CHAPTER 29MINING

Water uses in the mining and mineral processing industries may be divided intotwo parts. The first includes all uses of water and its treatment for removing themineral from its surroundings, both in underground environments and also insurface stripping operations. The second deals with the processing of theserecovered mineral ores. During these operations the beneficial mineral that hasbeen mined is separated from valueless companion materials, normally by grind-ing and processing fine ore particles as a slurry. Water is used to prepare the slurryand to transport the mineral during this processing operation. Both aspects ofwater usage—in mining and in mineral processing—will be discussed here.

The most widespread use of water in underground mining operations is fordust control. Most automatic mining equipment includes an integral water spraysystem. Spray nozzles are supplied by a water hose coupling at the rear of themining machine. The spray nozzles are set up to provide a water curtain aroundthe area being mined when the mechanized drill, cutter, or continuous miner isin operation, thereby confining the dust at the face (Figure 29.1).

One of the major problems with such underground water systems is cloggingof nozzles by scale and dirt particles in the water supply. Usually some type ofstrainer or filter immediately ahead of the mining equipment traps these particlesto prevent their reaching the small nozzle orifices. Often, stabilization reagentsare added to this water line to prevent scale formation if the water is hard orcontains iron or manganese.

Another use for underground water is hydraulic mining. With this technique,water under high pressure is directed toward the face of the mine. The energy ofthe water stream breaks the mineral from the mine wall and washes it into a col-lection system. This type of mining may gain popularity as it becomes betterunderstood. Currently its main use has been in the coal industry and to a limitedextent the uranium industry. The cutting action of the water stream can beenhanced by coalescing the water into a very narrow stream, focusing its energyon the selected area of the mine face. Certain types of water-soluble polymers areused to give this cohesiveness to the water stream, thereby increasing miningefficiency.

UNDERGROUND WATER

In these examples of underground water use—dust control and hydraulic min-ing—it may be necessary to pump fresh water underground. However, in mostinstances there is already extensive underground seepage, and it is necessary to

FIG. 29.1 This continuous coal-mining machine is equipped withwater spray nozzles to suppress dust at the working surface. (Courtesyof Lee-Norse Company.)

intimately mixed with the coal produce an acidic water, and treatment of acidmine drainage is a serious problem for this industry (Figure 29.2).

Bacterial action is believed to be responsible for the production of acid fromthe pyritic material, and the result is a water often at a pH below 3, containing upto 1000 mg/L Fe and with sulfate concentrations as high as 4000 mg/L. Not onlyis the cost of treatment a burden, but the by-product lime sludge is also a seriousproblem, often requiring large land areas for sludge impoundment.

It is often necessary to chemically treat underground water to prevent attackon pipes and pumps used to transport it to the surface. In many cases the water

remove this from the mine to continue efficient operations. This water must oftenbe pumped over appreciable distances from the mine floor to the surface for dis-posal. In many cases, it contains significant concentrations of dissolved solidsleached from geologic formations. It may be acidic as a result of the types of min-erals it has contacted. In the coal industry, reactions with forms of iron pyrites

FIG. 29.2 Flow sheet of a treatment system to handle about 300 gal/min (1635 m3/day) of coalmine drainage having an acidity of 1600 mg/L. Sludge recycle is essential to production of a dense,settleable precipitate.

Sludge recyc le Sludge to lagoon

Treatede f f l uen tTh ickener -c lan f ie rAera t ion-neut ra l i za t ion

Minedra inageFeeder

Limes i lo

Water

has run through muddy areas and picked up a significant load of suspended solids,making it necessary to provide flocculation and coagulation of the solids prior topumping the water to the surface.

Once at the surface, this drainage water is often added to the general watersystem for the mineral processing plant. Occasionally it requires further treatmentto adjust pH or remove suspended solids precipitated upon exposure to the air.After pH adjustment with lime, most heavy metals precipitate. Settling basins arethen required to clarify the water prior to discharge or recycling into the generalplant water system.

Often in underground mining, a shaft or tunnel provides the approach to theore body to be mined. As ore is withdrawn and the cavity (stope) becomesenlarged, it becomes necessary to regrade the floor of the stope at frequent inter-vals to make the mine face accessible to the miners and their machines and drills.

So, where the method of stope mining is practiced, tailings from the mineralprocessing plant are often used as backfill to provide the floor for the rising stope.These tailings compact in the stope, and water drains from beneath. This wateris often the predominant portion of the mine drainage discussed earlier, whichmust be clarified before being pumped to the surface. Flocculant addition to thetailings often improves drainage and provides a more compact backfill in thestope area.

Very few minerals are mined and used directly without further treatment. Theraw ore is either physically or chemically processed to remove the inert materials(gangue) from the desired mineral. This is normally done in a wet milling oper-ation. In wet milling, the ore is crushed, slurried with water, and subjected toseveral techniques capable of separating the valued mineral from the gangue.

COAL PROCESSING

In the coal industry, various types of shale and clay are produced as a mixturewith the coal. To increase the heating value of the coal and to reduce the haulingcost, a complex process of coal washing is normally used to reduce the total ashcontent. In this process the coal is graded to a certain size, usually less than 6 in,and then fed into a slurry bath in which the density of the media is closely con-trolled. The coal floats in this heavy media bath while the heavier rock sinks tothe bottom.

Following this heavy media separation, all the floated material is again sizedby vibrating screens for further purification. The smaller-sized fractions may beprocessed by shaking tables, hydrocyclones, or froth flotation. In each of thesesteps, coal is recovered and dried prior to shipment. The refuse is dewatered asmuch as possible by screens.

However, the final water effluent, after the larger materials have been removed,usually contains a significant concentration of very fine refuse in suspension. Thismight include some coal not recovered during the washing operations. Primarily,however, it contains sand and small pieces of rock and clay in a slurry of 3 to 15%solids. To close the coal preparation plant water system and minimize or elimi-nate effluent flow, it is necessary to dewater this refuse slurry as efficiently as pos-sible. Normally a thickener is used to compact the solids into a mud of 30 to 40%solids by weight. The overflow from the thickener is normally of good enoughquality to be reused in the coal washing plant.

FIG. 29.3 Flow sheet of coal washing and drying plant. (Reprinted from Coal Age, January1976. Copyright McGraw-Hill, Inc.)

FURTHER DEWA TERING

The underflow slurry from the thickener is still pumpable but not yet suitable forfinal discharge. In the past, this slurry was discharged into tailings ponds andlagoons, and allowed to settle under its own gravity. Today, coal washing plantsare closing their plant water systems entirely, requiring them to further dewater

Exhaust system

Self cleaning

magnet

Raw coal silos

Screen bowlcentrifuges

Classifying cyclones

Conditioner Dilution

Coarse coalflotation units

Fine coalflotation unitsMetering

pumps

Frothreagentfacilities

Meteringpumps

Stilling box

Distributor

Spiral classifier

Refusetransfer bin

Refusesurgebin Bypass

Surgehopper

Refusedisposalbin

Feeder

FIG. 29.3 (Cont.)

the refuse slurry to a dry, handleable form. The moisture must be reduced so thatthe refuse is dry enough to be trucked to a disposal site to be used as landfill.

Several techniques can accomplish this: vacuum filtration, centrifugation, andvibrating refuse screens. The key is to dewater the refuse slurry to the point wherethere is no subsequent water runoff when the dry solids are disposed of.

A complete water circuit for a modern coal washing plant is shown in Figure29.3. Analyses of a typical makeup water and concentrated circuit waters areshown in Figure 29.4. There is a consistent increase in sodium, sulfate, and chlo-ride. Lime is often added to the circuit to offset the development of acidity.

Legend

Raw coalClean coalRefuseHeavy mediaDilute mediaDilution waterFlocculantReagent

Magnetic separators

To heavy-media sumpPre wet screens

Sample cutterSplitter

Raw coalscreens

Distributors

Deister tables Fixedscreen

Fine coalhorizontalcentrifuge

Heavy mediasump

Screwfeeders

Rawmagnetite

Classifyingcyclone

feed sump

Pumps

Distribution header

Dip tubebox

DAR screen

Coarse coalhorizontalcentrifuge

Scrubberrecirculatingpumps

Exhaust fan

To thickener

Clean-coalsilo

Makeup

Two stage samplerScale

MixersFlocculantfacilities

Meteringpump

To screensflotation & tables

PumpsClarifiedwatersump

Clean coaldouble-rollcrusher

Clean coal 1transfer bin

Reciprocating

Dilute mediasump

FIG. 29.4 Selected coal mine water analyses.

METAL-CONTAINING MINERALS

In the base metal industries, such as copper, lead-zinc, and iron ore processing, itis necessary to grind the mineral to such a fine size as to produce discrete particlesof metal-rich minerals and quartz, feldspar, and other worthless materials. In thecopper, lead, and zinc industries, the liberated metal-rich particle normally con-sists of a sulfide mineral, which is then amenable to separation and recovery byfroth flotation techniques. In the iron ore or taconite industry, the iron is presentas the mineral magnetite, FeO-Fe2O3, which lends itself to separation by magnetictechniques. To recover minerals by either flotation or magnetic separation, theore is ground in a wet condition to improve efficiency. Large volumes of waterare used to slurry the dry ore.

Identification of Analyses Tabulated Below:

A Plant A—makeup D "B" recirculated water

B "A" recirculated water E Plant C—makeup

C. Plant B—makeup p. "C" recirculated water

Constituent

CalciumMagnesiumSodium

Total Electrolyte

Bicarbonate

CarbonateHydroxylSulfateChloride

Nitrate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDS (Conductivity)Color

As

CaCO,

CaCO,

CaCO,

CaCO,

SiOFe

A7431

644

749

170OO9

570

170O

7.8

160.2

1500

B350190

1368

1908

18OO

2901600

18O

7 .3

21.5

3800

C D E F60 44 490 95022 14 140 340

260 528 79 161

342 586 709 1451

88 118 220 O0 0 0 00 0 0 0

230 410 450 140024 58 39 51

88 118 220 O0 0 0 0

8.0 7 .8 7 .6 4 .0

3 4 15 235 28 0.1 0.7

650 1200 1200 2200

During the hard rock grinding process, steel balls and rods are used to pulver-ize the mineral ore (Figure 29.5). In these mills, the grinding media are consumedat the rate of about 1 Ib steel per ton of ore. This metal loss is due to physicalabrasion and chemical corrosion. Chemical inhibitors can effectively reduce therate of corrosion and subsequent wear of the steel balls and rods.

After separating the desirable mineral from the tailings, the final slurry is sentto a tailings thickener just as in the coal washing operation. Again, the tailingsslurry is thickened in the thickener and the clarified water returned to the pro-cessing plant for reuse. The underflow slurry may be disposed of in large opentailings ponds where natural evaporation and seepage occur. If there is any sig-nificant breakout of free water in the tailings pond, this water is either dischargedto a stream or recycled to the main plant water system.

The flow sheet for a copper mining operation is shown in Figure 29.6, includ-ing both the crushing and concentration steps.

In all thickening operations, the large demand for clear water and the size lim-itations of thickeners and settling ponds make it necessary to use synthetic floc-culants and coagulants to aid in the settling process. These synthetic separationaids enable the plant to obtain all the desired recycled water necessary along withincreased compaction and dewatering of the refuse solids. The size of thickenersand settling ponds is clearly illustrated in Figure 29.7, an aerial view of a coppermine in a mountainous area of Arizona.

However, no matter how efficient the thickening operation may be, a certainamount of process water always is lost to tailings with the slurry solids. Conse-quently it is necessary to add a quantity of fresh makeup water to maintain thebalance of the plant water system.

FIG. 29.5 Fine copper ore is ground in these ball mills, each 16 ft 6 in diameter by 19 ft long.Each mill is driven by a 3000-hp motor. (Courtesy ofDuval Corporation, Steams-Roger, Incor-porated, and Ray Manley Photography.)

In certain industries, such as copper, lead, and zinc where the sulfide mineralsare floated at high pH, the addition of fresh makeup water containing significantcalcium and magnesium hardness can result in unstable water (a low Ryznarindex, high Langelier index). The refuse slurry which is clarified in the thickenercontains sufficient alkalinity to react with the fresh calcium and magnesiumadded with the makeup water. Therefore, it is advantageous to add the makeupto the clarified recycled water in a vessel where precipitation of carbonates andhydroxides will not pose a major scaling problem. It has been found that additionof makeup directly to the thickener eliminates most problems.

However, it is also necessary to stabilize this combined water prior to sendingit back to the plant. If this is not done, a significant buildup of scale can form instorage tanks as well as pipes and pump parts, causing problems and increasingplant maintenance. Analysis of makeup and recirculated process water from sev-eral copper mills is shown in Table 29.1.

FIG. 29.6 Copper ore processing flow sheet, (a} Crushing sequence; (b) concentrating sequence.

Open pitmine

Crusher

Conveyer ' Coarseore pile

Secondarycrusher

Tert iarycrusher

/Fine orestorage

TailingsFroth

MoIy flotation

Waterreuse

Copper-molythickener

FrothFlotationcells

Ballmills

Fine

Tailingsthickener

Water

reuse

Tail

Tailing pond

Waterreuse

MoIythickener

Waterreuse

CopperthickenerFilter

MoIyconcentrate MoIy cleaner

flotation cells

Waterreuse

Water

reuseCopperconcentrate

PHOSPHA TE MINING

Phosphorus-bearing earth is mined at two major locations in the United States,Florida and the western mountain states, with minor production in Arkansas,Tennessee, and North Carolina. Ninety percent of the production is located inabout 2000 square miles of Florida.

The phosphate industry ranks with coal, iron, and copper as a major tonnageprocessor of bulk material, surpassed only by stone, sand, and gravel. The ore ismined by both surface and underground methods, but the former accounts foralmost 98% of annual tonnage. The overburden from surface mining exceeds 200million tons annually, for a crude oil production of about 100 million tons. Abouttwo-thirds of the crude ore is discarded in beneficiation.

FIG. 29.7 Aerial view of Duval copper-mining operation, Sahuarita, Arizona. (Courtesy ofDuval Corporation, Steams-Roger, Incorporated, and Ray Manley Photography.)

TABLE 29.1 Water Quality Comparison of Fresh Makeup Water and Recirculated MillProcess Water for Several Western Copper Concentrators

(Concentrations in milligrams per liter)

Mill

A. MakeupClarified

B. MakeupClarified

C. MakeupClarified

Ca

651410

601890

P

4280

20560

12214

M

136360

115630

60254

O

O200

O490

O164

PH

8.711.1

8.411.7

8.111.5

CO3

8160

40140

2490

HCO3

128O

75O

36O

TDS

3003000

2702000

5801300

The overburden is essentially silica sand overgrown with vegetation. Theunderlying phosphorus-bearing matrix has a variable mineral composition, beinga mixture of carbonate-fluorapatite (20 to 25%); quartz (30 to 35%); clays, chieflymontmorillonite (25 to 35%); and the balance feldspar, dolomite, and heavy min-erals. Huge draglines separate the overburden from the matrix, and the matrix isthen slurried by hydraulic guns and transported to the processing plant at about30 to 35% solids.

At the beneficiation plant, the maximum phosphate values are extracted fromthe matrix. The processes are relatively simple, yet no two plants are exactly alikebecause of differences in the matrix screen analysis and the proportions of phos-phate, clay, and sand in the matrix. The unit operations always include washing,feed preparation, and concentration (Figure 29.8). In the washing operation, par-ticle size classifiers produce a product of about 1 in to +14 mesh with a BPL(bone phosphate of lime) value of 65 to 70%. This portion of the matrix is knownas pebble product. The — 14 mesh fraction moves to the second stage where fineclays are removed so that they do not interfere with the effectiveness of down-stream chemical conditioning agents. This "desliming" process is accomplishedby hydraulic sizing devices, such as hydrocyclones. The slimy, fine clay fractionis discharged to slime ponds. Because the mineral composition of the phosphate-bearing matrix is variable, the slime composition is also variable, usually con-taining 10 to 20% P2O5 and high concentrations of silica, iron, lime, and alumina.

FIG. 29.8 Flow sheet of phosphate surface mining and beneficiation.

Phosphate matrix

Overburden

Matrix pile

Slurrypumps

HydraulicgunDragline

Storeoverburden

Water

Matrix slurryat 30-35% solids

Mud ballscalper

Pebble screen

Logwasher

+ 14Mesh

Cyclone

Screen 14 x 35 mesh14-150Mesh+ 14 mesh

Pebbleproductstorage

-35mesh

Classifier

Reagentconditioners flotation

Tailings

Acidagitator Waste

Fineconcentrate

De-oiling

Coarseconcentration

Reagentconditioner

SecondaryClassifier

CoarseconcentrateSilica

flotation

Froth to waste

De-oilingscrewclassifier

After desliming, the slurry is fractionated into 14 X 35 mesh and 35 X 150 meshportions for further concentration by removal of silica. This is accomplishedthrough the use of spirals, belt flotation, or froth flotation. Froth flotation is themost common practice and consists of:

1. Floating the phosphate with a tall oil fatty acid at an alkaline pH (float product:rougher concentrate; sink product: silica tailings).

2. De-oiling the rougher concentrate with sulfuric acid followed by rinsing.3. Floating residual silica with an amine at a neutral or slightly alkaline pH (float

product: silica tailings; sink product: final concentrate); the final concentrategenerally has a BPL value of 70 to 74%.

Wastes generated by the mining and beneficiation steps include the overbur-den, clay slimes, and silica tailings. The mines usually reclaim about 75% of themined acreage for citrus groves, timber stands, wildlife preserves, pasture land,and recreational use. The balance of the land is used for slime and tailings storageand for recirculation water reservoirs. Silica tailings can be used for slime ponddam construction; the clay slimes create a serious problem in that they cannotcompact beyond about 30% solids by natural gravity sedimentation. In the orig-inal matrix, the clays are present at >60% solids, but swell as they absorb waterduring the slurrying process.

MINERAL LEACHING AND DISSOLVING

Other mineral industries use aqueous solutions to leach or dissolve the desiredmineral from its ore. Included among these industries are uranium, bauxite forthe recovery of alumina, soda ash for the recovery of sodium carbonate, potashfor the recovery of potassium, and phosphate rock which processes naturallyoccurring phosphate ore to phosphoric acid for fertilizer intermediates. Aqueoussolutions containing various leaching materials (acids, alkalies, or brines) dissolvethe valuable mineral from the undesirable constituents in the ore. In most oper-ations of this type, a series of countercurrent decantation (CCD) thickeners areused to recover as much soluble mineral as possible from the mud refuse. Floc-culants are required in this process to compact the mud levels to the greatestextent and to yield as clear a pregnant liquor as possible for ultimate mineralrecovery.

In addition to chemical leaching processes, there are three somewhat relatedmineral recovery operations: bacterial leaching, hot-water melting, and waterdissolution.

Almost 20% of the copper recovered from tailings is put into solution by bac-terial action, and the extract is then concentrated by conventional methods torecover the copper. As in any biological digestion process (Chapter 23), it isimportant to maintain a rather uniform aqueous environment, as the bacterialactivity is upset by changes in temperature, pH, and nutrient levels; and the rateof activity depends on these factors and the food level, which in turn determinethe time the bacteria must be detained in contact with the copper tailings to effec-tively dissolve the copper from the mineral substrate. It is theorized that manymetallic compounds have been naturally concentrated in their ore bodies by bio-logical processes, so it is likely that even more applications of bacterial leachingwill materialize in the future.

The largest application of hot-water melting is the mining of native sulfur inthe United States by the Frasch process (Figure 29.9). Deposits of pure sulfur aregenerally located 500 to 2000 ft (150 to 600 m) below the surface of the earth,often off-shore below the sea. Six-inch shafts are bored into the deposit, and atriple, concentric pipe system is inserted; the center pipe delivers compressed airto the deposit; the outside annulus delivers water heated to 34O0F (1710C), abovethe melting point of sulfur; and the inner annulus collects the molten sulfur-watermixture, which is lifted to the surface in slugs by the bubbling compressed air.

FIG. 29.9 Frasch process for melting and air-lifting mol-ten sulfur from deposit to surface.

The mixture is fed into a bin, where the water drains off and the sulfur solidifies.The water required for the Frasch process is about 2000 gal per ton (8.34 m3/t) ofsulfur mined. If fresh water is used, it is usually lime-softened and deaerated toprotect the extensive piping from scale and corrosion. If seawater is used, it isdeaerated and stabilized. Since the water is heated to 34O0F (1710C), a large boilerinstallation is required for the energy needs.

Water dissolution is used to mine both salt and sodium carbonate-bicarbonatemixtures. In both cases, the final product is usually a crystalline product, so theamount of water used must be minimized to reduce the energy cost ofcrystallization.

In several of these industries, process steam is required to supply the heat nec-essary for maximum leaching. Operations of this type require an effective boilerwater treatment program to maintain maximum efficiencies. In those metalindustries in which smelters are used to reduce the ore to its elemental form,waste heat boilers are used to convert heat from smelter gases into steam. These

Native ,sulfurdeposit

Rock

Waterat 34O0F(1710C)

Hot compressed air

Froth of sulfur,water, and airto bins

waste heat units gather additional heat energy by cooling the sides of the furnacesand other liquid metal transferring vessels. The steam generated by the processheat is then utilized for power generation as well as other process purposes.

Associated also with these heavily process-oriented industries are significantcooling water applications. For instance, in many of the metal sulfide smeltingoperations, acid plants have been built to recover the sulfur dioxide from wastegases. These recovery plants yield sulfuric acid, which can in turn be used to leachother types of metal mineral ores. Cooling water is a major problem for thesesatellite plants because of the salinity of the limited water sources in mining areasand the atmospheric contamination.

In summary, the mineral processing industry is continuing to close as many ofits water systems as possible. To a great extent this industry has grown up withthe philosophy of water recovery, having been forced for many years to reusewater because of its large water requirements for milling and mineral processing.Many of these plants are in water-short areas of the country. Consequently mostmineral processing plants have always had thickeners to clarify their plant waste-waters (Figure 29.7). What is planned is to further dewater the thickener under-flow slurries to produce dry, handleable solids as required for landfill, completelyclosing the system. Large tailings ponds are still used in the western states whereenough land is available for long-term sedimentation. In these tailings ponds, thenatural process of evaporation usually produces a dry, disposable refuse material.

The mineral processing industry needs all the clear water it can obtain for usein the processing of its ores. It will continue to improve its techniques for therecycling of its process waters. The primary objective is clarity. It is necessary toremove suspended solids to convert the wastewater to a usable quality for furthermineral slurrying. Often pH control is necessary. Consequently, the addition ofacid or alkali to the recycled water is necessary to bring the water into balance forscale and corrosion control. In most cases very little is done to reduce dissolvedminerals in the effluent. The addition of fresh makeup water at a continuous ratemomentarily dilutes the buildup of dissolved minerals. But in a closed system,there is a large increase in dissolved solids in the circuit (Table 29.1), and the onlypoint of discharge of these solids is in the water lost with the final tailings dis-charge to landfill. The main water treatment process in the mineral processingindustry is solids/liquids separation in the tailings system (Figure 29.10). The

FIG. 29.10 Tailings clarifier-thickeners at a large western copper concentrator.

effective use of synthetic polymers to improve solids/liquids separation efficiencyenables the mineral processing industry to conserve water and minimize thestrain on the environment.

SLURRY CONVEYING

One water-related operation in the mining industry that is advancing rapidly ishydraulic transport of minerals. Several pipelines are already in operation, themost notable being the Black Mesa coal pipeline feeding an electric generatingstation in Nevada. There are many indications that additional slurry pipelineswill be needed to transport minerals such as coal and iron ore. The major concernin such slurry pipelines is that water is removed from the environment at onelocation and transported to another. Some slurry pipelines proposed for the morearid regions of Montana and Wyoming have encountered public oppositionbecause they represent an export of water from the region. Perhaps this may beresolved by intermittent use of the pipeline to return water and refuse to theiroriginal source.

The pipeline system lends itself to some interesting water treatment problems.Of course, a chemical that could be added to the slurry while it is in the pipelineto increase the pumping efficiency could be of importance to the operation. Var-ious materials which exhibit friction-reducing tendencies are being evaluated. Inaddition, there are usually periods when the pipeline is idle, normally filled withwater. Under such conditions, corrosion begins to take place under the layer ofsolids on the bottom. Therefore, it is necessary to chemically treat these waterslugs used to fill the gaps in the line between various quantities of the mineralslurries, to prevent this corrosive action. Because of the huge volumes of waterinvolved, economics of this once-through system are of great concern.