SURFACE MINING: AQUEOUS EXTRACTION METHODS
7.1 CLASSIFICATION OF METHODS In addition to mechanical extraction, there is a second class of surface mining methods. Aqueous extraction includes those methods which are reliant on water or a liquid solvent to recover minerals from the earth, either by hidraulick action or solution attack. Much less commonly employed than the mechanical methods (<10% of surface mineral production), the aqueous category consists of some of the oldest and newest mining methods. Restricted in their application, they neverthelees are attractive because of their very low relative cost: table 3.1 indicates relative costs of 5% or less. The aqueous extraction class in made up of two subclass, each consisting of two methods. Placer mining is one subclass, intended for the recovery of heavy minerals from mainly allvial of placer deposits, using water to excavate, transport,and/or concentrate the mineral. Solution mining is anther subclass, employed for the extraction from the earth of soluble or fusable minerals or those that can be slurried, using water or a liquid solvent. The two subclasses and four methods consist of following: A. Placer mining 1. Hydraulicking 2. Dredging B. Solution mining 1. Borehole extraction 2. Leaching In additionto being limited in their application, the four aqueous methods employ unique sequences of development and cycles of operations. This chapter discusses the major features of the four methods but, in view of their restricted use in modern mining, treats them more briefly than the mechanical references by Griffith (1960) and Macdonald (1983).
7.2 PLACER MINING: HYDRAULICKING Geologically, a placer deposit is a mechanical concentration of heavy mineral, which may be an ore deposit if commercial in value. Common occurrences of minerals in placers are gold, diamond, tin (cassiterite), titanium (rutile), platinum,tungsten (scheelite), chromite, magnetite, and phosphate (found in a placer-like deposit). Placers are classified by agent as alluvial (continental detrital), eolian (wind), marine, or glacial. By location, they are categorized as
residual (eluvial), bench (hillside),stream (fluvial), buried, or derert, commercially, the most important are, by agent, alluvial and marine and, by location, steam and beach. The distinctive qualities of placer deposits that permit aqueous mining are the following (Daily, 1968): 1. Material in place amenable to disintegration by the action of water under pressure (or mechanical plus hydraulic action) 2. Adequate water supply available at required head 3. Adequate space for waste disposal 4. Heavy concentrate is valuable mineral, amenable to simple mineral processing 5. Prevalent, low, natural gradient to permit hydraulic transport of mineral (if hydraulicking) 6. Can comply with environmental regulations regarding water clarity and waste disposal
The two principal methods of placer minig that lend themselves to high productivity are hydraulicking and dredging. In Hydraulicking, a high-pressure stream of water is directed against a placer bank to undercut and cave it (Fig. 7.1). As the bank disintegrates, the loosened placer material (mineral, sand, and gravel) is partially slurried in water and washed to a sluice, either a natural trough in the ground or a metal or wooden box, where it is transported by gravity to a riffle box or more elaborate concentrating device. Water is directed at the benk by a hydraulic monitor ar giant, a swivel-mounted nozzle, which is connected to a supply pipeline or header (Daily,1968). One of monitor, called an intellingiant, includes a section of curved pipe shaped in such a way that all reactive forces are absorbed in the until, thus eliminating anchors. The intelligiant can be automated to cycle through a vertical arc of 120 and near-full-circle horizontal rotation. In operation, two or more monitors are located close to the bank but at a safe distance and positioned so that they overlap while water- jetting a complete section of bank, cutting at an angle. Normally, the bank height is 15-50ft (5-15 m), but it may attain 150-200 ft (45-60m) (Morrison and Russell, 1973). Desigh specifications for monitors are the following: Nozzle diameter Pressure head Volume flow rate Water jet velocity: Gravel Boulders 1.5-in. (40-150 mm) 100-450 ft (30-140 m) or 45-200 Ib/in. (300-1400 kPa) 500-400 gal/min (30-250 L/sec) sand 30 ft/min (0.15 m/sec) 300 ft/min (1.5 m/sec) 600 ft/min (3.0 m/sec)
Hydraulicking is considered to have an intermediate production rate. It reportedly originated in Russia in 1830 ( macdonald, 1983). Sequence of Development In addition to possible stripping and the usual development work required in surface mining, there are several special steps necessary in hydraulicking, they relate to some of the distinctive qualities of placers just listed: 1. Provision of adequate water supply upstream 2. Location of suitable waste disposal area downstream 3. Control of water quality and reclamation of surface Stripping is usually minimal and performed by monitor or with any available stripping equipment. Preferably, water is supplied by gravity, fed in by stream or flume to a holding basin and stored at a pressure head sufficient to opeate the monitor (ordinarily,pumping is ruled out because of cost). A large-diameter header connects the basin to the monitors. Then a ground or box sluice is constructed to conduct the placer slurry away for processing and the barren debris remaining to the waste disposal area. Probably the most critical step in development for hydraulicking is environmental protection; provisions both to safeguard water quality and to reclaim the surface after mining must be made. Because of past pollution abuses, California and several other western states have outlawed hydraulicking, and the method has fallen into general disuse in this country (<1%of surface mineral production). Cycle of Operations Since both axcavation and handling are accomplished antrirely hydraulically, the production cycle is an abbreviated, unified one. No prior reakage and no subsequent transport are required. In fact, water is integral to the processing as well as to waste disposal. The simplicity of the cycle and multipurpose use of water accounts for the attractive productivity and cost of the method. Auxiliary operations other thn development are negligible, with reclamation incorporated into the cycle.
Conditions The following is based on Daily (1968a), Morrison and Russell (1973), and Hoppe, (1976): 1. 2. 3. 4. 5. 6. 7. 8. 9. Ore Strength: heavy mineral grains in unconsolidated soil or gravel matrix, few boulders Rock strength: unconsolidated Deposit shape: placer type, tabular, bank or beach Deposit dip: nearly flat (2-6% grade) Deposit size: small to intermediate (thickness 15-200 ft, or 5-60 m) Ore grade: can be very low Ore uniformity: fairly uniform Depth: very shallow, little overburden Other: large quantities of water (500-4000 gal/min, or 30-250 L/sec) at High heads (100-450 ft, 30-140 m)
Features The following is besed on Daily 1968a and Morrison and Russell (1973): Advantages 1. Fairly high productivity (100-300 yd , or 75-230 m, of gravel per employee-shift) 2. Low mining cost (relative cost:5%) 3. Intermediate production rate 4. Low capital cost; simple equipment and cycle 5. Can automate operations Disadvantages 1. Environmental damage savere unless elaborateprotection exercised (regardless, several states prohibit) 2. Extensive water requirements 3. Limited to unconsolidated deposists than disintegrate under hydraulic attack 4. Inefficient cutting action, difficult to contro
Applictions and Variations Hydraicking is little used today for placer mining in the Uited States (except in Alaska) but continues in use elsewhere (mainly in Australia for gold and southeast asia for tin). Occasionally, it finds application for other purposes, especially stripping. Specialized uses of stripping have bee removal of overburden in iron ore mining in Ontario (Li,1976b), frozen or thawed muck overlying gold placers in Alaska (daily, 1968b), and soft volcanicashes and tuffs which from the averburden of the bougainville copper deposit in Papua New Guinea (Thomas, 1978). In an unusual application, it is used in phosphate mining in Florida to slurry the ore matrix, a loose conglomerate (Hoppe,1976).
7.3 PLACER MINING : DREDGING The dredge may have been the first continuous mining machine invented; a crude device of this type was in use in the Netherlands in 1565 (macdonald,1983). Dredging is the underwater excavation of a placer deposit, usually carried out from a floating vessel which may incorporate processing and waste disposal facilities. The body of water may be natural or manmade;depending on the sizes of the dredge and deposit,from 200 to 2000 gal/min (13 to 125 L/sec) of water may be required for both mining and processing waste disposal (daily,1986a; Macdonald, 1983). Once popular in the united states, there are few placer mining dredges operating here today (except in one or two western states and Alaska ), but 130 are still used in mining elsewhere in the world. Foster (1984) estimates a total of 1350 are curerently active for mining and all other purposes. Dredges are classified as follows (Turner,1975): A. Mechanical 1.Bucket line (endless chain of buckets revolving along ladder) 2. Bucket- wheel suction (buckets discharge in suction pipeline) 3. Dipper (shovel,grapple, or dragline mounted on barge) B. Hidraulic 1. suction (open intake suction line) 2. Cutterhead (excavation by rotating cutter on suction line) The bucket-line dredge is the classical, wet, continous-excavating machine for poorly consolidated or loose materials with some boulders; hence its early application to placers. A recent, more versatile development , the bucket-wheel suction dredge ,mounts buckets on a rotating wheel and discharges excavators but are able to dig tougher, more consolidated materials with boulders; their use in the united states today is practically limited to sand and gravel deposits. Hydraulic dredges were adapted to placer mining from channel excavating; the suction type is restricted to sand and gravel, while the cutterhead type can manage consolidated materials. Both hydraulic dredges have limited application for placer mining because of low
heavy-metal recorvery and boulder restrictions, although they offer more continuous exvacation capability. We direct our attention mainly to bucket- line and bucket- wheel dredges because of their suitability for placer mining . in operation, a dredge carries its pond with it as it excavates the placer bankahead while depositing waste behind (Fig.7.2). Material dug by arotating bucket line is elevated on bord the dredge; the bucket ladder digs in a vertical arc up the placer bank, then steps over with the aid of a spud and shore lines to a fresh face. The sequence is pictured well by Stout (1980). The cut is about 1 ft (0.3m) deep and 5 ft (1.5m) wide. Dredges can excavate banks a maximum 50 ft (15m) above water level to 160 ft (50m) below (Morrison and Russell, 1973). With a bucket-line dredge, mineral processing is carried out on board. A tail sluice disposes of fine while a stacker conveyor discharges coarser material. The output of a bucket line dredge may be estimated from its design specifications. EXAMPLE 7.1 . Find the low to high output of a bucket-line dredge (in yd/day, or m/day) rated as following : Bucket capacity Bucket line spaad Shifts Bucket factor SOLUTION.
( )( )( )( )( )
10 ft (0.28 m) 22 bucket/min 3/day, 22.5 hr total 60-87%
Low ouput
= 6600 yd/day (5050 m/day)
High output
)(
)(
)(
)(
= 9600 yd/day (7340 m/day)
As arule thumb, one thus can estmate the output of a bucket-line dredge as 500-1000 yd/day per ft (13,500-27.000 m/day per m) of bucket capacity. A large dredge may excavate 9 milion yd (7 million m) of placer material per year. Bucket-line dredging is not only capable of production (it is a largescale method of mining),bt it is highly productive, perhaps the most productive of all methods. Capital costs for dredges in 1980 vary from $21,000 per m/hr) (Macdonald, 1983). Daily (1968b) quoes operating coasts as follows in 1962.
GAMBAR
BUCKET CAPACITY 6ft (4,6m) 7-8 (5.4-6.1) 12 (9.2) 13.5 (10.3)
18.0/yd 8.1-13.8 7.3-18.5 5.5-8.5
OPERATING COST (23.5 /m) (10.6-18.0) (9.5-24.2) (7.2-11.1)
Even corrected for current price levels,these are remarkably low costs, probably the cheapest for all mining,particulary in view of the fact that processing costs are included as well. The bucket-wheel suction dredge resembles and operates very much like its dry-land successor, the bucket-whell excavator (foster,1984). It can be built more compactly and cheaply than the bucket-line dredge (but processing must be done on shore), and it holds great promise in reviving placer mining in the united states. Its output can be estimated in a manner similar to that outlined in example 7.1. in excavating tailings, costs total about $0.75/ton ($0.83/tonne). Hydraulic dredges (fig.7.3) enjoy special applications in placer mining, mainly for excavating heavy beach sands and diamond ocean placer or stripping overburden under unusual circumstances-for example, sediment and glacial till overlying the steep rock iron core deposit in Ontario (pfleider,1973; turner,1975) Sequence of development There is little that differs in developing for placer mining,whether it is by hidraulicking or dredging. Again,provision of an adequate water supply is imperative (a pond is created by damming a stream or pumping water), as is suitable waste disposal and a sequenced reclamation plan. Overburden removal is minimal and done conventionally, unless overburden and placer are mined together without distinction (Pfleider,1973). Cycle of operations Since the dredge is a type of continuous mining machine ,no breakage is required , and materials handling (excavation plus transport) is conducted without interruption. Water aids in excavation, although muchthe digging is accomplished mechanically (except on a suction dredge). Mineral processing is almost always wet gravity concentration, ofter carried out on board the dredge. Waste is dumped sft of the dredge, fillng in the pond as it is excavated ahead of the dredge. Reclamation is conducted as an integral part of the operating cycle.
Conditions The Following is based on Daily (1968b), Morrison and Russel (1973), and Turner (1975): 1. Ore strength: mineral grains must be heavier than waste; placer-type deposit of unconsolidated soil or gravel matrix, with some boulders, depending on dredge type 2. Rock strength: unconsolidated 3. Deposit shape: placer type, tabular, bank or beach 4. Deposit dip: preferably flat (maximum 2-6% grade) 5. Deposit size: intermediate to large (thickness 25-200 ft, or 8-60 m) 6. Ore grade: can be very low 7. Ore uniformity: fairly uniform 8. Depth: very shallow, little overburden 9. Other: moderate quantities of water (200-2000 gal/min, or 13-125 L/sec, at atmospheric pressure) Features The following is based on Morrison and Russell (19730, Pfleider (1973a), Mocdonald, and Foster (1984): Advantages 1. Most productive of all methods ( 250-400yd, or 190-300 m,lat of gravel per employeeshift) 2. Lowest mining cost (relative cost: <5%) 3. High production rate (maximum 9 million yd, or 7 million m) 4. Low labor requirements (crew:10-30 employees) 5. Good recovery (approaches 90%), but accompanied by high dilution 6. Continuous operation, with no breakage required Disadvantages 1. Environmetal damage severe unless elaborate protection exercised (prohibited in some states) 2. Moderate water requirements (600-800gal/yd, or 3000 to 4000 L/m, of material mined) 3. Limited to unconsolidated deposits than disintegrate under hydraulic or comined attack 4. High capital investment with large dradges 5. Inflexible and unselective; limited to placer-type deposits
Applications and variations Little used today in the united states (2% of surface mineral production), dredging finds application for placer mining in other parts the world, mainly in South America,Southeast Asia,and the Far North (Daily, 1968b; Pfleider,1973a; Macdonald,1983; Foster, 1984; Lewis, 1984). Nearly all placer minerals are produced by dredges.In this country, dredging is occasionally employed in mining for stripping averburden ( e.g.,phosphate deposits in the U.S. Southeast). Dry-land dredging is a variation of placer mining used with conventional surface excavating equipment in arid regions. Case Study. Lee Creek phosphate mine, Texasguif company, Lee Creek, NC.
7.4 SOLUTION MINING: BOREHOLE EXTRACTION As conventional ore production becomes more difficult and costly, the attractions of solution mining as a primary or secondary exploitation methond increase. Solution mining is the subclass of aqueous surface mining methods in which minerals are recovered, usually in place, by dissolution, melting, leaching, or slurrying. (Even though certain development or exploitation. 6.6a. Modifly the design of the blasting pattern for the surface mine in Example 6.2 by increasing the powder factor to 0.6. what is the effect on hole spacing and number of holes required? b. Desigh a blasting pattern to conserve explosives (lower the powder factor) in Example 6.2 by modifying the hole burden,depth,and/or spacing.
