Assignment
Geoguy: "So how much of that super chunk chocolate is in a scoop of your super chunk chocolate ice cream?" Geogal: "Well, in order to tell you I need to take a scoop of the ice cream and drop it on this weighing scale (splat), weigh it, then pick out all those chocolate chunks and drop them on another weighing scale (thud), then take the weight of the chunks and divide by the weight of the scoop, multiply by 100 and there you have it: an eye-popping 42 percent (by weight) of super chunk chocolate in my super chunk chocolate ice cream"
In lecture we talked about the importance of mineral resources in sustaining our society and that every American consumes about 9 kilograms, or 20 pounds, of copper per year. That figure is based on an estimate of the national demand for copper to make such things as electrical wire, tubing, and electronic components (to name just a few) divided by the U.S. population. Like other metals, the copper you use comes from minerals, which in turn are extracted from earth's crust. Remember from lecture that valuable minerals do not occur in large pockets of pure, solid masses but are widely dispersed throughout earth's crust and mined where geological processes have enriched their concentration. This homework problem will help you appreciate the value of metal in minerals and the volume of rock material that must be processed to meet your annual demand for copper.
Background
A common "ore" of copper is the mineral chalcopyrite. Chalcopyrite is often the principal ore mineral in "porphyry copper deposits" as presented in class. You have analyzed the physical properties of chalcopyrite in laboratory so you already know some things about it or where to go to refresh your memory. Chalcopyrite is a sulfide mineral so this means it contains more than just copper. In addition to sulfur (S), chalcopyrite contains some iron (Fe) and the proportions of these metals in the mineral are expressed by its chemical formula: CuFeS2. So how much copper (Cu), say by weight percent, is in chalcopyrite?
Let's use another copper sulfide mineral as an example of how this calculation is made. How much copper is in the mineral chalcocite? Chalcocite is another ore mineral of copper. In order to determine this one needs to know the chemical formula and atomic weights of the included chemical elements. The chemical formula for chalcocite is Cu2S. The atomic weight of the chemical elements Cu and S are 63.55 and 32.07, respectively (this info can be retrieved from a Periodic Table of the Elements like the one presented in lecture). This means that the chemical formula weight of chalcocite is:
2 x 63.55 (Cu2) + 32.07 (S) = 159.17 grams (Cu2S)
and the weight percent of Cu in chalcocite is:
127.10 (Cu2) divided by 159.17 (Cu2S) x 100 = 79.85%
therefore, nearly 80% of the weight of any given chalcocite mineral is copper. So, if you had a 10 kilogram (2.2 pound) mega-crystal of chalcocite, it would include 8 kilograms (1.76 pounds) of copper.
Questions (clearly show your work):
1. What is the weight percent of sulfur (S) in chalcocite?
2. What is the weight percent of copper in chalcopyrite?
3. Which would contain more copper-a hundred kilograms (kg) of chalcopyrite or 100 kg of chalcocite?
4. Remember that porphyry copper deposits often have additional ore grade minerals such as molybdenite (MoS2). An average ore grade for the Chino porphyry copper deposit is 0.74 wt% copper and 0.012 wt.% molybdenum. Current metal prices can be found at:
https://www.infomine.com/investment/metal-prices/
Calculate the combined value of copper and molybdenum per metric ton (for pricing, assume molybdenum = molybdenum oxide).
5. In a typical copper deposit in the southwest U.S., about 14 kg of chalcopyrite is contained in every metric ton (1000 kg) of rock mined. How many metric tons of rock must be mined to meet your copper demand in a year? (hint: remember that chalcopyrite is not 100% copper!)
6. If we assume that U.S. population is 310 million people (2015), calculate the annual amount of rock mined (in metric tons) to meet total U.S. copper demand.
(To help you gauge the volume of your result--a full-size pickup can carry about 1 ton of rock.)
7. Describe what happens to all that mined rock (waste rock) after the chalcopyrite is extracted and then describe two specific environmental impacts of waste mine rock from this type of deposit. (You should strive for three meaningful paragraphs. No one or two sentence response. Show me you've researched the question.)