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# SURVEY OF U.S. MINERAL AND METAL PROCESS - page 9 / 12

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be reprocessed to recover some of the valuable components. The value (or cost) could be determined by the cost of transport and disposal of the sludge.

Table 2. Economic variables for separation of impurities from process residue

# Economic Variable

<50

40

Concentration of impurities, ppm 400

# A

Value of process residue before separating impurities

need data-may be negative

Value of process residue after separating impurities

I

Primary sludge)

process

residue

(e.g.,

acid

plant

need data

# Secondary process residue (e.g., mercury) need data

need data need data

need data need data

t

technology (G

-

# Net value of separating impurit

ies

1

-

I

Primary process residue (D

J

Secondary process residue (E

-

F)

calculate

calculate

calculate

calculate

calculate

calculate

• -

7

t

# L

nrocess

residue Primary process residue

# Secondary process residue

Highest value in row I Highest value in row J

The primary process residue (acid plant sludge) may have a market value that is a function of the concentration of impurities (mercury) (Table 2, Row B). For example, process residue may be sold to a smelter to recover commodity metals (lead, silver, gold). If the residue includes impurities, such as mercury, the price’the smelter pays the copper smelter could be more or less depending on the amount of impurity. Secondary process residue (mercury) may also have a market value (Table 2, Row C). Separation technology would tend to increase the value of the process residue to buyers because impurities would be removed. In our example, mercury separation technology reduces the concentration of mercury from 1300 to 250 ppm at a cost of \$500 (U.S.) per ton (Table 2, Row F).

Mines have existing methods for handling their process residue and could estimate these costs (Table 2, Row G). In our example, the copper mine used a less effective separations technology before using the innovative technology-the cost of this “old technology” would be the “next best” technology referred to in Row G.

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