Aqueous Extraction Methods in Surface Mining: Exhaustive Study Guide for Surface Mining

Classification and Overview of Aqueous Extraction Methods

Aqueous extraction encompasses a group of mineral recovery methods that utilize water or liquid solvents to extract minerals from the earth or broken rock. These methods typically leverage both the hydraulic action of the fluid, which is usually water, and chemical solution attacks on the minerals themselves. While these techniques account for approximately $10\%$ of surface mineral production and are used less frequently than mechanical methods, the percentage of minerals mined through aqueous methods is increasing significantly for certain mineral types. These methods are generally characterized as being comparatively inexpensive.

The aqueous extraction category is divided into two primary subclasses known as placer mining and solution mining. Placer mining involves the recovery of heavy minerals from alluvial or placer deposits, using water for excavation, transport, and concentration. This subclass includes hydraulic mining and dredging, which is further divided into shallow-water and deep-sea dredging. Solution mining is utilized for recovering minerals that are soluble, fusible, or can be converted into a slurry. This process typically employs ordinary water, heated water, or liquid solvents. Specific solution mining techniques include borehole extraction (multiple well methods), leaching procedures (such as dump, heap, and in-situ leaching), and evaporite or evaporation methods. Although these methods have historically had limited applications, leaching methods have seen increased use recently, particularly in the gold and copper mining industries.

Placer Mining: Technical Foundations and Hydraulic Methods

A placer deposit is a concentration of minerals re-deposited in an unconsolidated form through the action of a fluid, with most being sand and gravel deposits deposited by water. These deposits serve as sources for gold, diamonds, tin, titanium, platinum, tungsten, chromite, magnetite, coal, phosphates, and sand and gravel. For a placer to be minable by aqueous methods, several properties must be met: the material must be amenable to disintegration by water pressure or hydraulic transport; an adequate water supply must be available at the required head or pressure; there must be adequate space for waste disposal; there must be a sufficient density difference between the ore and gangue for separation; the natural gradient must support hydraulic transport; and the operation must comply with environmental regulations.

In hydraulic mining, a high-pressure stream of water, often referred to as water-jet cutting, is directed against a bank to undercut and cave it. As the material disintegrates, particles are carried in water as a slurry into a sluice—either a natural ground trough or a wooden/metal box—and then transported by gravity to a concentrating device like a riffle box. Water is applied via a hydraulic monitor or giant nozzle. A specific design known as the Intelligiant uses curved pipe sections to balance reactive forces, making anchoring and control easier. Some monitors are mounted on mobile carriers and programmed to move back and forth across the bank. The optimal distance for a monitor from the washing face is considered to be $100$ nozzle diameters to maximize impact energy, though safety and health considerations may necessitate a greater distance. Operational risks include stones being ejected if the jet hits a reflector like a flat stone.

Specifications, Cycle, and Characteristics of Hydraulic Mining

The design specifications for hydraulic monitors usually involve a nozzle diameter between $40$ and $150\,mm$, with operating pressures ranging from $300$ to $1,400\,kPa$. The volume flow rate typically falls between $77$ and $510\,Litres/sec$. Water jet velocities vary based on the material being moved: sand requires roughly $0.15\,m/sec$, gravel requires $1.5\,m/sec$, and boulders require $3.0\,m/sec$. Development for these operations requires a water supply (preferably upstream) and a waste disposal area (preferably downstream). Regulatory approval is difficult to obtain because hydraulic mining significantly disturbs land and water, leading many jurisdictions to outlaw the practice. Development requires a pump station or holding basin with sufficient head and large-diameter header pipes.

The production cycle is abbreviated because excavation and materials handling are both achieved hydraulically, requiring no prior breakage or separate transport. Productivity is fairly high, ranging from $75$ to $230\,m^3$ per employee-shift, with a low relative mining cost of about $5\%$. Capital costs are low due to simple equipment. However, the environmental damage is severe, water requirements are extensive, and the process is limited to unconsolidated deposits. While little used in the United States today outside of Alaska, hydraulic mining remains active globally for gold and tin and is used in Florida to slurry phosphate pebble mined by draglines. More recently, it has been applied to the re-mining of dewatered tailings dams.

Placer Mining: Dredging Technology and Output Calculations

Dredging is the underwater excavation of placer deposits from floating vessels, which may include processing and waste disposal facilities. Depending on the size, dredging requires between $13$ and $125\,L/sec$ of water. Dredges are categorized as mechanical (dipper, bucket, and ladder) or hydraulic (suction). Primary types used for mineral recovery are bucket-line dredges, cutter-head suction dredges, and cutter-wheel suction dredges. Bucket-line dredges were historically used for gold extraction at depths up to $50\,m$. They use a bucket ladder controlled by a crane, and waste is discarded via a stacking conveyor. The dredge moves using back-mounted spuds and wire ropes anchored to banks, often carrying its own human-made pond as it advances. Hydraulic suction dredges use slurry transport and typically perform processing on land rather than on the vessel, with cutter-heads freeing material for intake. Digging depths for these units are commonly up to $18\,m$.

The massive scale of dredging operations allows for high productivity. For example, the output of a bucket-line dredge can be calculated based on bucket capacity ($0.28\,m^3$), speed ($22\,\text{buckets/min}$), total shift time ($22.5\,\text{hrs/day}$), and a fill factor ranging from $60\%$ to $87\%$. The calculation for low output is as follows: $\text{Low output} = (0.28\,m^3) \times (22\,\text{buckets/min}) \times (60\,\text{min/hr}) \times (22.5\,\text{hr/day}) \times (0.60) = 5,000\,m^3/day$ The high output calculation is as follows: $\text{High output} = (0.28\,m^3) \times (22\,\text{buckets/min}) \times (60\,\text{min/hr}) \times (22.5\,\text{hr/day}) \times (0.87) = 7,200\,m^3/day$

Dredging is arguably the most productive of all mining methods, with gravel productivity between $190$ and $300\,m^3$ per employee-shift and annual production reaching $7\,million\,m^3$. However, capital costs are high, ranging from $\$9\,million$ for smaller bucket-line units to $\$50\,million$ for large dredges with $0.85\,m^3$ buckets. The water requirements for dredging are approximately $3,000$ to $4,000\,L/m^3$ of material mined.

Solution Mining: Borehole and Leaching Systems

Solution mining recovers minerals through leaching, dissolution, melting, or slurrying. Borehole extraction uses wells to access minerals, injecting water or a lixiviant to extract values. Key applications include the melting of sulfur, in-situ leaching of uranium, copper, gold, and silver, the dissolution of salt, potash, and trona, and the slurrying of phosphate, kaolin, oil sands, coal, and uranium. Leaching specifically refers to chemical extraction from a deposit or mined material, sometimes aided by bacteriological catalysts. Percolation leaching and flooded leaching are the two primary variations. In-situ leaching occurs on minerals in place, while heap or dump leaching involves stacking mined ore on impermeable pads and irrigating it with chemical solutions to create a "pregnant solution."

Modern heap leaching is the primary method for gold and silver recovery, often conducted on pads made of asphalt, impervious soil, or geomembranes (PVC or polyethylene). Heaps are typically $3$ to $100\,m$ thick and are leached for $30$ to $90$ days, though thicker heaps take longer. Lixiviants are applied via sprinklers, wobblers, or drip emitters. Environmental control is vital, requiring a water balance that accounts for rainfall and evaporation; gold leaching is ideally performed in climates where evaporation balances precipitation. Remediation involves neutralizing acids or using bacteria to destroy cyanide. Development sequence involves clearing vegetation, grading a $5\%$ slope for drainage, constructing pads and ponds, crushing ore, spreading it in lifts of $3\,m$ or more, and initiating irrigation. Advantages include low mining costs (averaging $5\%$ relative cost) and the ability to process low-grade deposits, while disadvantages include potential groundwater contamination and the hazard lixiviant ponds pose to wildlife, sometimes requiring protective netting.

Evaporite and Evaporation Operations

An evaporite is a sedimentary deposit, such as halite ($NaCl$), potash, or trona, produced by evaporation in a closed basin. These are recovered through solution mining by evaporating water from brines in solar ponds, leveraging solar energy in warm, dry climates. The sequence of development for a lake brine operation includes locating large flat areas, laying a fine-grained bed (sand or clay) for a geomembrane, ensuring containment integrity, and initiating solution flow. The cycle involves matching the inflow rate with the net evaporation rate, allowing solar energy to crystallize minerals, and harvesting the minerals while leaving a protective salt layer over the pond lining.

Evaporation methods are highly specialized and require large land areas and moderate water supplies, but they offer the advantages of low labor costs, the use of free solar energy, and high recovery rates. In the United States, a large percentage of lithium, boron, and magnesium production is derived from solution mining. Lithium and boron are typically extracted from lake brines, while magnesium is sourced from lake brines, well brines, and seawater. These operations represent a growing segment of the mining industry as conventional ore production becomes more costly and difficult.