The grade of an ore

The grade of an ore is the concentration of the desired material within the rock. There is more metal (a
higher concentration) in higher grade metal ores. Ore grades are often given in percentages or in units of
ppm (defined below).
Percentages, which most of us are familiar with through our class grades, are actually a measurement of
“parts per total parts.” In the case of grades on an exam, if you received a 92% on your 100-point exam,
you received 92 points out of a possible 100 points. This is similar to ores: If a nickel ore has a grade of
2%, it means there are 2 pounds of nickel for every 100 pounds of ore. It also means that there are 2
grams of nickel for every 100 grams of ore, etc., so long as the unit of comparison remains the same
within the percentage calculation (pounds to pounds or grams to grams or ounces to ounces, etc.).
1) Some sources say that the average single family home in the United States uses about 420 pounds
of copper within the plumbing, appliances, building wire, and more. In the twenty-first century,
an average copper grade ore might be ~0.6%.
a. How many pounds of 0.6% grade copper ore needs to be mined in order to obtain 420
pounds of copper? Show your work here
Another frequently used unit of measurement for ore grades is “parts per million” or ppm. Instead of
finding the concentration per 100 parts, like the percentage, ppm finds concentration out of one million
parts. This unit is used to represent metals that often occur in smaller concentrations. A gold ore with a 2
ppm ore grade would have 2 pounds of gold for every 1,000,000 pounds of ore. An equivalent unit is
grams/ton (since there are 1,000,000 grams, or 106 grams, in a metric ton). Thus a 2 ppm grade gold ore
would also have 2 grams of gold for every ton of ore.
2) A gold coin called a Krugerrand has approximately 30 grams of gold in it.
a. How many metric tons of 15 ppm grade gold ore need to be mined in order to get enough
gold for a single Krugerrand? Show your work here. Remember that 1 metric ton = 1x106
grams.
b. How much waste product (in metric tons) is created? Show your work here