Calif.; and Savannah, 

Ga. 2LOI is loss on ignition. 

4 

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 

H20 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 water5 

insoluble 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 P2o5 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)~ 
Table 4.--Amounts of elements dissolved in HCL 

[Samples 1A and 1B from table 2. Part in A. I. is 

amount that remains in acid-insoluble part of 1A 

(percent in 1B multiplied by .53, the proportion of 

total in 1A that is acid insoluble). Part dissolved is 

total in 1A minus part of 1A that is acid insoluble. 

Percent dissolved is part of 1A dissolved relative to 

total in lA.] 

Element Sample Sample 

Si 

Al 

Fe 

Mg ----

Ca ----

Na ---- K 

Ti 

p 

Ag ----

Au ----

B ---- Ba ----

Be ----

Cd 

Ce 

Co 

Cr 

Cs 

Cu ----

Ga ----

Hf ----

La ----

Mn ---- Mo ----

Nb ----

Nd ----

Ni ---- Pb ----

Pd ----

Sb ----

Sc ----

Sm ----

Sn ----

Sr ----

Ta ----

Th 

u 

v 

y 

Yb ----

Zn ----

Zr ----

1A 1B 

17 

4.4 

5.4 

1.7 

8.7 

1.3 

.5 

.3 

7.4 

760 

34 

59 

2750 

5 

80 

200 

435 

1700 

22 

8200 

100 

6 

81 

650 

14 

17 

55 

2200 

2300 

5 

18 

4 

4 

3600 

400 

9 

7 

12 

33 

7 

.6 

4500 

240 

6.4 

3.2 

0.9 

0.4 

0.9 

0.6 

0.6 

0.3 

710 

64 

51 

1850 

8 

62 

170 

2150 

22 

6400 

84 

11 

35 

270 

7 

18 

1500 

770 

11 

14 

4 

2 

3500 

61 

17 

7 

2 

33 

6 

.7 

3100 

580 

Part Part Percent 

in A.I. dissolved dissolved 

3.4 

1.7 

.5 

.2 

.5 

.3 

.3 

.2 

380 

34 

27 

980 

4 

33 

90 

1100 

12 

3400 

44.5 

6 

19 

140 

4 

9.5 

795 

410 

6 

7 

2 

1 

1900 

32 

9 

4 

1 

17.5 

3 

.4 

1600 

310 

Percent by weight 

1.0 

3.7 

1.2 

8.5 

.8 

.2 

0 

7.2 

Parts per million 

23 

68.5 

71 

98 

62 

40 

0 

97 

380 50 

0 0 

32 54 

1770 64-

1 20 

167 

345 

600 

10 

4800 

55.5 

0 

62 

510 

10 

45.5 

1405 

1890 

0 

11 

2 

3 

1700 

368 

0 

3 

11 

15.5 

4 

.2 

2900 

0 

93.5 

79 

35 

45 

59 

55.5 

0 

77 

78 

71 

83 

64 

82 

0 

61 

50 

75 

47 

92 

0 

43 

92 

47 

57 

33 

64 

0 

Strontium and uranium probably occur in the 

calcium phosphate compounds as they do in 

natural phosphate deposits. Nearly 

insoluble .elements are gold, titanium, 

hafnium, palladium, and zirconium. The more 

abundant rare elements in the ash are 

dissolved in significant amounts: silver, 

50 percent; barium, 64 percent; chromium, 35 

percent; copper, 59 percent; nickel, 64 

percent; lead, 82 percent; tin, 47 percent; 

and zinc, 64 percent. 

It seems likely from these data that 

any process for making fertilizer from the 

ash will have to include a means of 

separating undesirable elements from the 

fertilizer product. The amount of dissolved 

6 

lead, for example, is especially large and 

potentially hazardous to health if it enters 

the food chain in fertilizer. 

Treatment of the ashes that were 

studied by Diosady (1975) and Oliver and 

Carey (1976) includes the recovery of a 

number of rare elements as well a high-quality 

phosphate fertilizer product. In general, 

the principal procedures include the solution of rare elements and phosphate in hot 

sulfuric acid solutions; separation of the 

rare elements by ion exchange and sulfide 

precipitation, or electrolysis; and the 

recovery of the phosphorus as phosphoric 

acid or ammonium phosphate. 

Phosphorus was found to be the resource 

of greatest value in the ashes investigated 

by Diosady (1975) and Oliver and Carey 

(1976), and Diosady concluded that only the 

recovery of phosphorus seemed economically 

possible. The recovery system for phosphorus and some other elements that was 

proposed by Oliver and Carey was considered 

likely to be economically feasible only for 

very large sewage plants. 

CONCLUSION 

Recycling of mineral resources is of 

increasing importance as the primary 

deposits become depleted or more costly to 

develop, as environmental concerns restrict 

or prohibit the development of new deposits, 

and as the nation strives to maintain or 

gain independence from foreign sources for 

some resources. 

No resources are recovered presently 

from ash of incinerated sewage sludge in the 

United States, although a potential is 

recognized, some research into recovery 

processes has been done, and additional work 

is underway. Part of the impetus for the 

recovery of element resources arises from 

the need to ensure that the ash is safe for 

disposal in the environment. It is the 

large number of potential resources in ash 

that poses difficult recovery problems as 

well as offering opportunities for 

recycling. 

The Palo Alto ash is especially rich in 

resources. The remarkably large amounts of 

gold, silver, phosphorus, tin, and copper, 

in particular, present an unusual opportunity for pioneer work in profitable 

recycling, which could lead also to the 

development of processes for the recovery of 

other element and mineral constituents in 

the ash. 
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. 

7 

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.