Mining in one form or another has existed since ancient times. The modern industry has evolved by incorporating gradual improvements into common practice. Mining in the United States can be classified in several ways. The classification used in this chapter recognizes four segments:
• Hard rock
• Sand and gravel
• Industrial (soft rock) minerals
Each of these categories can be further be sub categorized; moreover, each mine or deposit
has unique features. This chapter must neces sarily provide only overview discussions of each major category, but acknowledges the diversity of the industry, and of deposits and methods for any one type of product.
Hard rock mining produces ore for a variety of metals and minerals in the United States. Typi
cal operations at hard rock mines, whether underground or open pit, include drilling, blast
ing, ore transporting and stockpiling, and, usually, size reduction.
Water use in the context of hard rock mining refers to process water that is necessary for
routine functioning of the mine-mill complex, and not to incidental water such as excess mine
water, accumulated precipitation, or other “nuisance” sources of water that must be dissi
pated. Nevertheless, incidental water, including mine water or natural precipitation, may be
used for routine operation of the mine, if the mine is located in a water-short region.
Hard rock mines typically require water for drilling, and for any associated size reduction
facilities. Water consumption can be stated in terms of gallons of water per ton of ore pro
duced, except for production drilling and site dust control. For present purposes, size reduction is assumed to consist of crushing, wet screening, semi-autogenous grinding, and ball and rod mills (McNulty, pers. comm.)
Sand and Gravel
Sand and gravel are widely used as bedding material, in preparation of concrete mixes, and
in many other construction applications. An estimated 90 percent of commercial sand and
gravel is produced from “loose material.” Only about 10 percent comes from hard rock. The
following discussion describes water use during sand and gravel production from loose de
Step 1. In a typical operation, rock less than 12 inches, long dimension, is screened through
coarse bar screens (“grizzlies”) and the passing material is crushed in a jaw crusher to inter
mediate size rock.
Step 2. Coarse-crushed rock passes through a three-level screen, and oversize material is re
turned to the jaw crusher.
Step 3. The smallest, sand size fraction is stockpiled for use in concrete, while the inter mediate size rock fraction is either stockpiled for aggregate (nominally 1 inch and below), or
is further crushed in a gyratory cone crusher.
Step 4. Crushed intermediate material is screened, and oversized material is returned to the cone crusher, or further processed in a rolling mill or a vertical impact mill, depending on product specifications.
Step 5. a. The size fraction that passes the screen drops into a tank (or vat) from which sand size material (3/16 inch to ¼ inch and below) is withdrawn with a sand screw (about half of the installations). b. As an alternative, the fraction passing through the screen may be classified according to size in a gravity classifier (about half of the installations) to recover the sand fraction.
Step 6. Clays and silts are sent to a settling pond, from which decanted water is returned for use in the process.
Overall, a typical sand and gravel plant might produce 70 to 80 percent of its processed material as gravel and 20 to 30 percent as sand.
Clays and silts normally comprise less than 10 percent of a viable loose material deposit; the
settled clay mass might contain around 5 percent solids and 95 percent moisture.
Industrial Mineral Mining
A variety of minerals are mined for use in manufacturing, in construction, and for purposes other than heating value (coal) or metal recovery. Industrial mineral (“soft rock”) mining practices vary widely, according to the mineral produced and the nature of the deposits.
Two familiar examples of industrial minerals are kaolin (clay) and silica sand (used in glass
making). Each is mined and processed with different methods. Kaolin clay mining and processing serve as an example. Kaolin is used in a variety of industries, including paper manufacture, ceramics, and paint formulations. Papermaking uses a large amount of kaolin clay, and crude kaolin is not useful as a mined product until it is processed to remove impurities.
In Georgia, kaolin deposits are normally located and mapped from exploratory cores to depths typically from a few tens to about 200 feet. During mining, the overburden is stripped
from one to a few acres to expose the kaolin layer. No water is used in actual drilling, but up to 1,000 gallons per core may be used in exploration and mine development.
Relatively large volumes of water are used at kaolin processing plants, which are usually located some distance from the mining area. Mined kaolin is usually slurried near the mine
and transported to processing facilities through a pipeline as a clay suspension dispersed in
water. Although processing methods vary with the run-of-mine clay quality and the end use
for the processed clay, they usually include suspension or dispersion (deflocculation), screening, grit removal (e.g., gravity separation, centrifugation), flotation, brightening (e.g., magnetic separation, oxidation), flocculation, filtration, drying, and packaging.
Water usage varies with the specific operations needed to refine the clay for its end use, but a
nominal estimate from one source indicates typical usage is ~2,000 gallons per ton of finished product. Approximately 80 percent of the finished kaolin shipped to the paper industry is inslurry form, which is 70 percent kaolin and 30 percent water.
Coal is mined in a number of areas in the United States. It is used most extensively in electrical power generation, with coke making and byproduct chemical recovery among other
uses. In the eastern United States, coal is often mined underground, where risks of gas buildup cannot be tolerated. In the western United States, more coal is strip mined.
Water use in coal mining varies according to the method of mining, the equipment used, and the availability of water. Underground coal mines in West Virginia rely on the use of water for cooling the cutting surfaces of mining machinery and for inhibiting friction-induced ignition of coal fines or gas. Surface mines in the Western United States do not use water in actual mining, but they do suppress dust on haul roads with water and aqueous solutions of calcium chloride and magnesium chloride.
Statistical information about the use of water in coal mining is not available from readily accessible sources. However, one surface mine operator reported that aside from minor uses
for personnel (sanitary, showers, potable), equipment maintenance, and miscellaneous uses, the overwhelming use was for dust control. Dust control consumed about 5.2 gallons per ton of coal produced. In addition, small amounts of magnesium chloride solution (~0.01 gallon of solution per ton of coal) and calcium chloride (~0.003 gallons solution per ton of coal) were used to retain moisture, since both these salts are hygroscopic (take moisture from the air).