Evaluating precious metal content in the processing of scrap materials

A method for evaluating a batch of scrap material, subsequent to processing of the batch, to determine the content of precious metal present in the batch prior to processing of the batch, includes introducing a predetermined amount of a tracer into the batch, prior to processing, processing the batch and the tracer into a homogenous mixture, assaying a sample portion of the homogenous mixture to assess the amount of tracer in the sample portion, and to assess the amount of precious metal in the sample portion, and ascertaining the content of the precious metal in the batch prior to processing of the batch by applying to the assayed amount of the precious metal a ratio between the predetermined amount of tracer introduced into the batch prior to processing and the assessed amount of tracer in the sample portion. Where the precious metal includes gold, silver, platinum, palladium or rhodium, a tracer includes one or more of the metals bismuth, antimony, lead, cadmium, tellurium, barium, cobalt, nickel, hafnium and selenium.

The present invention relates generally to the recovery of precious metals 
from scrap materials which bear precious metals and pertains, more 
specifically, to method and materials used in the method for verifying the 
amount of precious metal present in a batch of scrap material to be 
processed for recovery of the precious metal, by evaluation of a sample of 
the batch taken subsequent to processing the batch. 
A variety of industries generate scrap materials which contain precious 
metals. Usually, these scrap materials are sold to scrap processors who 
will recover the precious metals from the scrap materials. Thus, jewelry 
manufacturers, semiconductor producers, precious metal plating operators 
and photographic processors are some of the industries which generate 
scrap materials bearing precious metals such as, for example, gold, 
silver, platinum, palladium and rhodium. 
The handling and evaluation of precious metal bearing scrap material 
presents problems to both the seller of the scrap material and the 
purchaser in that neither the seller nor the purchaser can know the 
precise amount of precious metal in a batch of scrap material prior to 
processing of the batch. Often, a seller must rely upon the purchaser to 
report the amount of precious metal recovered from a particular batch of 
scrap material, leading to some uncertainty in the value of the scrap 
material prior to processing. This uncertainty in the value of the scrap 
material has created an atmosphere of skepticism, insecurity and even 
distrust among sellers and buyers of scrap materials. 
The present invention enables determining, with a high degree of certainty, 
the amount of precious metal present in a given batch of scrap material 
prior to processing of the batch, by evaluation of a sample of the 
processed batch. As such, the present invention attains several objects 
and advantages, some of which are summarized as follows: Enables the 
determination, with precision, of the amount of precious metal contained 
within a heterogeneous mixture of scrap materials in a batch of scrap 
material, prior to processing the batch, by evaluating a sample of the 
processed scrap material; assures that all of the amount of precious metal 
in a batch of precious metal bearing scrap material is accounted for 
subsequent to processing of the batch; instills confidence in both the 
seller and in the purchaser of precious metal bearing scrap materials in 
that the value of the scrap material is verified with accuracy and 
reliability; deters unauthorized removal of any quantity of precious metal 
from precious metal bearing scrap material; provides better control over 
the processing of scrap materials for the recovery of constituents of the 
scrap material; allows a seller of scrap material bearing precious metal 
to assure compensation for the full value of the scrap material with 
relative ease and minimal expense. 
The above objects and advantages, as well as further objects and 
advantages, are attained by the present invention which may be described 
briefly as a method for evaluating a batch of scrap material of known 
quantity, subsequent to processing of the batch, to determine the content 
of at least one precious metal present in the batch prior to processing of 
the batch, the precious metal including one or more of the metals gold, 
silver, platinum, palladium and rhodium, the method comprising: 
introducing a predetermined amount of a tracer into the batch of scrap 
material of known quantity, prior to processing of the batch; processing 
the batch of scrap material and the tracer into a homogenous mixture; 
assaying a sample portion of a prescribed quantity of the homogenous 
mixture to assess the amount of tracer in the sample portion, and to 
assess the amount of precious metal in the sample portion; and 
ascertaining the content of the precious metal in the batch of scrap 
material prior to processing of the batch by applying to the assayed 
amount of the precious metal a ratio between the predetermined amount of 
tracer introduced into the batch prior to processing and the assessed 
amount of tracer in the sample portion. 
Further, the invention includes a tracer for use in evaluating a batch of 
scrap material of known quantity, subsequent to processing of the batch, 
to determine the content of precious metal present in the batch prior to 
processing of the batch, the precious metal including one or more of the 
metal gold, silver, platinum, palladium and rhodium, the tracer comprising 
at least one metal selected from the group consisting of bismuth, 
antimony, lead, cadmium, tellurium, barium, cobalt, nickel, hafnium and 
selenium. 
Ordinarily, a seller of precious metal bearing scrap material will deliver 
a lot of heterogeneous scrap material of known quantity to a processor who 
then processes the lot to recover the precious metal. In order to evaluate 
a lot, the lot is processed into a homogeneous batch of scrap material and 
a sample portion of a prescribed quantity of the homogenous batch is 
assayed, either by the processor or by the seller, in order to assess the 
amount of precious metal in the sample portion. By directly relating the 
prescribed quantity of the sample portion to the known quantity of the lot 
of scrap material, the amount of precious metal in the lot, prior to 
processing the lot, and hence the value of the lot, can be calculated. 
Thus, if B is the weight of the lot of scrap material delivered to the 
processor, S is the weight of the sample portion, and P.sub.a is the 
weight of the precious metal actually derived from the sample portion, 
then the total amount P of the precious metal in the lot of scrap material 
can be expressed as follows: 
EQU P=P.sub.a B/S 
However, the accuracy of the amount P derived from the above expression 
requires that the full lot B be homogenized before a sample portion S is 
taken so that the calculation yields a true amount for P. Should a part of 
the lot be deleted prior to processing the scrap material into a 
homogenous batch, either deliberately or by an inaccuracy in processing, 
or by some other error, the amount of precious metal P.sub.a actually 
derived from the sample portion will be reduced, with a proportional 
reduction in the calculated amount P, and a concomitant reduction in the 
reported value of the lot of scrap material. The present invention enables 
detection of any discrepancy between the full amount of precious metal 
actually present in a batch of precious metal bearing scrap material prior 
to processing of the batch, and the amount of precious metal reported to 
be in the batch prior to processing of the batch, as a result of the assay 
of a sample portion of the batch subsequent to processing of the batch. 
The term "processing" as employed herein includes at least homogenizing 
the lot of scrap material, as outlined above. 
In the practice of the present invention, a measured, predetermined amount 
of a tracer is added to a lot of scrap material bearing precious metal. 
The lot is delivered for processing in the form of a batch containing a 
heterogeneous mixture of materials, and the tracer includes at least one 
metal, and preferably more than one metal, not usually found in the batch. 
The batch then is processed to homogenize the materials in the batch. For 
example, metallic scrap is melted and formed into ingots which then are 
weighed. A sample portion in the form of a prescribed quantity of the 
processed batch then is assayed to determine the amount of tracer in the 
sample portion and to determine the amount of precious metal in the sample 
portion. 
A ratio then is determined between the assessed amount of the tracer and an 
amount of the tracer expected to be found in the sample portion, based 
upon the predetermined amount of tracer originally placed in the batch, 
prior to processing of the batch, the known quantity of material in the 
batch prior to processing, and the prescribed quantity of material in the 
sample portion. The ratio then is applied to the assayed amount of the 
precious metal to determine the content of the precious metal in the batch 
of scrap material prior to processing of the batch, based upon the known 
quantity of material in the batch prior to processing and the prescribed 
quantity of material in the sample portion. 
While the tracer may be a single metal not found in the scrap material, the 
preferred tracer is in the form of a mixture of metals, each of which 
metals is chosen on the basis of the chemical behavior of the tracer 
materials being similar to the chemical behavior exhibited by the precious 
metal in the scrap material when subjected to the processing techniques 
and evaluation procedures ordinarily conducted during the processing of 
the scrap material. In this manner, the tracer will "follow" the precious 
metal during processing so as to be present in the processed batch in the 
same predetermined amount as introduced to the batch prior to processing. 
Preferred tracers include a mixture of constituents selected from the 
group consisting of bismuth, antimony, lead, cadmium, tellurium, barium, 
cobalt, nickel, hafnium and selenium. In view of the uncertainties in the 
composition of any lot of scrap material, the use of a mixture of 
constituents in the tracer assures that at least one of the constituents, 
and preferably more than one, will be a metal not found in a particular 
lot of scrap material and will therefore serve as an accurate indicator 
for the purpose of auditing precious metal content of the scrap material 
in accordance with the present invention. One mixture found to be 
exceptionally effective in the practice of the present invention is made 
up of about 65% by weight of tellurium, about 25% by weight of bismuth and 
about 10% by weight of selenium. Tracers are prepared from commercially 
available certified pure metallic powders. The selected powders are 
weighed, blended together and then a sample of the blended mixture is 
analyzed, as by atomic absorption spectroscopy (AAS), to verify the 
content of the blended powders. 
The manner in which a tracer is introduced into the scrap material is 
dependent upon the nature of the scrap material. Thus, where the scrap 
material is a lot of metallic scrap to be processed by melting, the tracer 
preferably is introduced in a jellied form prepared by mixing the blended 
powder mixture with petroleum jelly. The resultant jellied tracer powder 
is assayed to certify the content of the constituents and a measured 
amount of the jellied tracer powder is applied to the bulk of the scrap 
material and allowed to flow around all surfaces. Care is taken to assure 
that the amount of tracer added to the lot is known with precision. In the 
preferred preparation, an aliquot of blended tracer powder includes 250 
grams of blended powder which is mixed with 500 grams of petroleum jelly 
and the resultant jellied tracer is placed into dispensing tubes for 
subsequent delivery. The amount of tracer introduced is governed by the 
lot size of the scrap material. Preferably, enough tracer is added to the 
lot to yield at least fifty parts per million (50 ppm) of tracer in the 
final processed lot. 
Where the lot of scrap material is to be processed by incineration, with 
the resultant ash ground and screened to a homogeneous residue for 
sampling, the tracer preferably is delivered to the lot by combining a 
known weight of blended tracer powder mixture with a urethane resin and 
placing the combined powder mixture and urethane resin into a container, 
such as a commercially available plastic bag, which is then sealed for 
delivery to the lot of scrap material. In the preferred preparation, an 
aliquot of blended tracer powder includes 250 grams of blended powder, 250 
grams of urethane resin is added to the aliquot, and the combined blended 
powder and urethane resin is sealed in the plastic bag for delivery. A 
jellied tracer, as described above, also may be used in connection with 
the lot. Preferably, enough tracer is added to the lot to yield at least 
one hundred parts per million (100 ppm) of tracer in the residue. 
Where the lot of scrap material is a solution which is to be sampled as a 
solution, the tracer preferably is delivered in the form of a tracer 
solution. An aliquot of blended tracer powder is dissolved in warm 
concentrated nitric acid. The resultant solution is diluted with deionized 
water, sampled to verify content, and then sealed in ampules for delivery. 
In the preferred preparation, the aliquot of blended tracer powder 
includes 150 grams of blended powder and the dissolved powder is diluted 
to one liter, into which one mole of NaCN has been dissolved, and the 
solution is sealed within 100 ml. ampules. Upon delivery, the tracer 
solution is added to the lot of scrap material. The preferred 
concentration of tracer solution in the lot of scrap material is at least 
100 cc. (one ampule) per 100 gallons of scrap material. 
Subsequent to processing a batch of scrap material to which a tracer has 
been added, as described above, a sample portion comprised of a prescribed 
quantity of the material of the processed batch is assayed for tracer 
content and either the same sample or another sample of the processed 
batch is assayed for precious metal content. In one method, the tracer is 
assayed by inductively coupled plasma spectroscopy (ICP) analysis of a 
solution derived from nitric acid leaching. In an alternate method, atomic 
absorption spectroscopy (AAS) is used to assay the solution derived from 
nitric acid extraction of the sample. In either method, the extraction is 
proven to be quantitative. 
Once the actual amount of tracer in the sample portion is ascertained, a 
ratio R is determined between an amount of tracer expected to be found in 
the sample portion, based upon the predetermined amount of the tracer, the 
known quantity of material in the batch prior to processing, and the 
prescribed quantity of material in the sample portion assayed for the 
tracer, and the actual amount of tracer found in the sample portion. Thus, 
if T is the weight of the predetermined amount of tracer delivered to the 
batch of scrap material prior to processing of the batch, T.sub.e is the 
weight of tracer expected to be found, and T.sub.a is the weight of tracer 
actually found in the assayed sample, and if B is the weight of the known 
amount of material in the batch prior to processing and S is the weight of 
the prescribed amount of the sample portion assessed for tracer, then the 
ratio R is T.sub.e /T.sub.a where T.sub.e =T S/B. If the weight (T.sub.a) 
of tracer actually found in the assayed sample matches the weight 
(T.sub.e) of tracer expected to be found, then the ratio R is 1, 
confirming that the actual amount of precious metal in the batch, prior to 
processing, can be determined directly from the amount of precious metal 
derived from an assay of a sample of prescribed quantity, on the basis of 
a comparison of that prescribed quantity with the known amount of material 
in the batch prior to processing, as described above. However, if the 
ratio R is greater than 1, that is, if the weight (T.sub.a) of tracer 
actually found falls short of the weight (T.sub.e) of tracer expected to 
be found in the assayed sample portion, that becomes an indication that 
there is a shortage of precious metal in the processed batch of material. 
Should a shortage be detected, an investigation can be made into whether 
the shortage is a result of inadvertent processing errors or deliberate 
acts occurring between submission of the lot of scrap material for 
processing and the taking of a sample portion subsequent to processing of 
a batch. 
Using the ratio R, the true, full precious metal content of the batch, 
prior to processing, can be determined. Thus, if P.sub.a is the weight of 
the precious metal actually derived from assaying a sample portion of 
prescribed quantity S of the material of the batch, then the full amount 
P.sub.f of precious metal in the batch prior to processing can be derived 
on the basis of the weight B of the known quantity of material in the 
batch prior to processing, the prescribed quantity S of material in the 
sample portion, and the ratio R. Hence, the full amount of precious metal 
in the batch prior to processing can be expressed as follows: 
EQU P.sub.f =R P.sub.a B/S, or 
EQU P.sub.f =P.sub.a T/T.sub.a 
Accordingly, through the use of a tracer, as described above, a seller of 
scrap material can determine, through analysis of a sample portion of the 
scrap material taken subsequent to processing, the amount of precious 
metal in the batch of scrap material prior to processing. For example, 
where a seller of precious metal bearing scrap material introduces a 
tracer into a lot of scrap material and then delivers the lot of scrap 
material to a processor, the processor will return a sample portion of the 
processed lot to the seller. The seller then can assay the sample portion 
for precious metal and for the tracer introduced by the seller into the 
lot, thereby auditing the lot for an accurate determination of the amount 
of precious metal in the lot prior to delivery of the lot to the 
processor. Any shortage of precious metal indicated by the analysis of the 
sample portion then can be investigated to determine if the shortage is a 
result of deliberate removal of precious metal content, of processing 
errors or some other inaccuracies in the processing of the scrap material. 
In any event, the procedure enables an accurate determination of the 
amount of precious metal present in the lot of scrap material prior to 
delivery of the lot to the processor.

The following examples demonstrate the practice of the present invention: 
EXAMPLE 1 
A batch of scrap material of known quantity in the form of a heterogenous 
mixture of scrap metal having a weight of 1000 kg was processed for 
recovery of an unknown amount of precious metal content. Prior to 
processing, 100 grams of a tracer was added to the batch. The tracer 
included a mixture of 65% by weight of tellurium, 25% by weight of bismuth 
and 10% by weight of selenium. The known quantity of scrap metal in the 
batch, together with the predetermined amount of tracer, then was melted 
to form a homogenous mixture of constituents. A sample portion of 31.1 
grams of the homogenous mixture was assayed and found to include 3.11 
grams of precious metal in the form of gold, together with 6.5 ppm of 
tellurium, 2.5 ppm of bismuth and 1.0 ppm of selenium. 
Based solely upon the actual weight of gold found in the assay of the 
sample portion, the weight of the sample portion itself, and the weight of 
the known quantity of scrap metal in the batch, the amount of gold 
(P.sub.i) indicated to be in the batch prior to processing can be 
calculated as follows: 
EQU P.sub.i =P.sub.a B/S 
The ratio R is determined as follows: 
EQU R=T.sub.e /T.sub.a where T.sub.e is T S/B so that 
EQU R=1. 
The full amount of gold in the batch prior to processing is derived as 
follows: 
EQU P.sub.f =R P.sub.a B/S=100 kg; or 
EQU P.sub.f =P.sub.a T/T.sub.a =100 kg. 
Thus, the assay of the sample subsequent to processing of the batch 
confirms that the amount of gold in the batch, prior to processing, indeed 
was 100 kg and that all of the gold is present in the homogenized mixture. 
EXAMPLE 2 
A batch of scrap material of known quantity in the form of a heterogenous 
mixture of scrap metal having a weight of 1000 kg was delivered for 
processing for recovery of an unknown amount of precious metal content. 
Prior to processing, 100 grams of a tracer was added to the batch. The 
tracer included a mixture of 65% by weight of tellurium, 25% by weight of 
bismuth and 10% by weight of selenium. Subsequent to adding the tracer, 50 
kg of the batch of scrap material deliberately was removed and set aside. 
The remaining quantity of scrap metal in the batch, together with the 
predetermined amount of tracer, then was melted to form a homogenous 
mixture of constituents. A sample portion of 31.1 grams of the homogenous 
mixture was assayed and found to include 2.954 grams of gold, together 
with 6.17 ppm of tellurium, 2.37 ppm of bismuth and 0.95 ppm of selenium. 
Based solely upon the actual weight of gold found in the assay of the 
sample portion, the weight of the sample portion itself, and the weight of 
the known quantity of scrap metal in the batch, the amount of gold 
(P.sub.i) indicated to be in the batch prior to processing can be 
calculated as follows: 
EQU P.sub.i =P.sub.a B/S=94.983 kg 
The ratio R is determined as follows: 
EQU R=T.sub.e /T.sub.a where T.sub.e is T S/B so that 
EQU R=1.053 
The full amount of gold in the delivered batch prior to processing is 
derived as follows: 
EQU P.sub.f =R P.sub.a B/S=100 kg; or 
EQU P.sub.f =P.sub.a T/T.sub.a =100 kg. 
Thus, the assay of the sample subsequent to processing of the batch 
revealed a discrepancy between the amount of gold indicated to be in the 
delivered batch, prior to processing, and the full amount of gold 
determined by applying to the assayed amount of gold (P.sub.a) a ratio 
between the predetermined amount of tracer (T) introduced into the batch 
prior to processing and the assessed amount of tracer (T.sub.a) in the 
sample portion, demonstrating that not all of the gold was present in the 
processed homogenized mixture from which the sample portion was taken. The 
discrepancy introduced by deliberate removal of a portion of the delivered 
batch was detected and the true value of the delivered batch is 
ascertained. 
It will be seen that the present invention attains the objects and 
advantages summarized above, namely: Enables the determination, with 
precision, of the amount of precious metal contained within a 
heterogeneous mixture of scrap materials in a batch of scrap material, 
prior to processing the batch, by evaluating a sample of the processed 
scrap material; assures that all of the amount of precious metal in a 
batch of precious metal bearing scrap material is accounted for subsequent 
to processing of the batch; instills confidence in both the seller and in 
the purchaser of precious metal bearing scrap materials in that the value 
of the scrap material is verified with accuracy and reliability; deters 
unauthorized removal of any quantity of precious metal from precious metal 
bearing scrap material; provides better control over the processing of 
scrap materials for the recovery of constituents of the scrap material; 
allows a seller of scrap material bearing precious metal to assure 
compensation for the full value of the scrap material with relative ease 
and minimal expense. 
It is to be understood that the above detailed description of preferred 
embodiments of the invention is provided by way of example only. Various 
details of procedure and composition may be modified without departing 
from the true spirit and scope of the invention, as set forth in the 
appended claims.