What impact did the products of mines described in the passage along with metals extracted from other mines have on Spain and the rest of the world?

8.2.3.3 In situ leaching

ISL (also often referred to as “in-situ recovery”) is becoming the more prevalent approach to mine uranium in comparison to open-pit and underground mining techniques. The premise of ISL is to exploit the local geology of some orebodies that may be in somewhat porous material by dissolving the ore in solution, which can then be relatively easily recovered. These uranium-bearing species are typically insoluble in groundwater. The general extraction process involves injecting a leaching agent called “lixiviant” into the ore body (below the water table), dissolving uranium bearing species in solution, recovering the uranium bearing solution (sometimes referred to as “pregnant solution”) to the surface by pumping the material through production wells.

There are three principle advantages of ISL in comparison to conventional methods: (1) reduced risk to operators by avoiding underground mining; (2) stripping, mining, and milling of the ore on the surface is unnecessary, which is a great advantage as it minimizes the burden of dealing with tailings; (3) as a concomitant effect to the last point, there is a reduced amount of radioactivity reaching the surface; (4) one is able to extract lower grade ore than is possible with other techniques; and (5) there is minimal surface disturbance with the least environmental impact [6]. The greatest disadvantage of ISL is that there is a lower recovery rate in comparison to other methods. Also, there is a risk of potentially contaminating ground water. Clearly, this operation must be far removed from any source of water that may become potable.

Uranium deposits suitable for ISL are in permeable sand or sandstone that are confined by adjacent impermeable structures, which facilitates the chemical solution injection and recovery. A lixiviant is pumped into the orebody (below the water table) via a borehole for the purpose of dissolving the uranium in solution, thereby mobilizing it. A large number of lixiviant materials are used that broadly fall under two categories: acids and carbonate reactants. Commonly used reactants include sulfuric acid, nitric acid, hydrochloric acid, sodium bicarbonate, and hydrogen peroxide [7]. Sulfuric acid offers high leach performance at a relatively low cost but produces residues (e.g., gypsum), which degrades performance [7].

The choice of lixiviant often depends on the amount of calcium in the ore. If calcium content is significant, alkaline leaching is typically used; otherwise, acid leaching is used, which typically yields higher recovery rates at a lower cost [7]. One advantage of alkaline leaching is that undesired impurities are less likely to be dissolved in solution; however, this can also be a disadvantage as uranium minerals must be directly exposed to the solution in order to permit extraction [4].

The general process used in acid leaching is illustrated in Fig. 8.3. After the ore is received in the mill, it may go through a grinding process, as described in the previous section. Then, the chemical process involving sulfuric acid in ISL is described in the following partial reactions, beginning with dissolution [7]:

(8.1)UO3,s+2Haq+→ UO2,aq2++H2O

then oxidation,

(8.2)UO2,s+0.5O2+2Haq+→UO2,aq2++H2O

a ferrous oxidation reaction,

(8.3)UO2,s+2Feaq3+→UO2,aq2++2Fe aq2+

with sodium chlorate oxidation,

(8.4)6Fe2++NaClO 3+6H+→6Fe3++NaCl+3 H2O

Dissolution of uranium by sulfuric acid involves the following three partial reactions:

(8.5)UO2,aq2++SO42−→UO2SO4

(8.6)UO2SO4+SO42−→[UO2(SO4)2]2−

(8.7)[UO2(SO4)2]2−+ SO42−→[UO2(SO4)3]4−

The composition of the solution obviously depends on a number of factors, such as the pH and redox potential, which is affected by the local geochemistry and lixiviant added to the system. Uranium bearing species in solution may include UO2,aq2+, UO2SO4,aq, UO2(SO4,aq)22−, and UO2(SO4,aq)34− .

Alkaline leaching with a carbonate lixiviant, such as bicarbonate (HCO3−), carbon dioxide gas (CO2), or sodium carbonate (Na4CO3), involves a neutral or slightly alkaline solution. Fig. 8.4 gives an overview of the general alkaline leaching process. This process is generally preferred when sulfuric acid is largely consumed by the host rock, rendering acid leaching less desirable from an economic perspective. The predominant species in carbonate leaching is UO2(CO3)3.

Chemical reactions pertinent to the fortification of lixiviants in alkaline leaching begins with oxidation [7,8]:

(8.8)UO2+0.5O2→UO3

followed by

(8.9)UO2+0.5O2+3CO32−+H2O →UO2(CO3)34 −+2OH−

or with bicarbonate,

(8.10)UO3+CO 32−+2HCO3−→[UO 2(CO3)3]4− +H2O

or with sodium carbonate,

(8.11)UO3+Na2CO3+2NaHCO3→Na4[UO2(CO3)3]+H2O

Once uranium in the ore has been dissolved in solution and is now mobile, it can be more easily transported by submersible pumps in the production well. Uranium is removed from solution at a surface processing facility using an ion exchange resin or polymer [9]. Ion exchange involves the following two stages: (1) loading/absorption and (2) elution. During stage (1) the solution is in contact with the ion exchange resin permitting selective adsorption of uranium. During stage (2), uranium is stripped from the resin yielding a uranium-rich solution called “eluate.” Then, hydrogen peroxide may be added to assist with uranium precipitation, which, after drying, produces yellow cake. Different ISL plants may execute this process slightly differently, which may yield ammonium diuranate ((NH4)2U2O7) or uranyl peroxide (UO4nH2O) [9]. Residual material is typically combined with fresh leaching chemicals and then recycled into the injection wells to continue the ISL cycle. The interested reader is referred to several IAEA technical reports that have thoroughly covered a number of details pertaining to ISL processes related to uranium [7,9].

1.3 In Situ Leach (ISL) Mining

In situ leaching (ISL), also known as solution mining, involves leaving the ore where it is in the ground and using liquids that are pumped through it to recover the minerals out of the ore by leaching. Consequently there is little surface disturbance and no tailings or waste rock generated. However, the orebody needs to be permeable to the liquids used and located so that they do not contaminate groundwater away from the orebody.

Some ISL mining in the past, notably in the eastern bloc, has occurred in broken rock with insecure containment of fluids, and this has resulted in considerable pollution. ISL mining was first tried on an experimental basis in Wyoming during the early 1960s. The first commercial mine began operating in 1974. About a dozen projects are licensed to operate in the United States (in Wyoming, Nebraska, and Texas), and most of the operating mines were less than 10 years old in the early 21st century. Most are small, but they supply some 85% of the U.S. uranium production. About 16% of world uranium production is by ISL (including all Kazakhstan and Uzbekistan output).

ISL can also be applied to other minerals such as copper and gold. Uranium deposits suitable for ISL occur in permeable sand or sandstones, confined above and below by impermeable strata and below the water table. They may either be flat, or roll front—in cross section, C-shaped deposits within a permeable sedimentary layer. Such deposits were formed by the lateral movement of groundwater bearing oxidized uranium minerals through the aquifer, with precipitation of the minerals occurring when the oxygen content decreased, along extensive oxidation-reduction interfaces. The uranium minerals are usually uraninite (oxide) or coffinite (silicate) coatings on individual sand grains. The ISL process essentially reverses this ore genesis in a much shorter time frame. There are two operating regimes for ISL, determined by the geology and groundwater. If there is significant calcium in the orebody (as limestone or gypsum), alkaline (carbonate) leaching must be used. Otherwise, acid (sulfate) leaching is generally better.

Techniques for ISL have evolved to the point where it is a controllable, safe, and environmentally benign method of mining, which can operate under strict environmental controls and which often has cost advantages. The mine consists of well fields, which are progressively established over the orebody as uranium is depleted from sections of the orebody after leaching. Well-field design is on a grid with alternating extraction and injection wells, each of identical design and typical of normal water bores. The spacing between injection wells is about 30 m with each pattern of four having a central extraction well with a submersible electric pump. A series of monitor wells are situated around each mineralized zone to detect any movement of mining fluids outside the mining area. The wells are cased to ensure that liquors only flow to and from the ore zone and do not affect any overlying aquifers. They are pressure-tested before use.

The submersible pumps initially extract native groundwater from the host aquifer prior to the addition of uranium complexing reagents (acid or alkaline) and an oxidant (hydrogen peroxide or oxygen) before injection into the well field. The leach liquors pass through the ore to oxidize and dissolve the uranium minerals in situ.

While uranium production in Australia uses acid leaching of the crushed ore, ISL elsewhere normally uses alkaline leaching agents such as a combination of sodium bicarbonate and carbon dioxide. At Beverley and Honeymoon in South Australia the process is acid leaching, with weak sulfuric acid plus oxygen. The leach solution is at a pH of 2.0 to 3.0, about the same as vinegar.

The pregnant solution from the production wells is pumped to the treatment plant where the uranium is recovered (see Section 1.4). Before the process solution depleted of uranium (i.e., barren liquor) is reinjected, it is oxygenated and if necessary recharged with sulfuric acid, or with sodium bicarbonate or carbon dioxide, to maintain its pH.

Most of the solution is returned to the injection wells, but a very small flow (about 0.5%) is bled off to maintain a pressure gradient in the well field and this, with some solutions from surface processing, is treated as waste. It contains various dissolved minerals such as radium, arsenic, and iron from the orebody and is reinjected into approved disposal wells in a depleted portion of the orebody. This bleed of process solution ensures that there is a steady flow into the well field from the surrounding aquifer and serves to restrict the flow of mining solutions away from the mining area.

In the United States, the production life of an individual ISL well pattern is usually less than 3 years, typically 6 to 10 months. Most of the uranium is recovered during the first 6 months of the operation of those wells. The most successful operations have achieved a total overall recovery of about 80% of the uranium from the ore. Over time, production flows decrease as clay and silt become trapped in the permeable sediments. These can be dislodged to some extent by using higher pressure injection or by reversing the flow between injection and production wells. At established operations in the United States, after ISL mining is completed, the quality of the remaining groundwater must be restored to a baseline standard determined before the start of the operation so that any prior uses may be resumed.

In contrast to the main U.S. operations, the water quality at the Australian sites is very low to start with, and it is quite unusable. At Beverley the groundwater in the orebody is fairly saline and orders of magnitude too high in radionuclides for any permitted use. At Honeymoon, the water is even more saline and high in sulfates and radium. When oxygen input and leaching are discontinued, the water quality soon reverts to its original condition. With ISL, no tailings are involved and very little waste is generated. ISL thus has clear environmental advantages in the places it can be applied.

7.1.3.3 Eastern Europe

In Bulgaria, in situ leaching (ISL) using sulfuric acid chemistry was used in many locations. Nedyalkov (1996) reported the first ISR activity at the Sclishte in 1961. From 1981, ISL of broken hard rock was also used to increase the yield from mined-out conventional underground mines. In 1990, 70% of the uranium produced was from ISL of ore deposits with very low grades of 0.02–0.07% of uranium (WISE Uranium Project). The Plovdiv ISR facility provided resin loaded with uranium solution to a mill at Elesnica Production Center for final processing. ISR was practiced at 21 sites by 4 companies. Production was reported at approximately 345 tU/year (0.761 mm lb) (Norman, 1993). In 1992, uranium development in Bulgaria was closed by decree. (Nedyalkov, 1996). Norman (1993) reported 14,000 wells in 15 well fields and 4 satellite recovery units.

Czech Republic. ISL using sulfuric acid chemistry began in Stráz pod Ralskem in North Bohemia in 1967 (Ekert and Muzak, 2010). Tomas (1997) reported the lesser use of nitric acid and hydrofluoric acid and ammonia in the leach solutions as well. The ore deposit is located in Cretaceous sandstones at depths of up to 488 m (1600 ft) with grades of about 0.12% uranium. The Stráz ISR project is adjacent to the Hamr underground uranium mine. Flowrate at the Stráz project was about 37,854 L/min (10,000 gal) at a production rate of about 575 tU/year (1.27 mm lb U) (Norman, 1993) (Fig. 7.6, Photo 7.5).

Germany. Germany has been the most secretive about uranium production in Eastern Europe (Norman, 1993). In Eastern Germany, an underground mine converted to an ISL facility was in operation at Königstein near Dresden until the end of 1990, when all uranium production ceased in Germany. It produced a total of 18,000 tU (39.69 mm lbs), 30% of which were from ISR with sulfuric acid (http://www.wise-uranium.org).

Ukraine. In 1961, Ukraine began testing ISR. From 1966 to 1983, uranium was produced by ISR in the Devladovo of Sofiivela District, Drivipropetrovska Provence, and Bratske of Mikolaivska Provence using sulfuric acid chemistry. Future plans call for changing to alkaline chemistry (Sukhovarov-Jornoviy, 2005; Rudy, 1997; OECD, 2014).

16.2.2 Mining methods

Mining methods used include open cut, underground and ISL methods. Each of these has very different potential environmental impact advantages and disadvantages with regard to site remediation requirements. The deposition of solid wastes into a surface or underground waste management facility (WMF) is of fundamental importance in regard to the stability and permeability of the final facility and the longer-term environmental impact of its remediation.

The deposition of waste (tailings and other process wastes, contaminated equipment and mine waste) either onto a surface or into an underground facility must produce a waste structure with suitable characteristics for the long term. Backfill or cemented backfill of tailings as a paste into the WMF are techniques used to dispose of tailings and to stabilize the mine to improve mining extraction. These deposition methods can produce a structure with reduced permeability to surface and/or ground waters and to erosion and therefore the likelihood of impurity (including radioactivity) dispersion into the environment. The surface disposal of slimes containing higher radionuclide concentrations, with greater propensity to dispersion as dust and/or to erosion has led to the development of “whole-of-tailings” in paste techniques being used as backfill underground or into pits. These deposition methods are likely to be accompanied by barriers, on the surface or underground, to limit gaseous (radon) and solution migration through the waste.

1.5.3 Increasing importance of in situ leaching (ISL) operations

Nearly two-thirds of the uranium produced now is produced from ISL mine operations (OECD-NEA/IAEA, 2014). This trend is likely to continue, as long as mineralizations in porous host rocks can be mined. ISL is not feasible where the uranium minerals form a constituent of a largely impermeable rock matrix. In matrices, such as granites, even permeability-enhancing techniques, such as fracking, would not provide sufficient access to the uranium minerals that need to be dissolved. Otherwise, given diligent process control by the operator and adequate regulatory oversight, ISL has the potential for low-impact mining (NRC, 2009). From a resource use–efficiency perspective remains the concern that the percentage of uranium recoverable from the formation by ISL is smaller than when using traditional mining techniques. This is a problem that is also faced by other mining industries that use ISL, namely, the copper mining industry, and research and development efforts are being made to improve the effectiveness of the process.

IV.B Mining

Uranium ore is mined using open pit and underground methods similar to those used for mining other minerals and coal. In situ leaching of permeable sandstone deposits accounts for an increasing share of production. The depth, grade, and size of the deposit determines the type of mining to be used.

Depths of open pit uranium mines range from a few feet to depths in excess of 300 ft. Open pit mining is currently in use today in northern Australia, Niger, Namibia, Saskatchewan Canada, Spain, and Argentina.

Access to deeper ore deposits is by vertical shafts or by subhorizontal declines. Vertical shafts are used in Australia, France, Canada, Russia, and South Africa. Vertical shafts in South African gold mines where uranium is produced as a by-product of gold may exceed 7000 ft in depth.

Declines sloping 15–25° are used in Niger and the United States for access to relatively shallow underground mines. These declines require cheaper hoisting installations than vertical shafts, and for that reason find favor among small-scale operators, especially on the Colorado Plateau region of Colorado and Utah.

Horizontal access by adit is common where ore bodies are near or crop out of canyon rims. Where feasible, such access provides the simplest and cheapest method of reaching and extracting ore in underground mines. This method of mining is used throughout the Colorado Plateau especially in Colorado and Utah. Radon gas, emitted by uranium ores, is a hazard to sminers and must be monitored and controlled by circulating large amounts of fresh air in underground mines.

Uranium is recovered from the groundwater that enters deep underground mines such as those in New Mexico. The water is collected in sumps and pumped to the mill for processing. Heap leaching is also used at some mines to recover uranium from lower grade ores. In this procedure, acid solutions are allowed to percolate through a mound of crushed ore. The uranium-bearing solutions are collected in a sump at the bottom of the mound and sent to the mill.

Uranium is also produced by solution or in situ mining methods. This process involves circulating weakly alkaline or acid solutions through an ore body continued within a permeable sandstone aquifer. The uranium in the rock is dissolved by the chemicals and the uranium-bearing solution is pumped to the surface from a production well. On the surface the uranium is precipitated as yellowcake. A “five spot pattern” of wells is commonly used. The injection wells are at the corners of a square pattern with side dimensions ranging from 50 to 200 ft. The production well is in the center of the square. Wells are 6–8 in. in diameter and are lined with polyvinyl chloride or fiberglass-reinforced pipe. A series of wells are grouped into well-field units. Several wellfield units are usually required to mine a deposit. Uranium is recovered from solution by ion exchange in a small surface plant and precipitated, dried, and packaged into steel drums. This technique results in little surface disturbance and only small amounts of waste are generated.

After the uranium has been extracted, additional solutions are injected into the host rock to flush and restore the aquifer to its original geochemical state. In the United States, solution mining, or in situ leaching, has been used successfully in south Texas, Wyoming, and Nebraska at depths up to 900 ft. Uranium production in the central Asian republics of Kazakhstan and Uzbekistan is now solely by solution mining. Solution mining accounted for 18% or world uranium production and 78% of United Ststes production in 1999.

9.7.4.1 In situ leaching of uranium

Conventional mining involves removing mineralized rock (ore) from the ground, breaking it up, and treating it to remove the minerals. In situ leaching (ISL), also known as solution mining, or in situ recovery, involves leaving the ore where it is in the ground, and recovering the minerals from it by dissolving them, and then pumping the pregnant solution to the surface where the minerals are recovered. Consequently, there is little surface disturbance and no tailings or waste rock are generated. However, the ore body must be permeable to the liquids used, and located such that the liquids do not contaminate the groundwater away from the ore body. ISL can also be applied to other minerals, such as copper and gold, for uranium- and other radionuclide-contaminated soils. ISL techniques were developed where it is a controllable, safe, and environmentally benign method of mining, operating under strict operational and regulatory controls. Due to the low capital costs (relative to conventional mining), it often proves to be an effective method of mining low-grade uranium deposits.

12.4 Mining and Milling

Uranium ore is extracted from opencast mines, underground mines and by in situ leaching. The ore from mining is processed into a refined form of U3O8 known as yellowcake or UOC (uranium ore concentrate). This is usually done near the mines at facilities referred to as uranium mills prior to transport to fuel manufacturing facilities. The solid wastes from this process are known as mill tailings and are usually stored on the site of the mine. Significant quantities of ore have to be processed because the uranium content is relatively low, at between 0.1% and 0.2%.

The extracted ore is crushed into small pieces and the uranium is leached into solution from it, usually with sulphuric acid (alkaline leaching is also used dependent on the nature of the ore), as per the following:

UO3+2H+→UO22++H2O

(12.1)UO22++3 SO42−→UO2SO4 34−

Ammonium carbonate can also be used to yield uranium trioxide, UO3, or uranyl tricarbonate, UO2 CO334−. In situ leaching (also known as in situ recovery) involves dissolving the ore with an oxidant (such as hydrogen peroxide) while it is still in the ground and fixing the uranium-containing minerals with a complexing agent (either acidic or alkaline depending on the mineral content of the local geology). By pumping the leachate into underground rock deposits, the uranium-containing solution can be pumped out.1 In situ leaching is regarded as environmentally preferable and cost effective, with almost half of all uranium mined in the world being leached, especially in United States, Kazakhstan and Uzbekistan.

To extract the uranium from its solution, either a solvent extraction or ion exchange process is used. Solvent extraction is an approach common to several elemental separations applications used in the nuclear fuel cycle, nuclear reprocessing and elsewhere in the process industries for metals extraction: two immiscible liquid phases—an organic solvent phase and an aqueous phase—are combined by mixing or by forcing them to move counter-currently to one another. The extraction process exploits the difference in solubility of an element or compound in the aqueous stream as opposed to the organic medium. The element or compound of choice (in this case uranium) is transferred (or partitioned) from one liquid to the other, in order to separate it from the other elements and compounds in the primary aqueous stream (that is in this case derived from the ore). When the transfer is complete, the mixing process is halted and the loaded immiscible media separate under gravity.

In the first stage, a tertiary amine in a kerosene diluent is used to transfer the uranium from the aqueous to the organic phase. Subsequently, the uranium is stripped from the loaded solvent by mixing with ammonium sulphate. This returns the uranium back into solution and the solid form ammonium diuranate, (NH4)2U2O7, is precipitated by mixing it with ammonium hydroxide. It is this form that is bright yellow in colour as per the name yellowcake, although this reference tends to apply to all of the general products of milling irrespective of whether they are actually yellow. The ammonium diuranate is then calcined to remove the ammonium ions that yields triuranium octoxide, U3O8, which is the main form of uranium produced by most milling operations.

In situ recovery/leaching

Some orebodies lie in groundwater in porous unconsolidated material (such as gravel or sand) and may be accessed simply by dissolving the uranium and pumping it out – this is in situ leaching (ISL) mining (also known in North America as in situ recovery – ISR). It can be applied where the orebody’s aquifer is confined vertically and ideally horizontally. It is not licensed where potable water supplies may be threatened. Where appropriate it is certainly the mining method with least environmental impact.

ISL mining means that removal of the uranium minerals is accomplished without any major ground disturbance. Weakly acidified groundwater (or alkaline groundwater where the ground contains a lot of limestone such as in the USA) with a lot of oxygen in it is circulated through an enclosed underground aquifer, which holds the uranium ore in loose sands. The leaching solution dissolves the uranium before being pumped to a surface treatment plant where the uranium is recovered as a precipitate. Most US and Kazakh uranium production is by this method.

In Australian ISL mines the oxidant used is hydrogen peroxide and the complexing agent sulfuric acid to give a uranyl sulfate. Kazakh ISL mines generally do not employ an oxidant but use much higher acid concentrations in the circulating solutions. ISL mines in the USA use an alkali leach to give a uranyl carbonate due to the presence of significant quantities of acid-consuming minerals such as gypsum and limestone in the host aquifers. Any more than a few per cent carbonate minerals means that alkali leach must be used in preference to the more efficient acid leach.

In either the acid or alkali leaching method the fortified groundwater is pumped into the aquifer via a series of injection wells where it slowly migrates through the aquifer leaching the uranium bearing host sand on its way to strategically placed extraction wells where submersible pumps pump the liquid to the surface for processing.

Acid consumption in acid leach environments is variable depending on operating philosophy and geological conditions. In general, the acid consumption in Australian ISL mines is only a fraction of that used in a Kazakh mine (per kilogram of uranium produced). A general figure for Kazakh ISL production is up to 80 kg acid per kgU, though some mines are a bit lower. This is becoming a significant cost constraint there. Beverley in Australia is reported to be 3 kg/kgU.

For very small orebodies that are amenable to ISL mining, a central process plant may be distant from them so a satellite plant will be set up. This does no more than provide a facility to load the ion exchange (IX) resin/polymer so that it can be trucked to the central plant in a bulk trailer for stripping. Hence very small deposits can become viable, since apart from the wellfield, little capital expenditure is required at the mine and remote IX site.

Methods in Biohydrometallurgy

Methods in biohydrometallurgy are illustrated in terms of commercially relevant bioleaching techniques. Science and technology of industrially adapted bioleaching processes are discussed and relevant laboratory and research approach for the development of such processes analyzed.

Methods in biohydrometallurgy can be generally categorized as agitation (for fine particles) and percolation (for coarser particles) leaching processes [5].

Depending on ore and mineral deposit characteristics, percolation leaching can be classified as

Heap leaching: Crushed/agglomerated ore on prepared leach pads.

Dump leaching: Run of mine ore without size reduction

In situ leaching (ISL) – underground solution mining. A leaching process to solubilize minerals from host rocks without removing the desired mineral from the ore body.

Vat leaching – Crushed ore/concentrate in submerged leach solution – The crushed materials collected in concrete vats. The solution percolates through the ore mass, overflows from a designed weir and pumped to the next vat.

Heap leaching can be grouped among the percolation leaching technologies which also include dump leaching, ISL, as well as vat leaching. In ISL, the ore underground is leached by percolating solutions through natural porosity of rocks or porosity created by blasting and selective fracturing. In dump leaching, the as-mined ore is piled up as dumps and irrigated with leaching solution that percolates through the bed and effluents collected from the base. No crushing is involved before stacking and all sizes of the mined ore ranging from large boulders to few centimeters and more form part of the dump. In heap leaching, the mined ore is crushed to size, usually below 20–25 mm, and heaps are scientifically prepared and engineered on prepared bottom pads. In vat leaching, more finely crushed ore (1–10 mm) is placed in a large basin (vat) flooded with leachant solution and left to react with time. The solution is drained off after treatment for metal recovery.

Among the above methods, only the first three are illustrated below, because the vat leach method is still not very prominent in commercial bioleach operations.

Heap Bioleaching

Heap bioleaching of different ores is a rapidly developing metal extraction technology because it has a major impact on base and precious metal industries. This technology made initial impact in Chile where acid-soluble copper oxide ores were first heap leached and subsequently secondary copper sulfides were heap bioleached. The process got evolved based on practical operation data acquired over several years of experience both in Chile and Australia. Better operational controls of bioheaps were incorporated with the design of aerated and high temperature heaps with provision for different irrigation systems. Heap bioleaching is now commercially used to treat copper, uranium, and gold ores as well as polymetallic ores containing nickel, zinc, cobalt, and copper. Significant advances have been made in several operational aspects of heap bioleaching, such as agglomeration of crushed ores, inoculation strategies, acid curing, forced aeration, irrigation management, in-heap heat generation, maintenance and control, as well as close monitoring of all parameters controlling leaching kinetics [6]. A simplified heap leach scheme is shown in Fig. 5.1.

Figure. 5.1. Heap leaching scheme.

Several factors need to be considered for the choice of a heap reaching method depending on the size and mineralogy of the deposit, grade of the ore mineral and transport, power, and labor costs. For example, for low-grade copper oxide ores, conventional heap acid leaching may suffice. However, for lower grade secondary copper sulfides, heap bioleaching is the most likely process route. In recent years, heap bioleach processes are being developed to treat primary copper sulfides such as refractory chalcopyrite. Typical pilot heap for bioleaching of low-grade chalcopyrite ores is shown in Fig. 5.2.

Figure. 5.2. Pilot heap bioleaching for chalcopyrite ores. Photographs and information provided by Mintek of Randburg, South Africa, http://www.mintek.co.za.

Basic efficiency in a heap leaching process is evaluated in terms of percent metal dissolution and the time required for such dissolution. Metal dissolution is controlled by the degree of liberation of the desired mineral component in the heap, ore particle size, contact between the ore particles, and the lixiviant (heap permeability), leach kinetics based on dissolution mechanisms, dissolution potentials, and composition of minerals and electrolytes, bacterial growth and activity, availability of oxygen and other gaseous reagents, and rate of irrigation. For heap leaching, the mined ore is subjected to size reduction by crushing before stacking on impermeable under liner fitted pads. If the ore is very permeable, little or no crushing may be required, Lixiviant contact with the mineral particles needs to be ensured through efficient percolation. Percolation rate should be slow enough to facilitate necessary contact periods between the ore particles and the lixiviant. Uniform permeability is essential to ensure optimal flow of leach solutions throughout the heap. Whenever excessive fine particles are present in the ore feed, prior agglomeration using binding agents becomes necessary before stacking. The height of the heap also controls consolidation and permeability of the stacked bed. Stacking methods such as the use of conveyers, trucks, or trippers are employed depending on proposed heap designs with respect to consolidation and height [7,8].

Mineral dissolution from the heap depends on nature of the mineral, lixiviant type and concentration, pH, temperature, presence of other cations and anions in solution, oxygen levels, and bacterial activity. The nature and amount of gangue constituents present in the ore feed are very important because they govern the lixiviant (acid) consumption and extent of impurity dissolution. In addition to acid consumption, the presence of gangue constituents such as gypsum, silica, and jarosite has potential detrimental effects as plugging up of pores affecting leach permeability and creating problems in downstream filtration and metal extraction. Heap heights range between 6 and 10 m in general cases; however, taller heaps are also constructed [7,8].

Basic components of a heap bioleaching system include agglomerated ore on the heap, lined bottom pad, solution (effluent) collection system, lixiviant solution storage, and recirculation as well as ponds (irrigation systems). Percolation and subsequent drainage of the leach solution is driven by gravity. In flat-bed pads, the internal drainage is fed to collection ponds. In mine valley heap leach pads, downgradient embankment is used for the collection of effluents. Collection of effluents in sumps has multiple base liner systems. Leach pads should be so designed and constructed as to provide:

•

environmental protection through prevention of solution leakage to ground table and surrounding environment,

•

stable foundation for the height and weight of the heap,

•

efficient collection of leached solution, and

•

site optimization.

Ore stacking can be done by conveyor systems or trucks. Truck dumping may cause ore segregation with respect to distribution of coarse and fine particles. Short lifts can cause less segregation. Conveyer stacking systems which are continuously moved as the heap is being built through mechanization are often used [7,8].

Application of leach solution is to achieve complete and uniform ore wetting through continuous percolation between entire ore particles. Solutions can be applied on top surfaces of the heaps through drip irrigation or spray techniques or sprinklers based on climatic conditions. Irrigation ponds on the top surfaces can also be provided suitably. Typical irrigation rates are 5–20 L/m2/hours, while aeration at 0.1–0.5 m3/m2/hours.

Types of pad configurations can be

•

Dedicated single use pad:

•

Suitable for various types of ores and leach cycles-flat topography.

•

On/off or reusable pad:

•

Suitable for ores with short leach cycles and require a rinsed ore site. A range of climatic conditions can be used.

•

Valley fill:

•

Suited for hard ores with good drainage and used in steep terrains.

•

Hybrid:

•

Hybrid pads are a combination of single use and on–off pads.

Heap leach kinetics involves complex interplay between solution transport to and fro, air solution mass transfer, and migration through stagnant solution in agglomerates and particle porosities. Microbial colonization behavior, desired mineral location and liberation in the heap mass, biooxidation rates, and heat reaction balance also influence heap leach kinetics [7]. Heap leach periods vary depending on the type of ore, and leaching method ranging from a few days to months and even a couple of years. Subprocesses in heap bioleaching have been studied to understand the complex nature of interactions. Four different scales of reaction–transport phenomena have been distinguished [9,10].

•

Heap scale: Packed bed solution flow, advection of gas, heat balance, and water vapor transport

•

Agglomerate scale: Adsorption of gas, diffusion through pores, bacterial growth and attachment, and biooxidation.

•

Particle scale: Shape of particles, intraparticle diffusion, and distribution of grain size and particles.

•

Grain size: Mineral oxidation, ferric reduction, oxidation of sulfur/sulfides, and surface reactions.

The transport effects more specifically include

•

Solution flow

•

In coarse packed beds which are unsaturated, solution flow follows different difficult pathways and remains stagnant in crevices and pores between particle aggregates. Reagent delivery and removal of reaction products from heap inner sites are strongly affected by this phenomenon.

•

Gas flow

•

Gas is generally well distributed in aerated heaps. However, there may be pockets where oxygen and CO2 supply are limited.

•

Heat of reactions.

•

Exothermic reactions are significant in sulfide mineral bioleaching. Temperature profiles in heaps depend on air and solution flow rates.

•

Diffusion is the main transport mode for dissolved constituents to and from flowing liquid into pores and crevices between particles. Pore diffusion influences overall kinetics.

•

Microbial mass and population encompasses complex synergetic interactions both in liquid and solid phases. Temperature, O2 and CO2 availability, solution constituents, and concentration influence microbial growth and activity.

•

Mineral surface area available influences leaching rate. Mineral topology refers to distribution of different size grains and their accessibility within the ore matrix.

Biooxidation reactions as well as transport network between gas, solid, and liquid phases in heaps has been modeled [9,10].

Heap leaching is considered to be an economical, cost-effective process to treat lean-grade ores. Its advantages compared to conventional flow sheets can be understood in terms of

1.

elimination of costly fine grinding and

2.

ability to treat even very lean-grade ores (e.g., 0.4%–0.5% for copper sulfides and 3–5 g/t for gold ores).

However, longer extraction periods are required along with generally lower overall recoveries. The process is further restricted to treat only coarser particles (not fines) through percolation leaching.

Heap leaching poses several challenges as well.

The slow rate of metal recovery is a major constraint and heap permeability is a key factor in this regard. Various processes can contribute towards this anomaly, such as the presence of fines and reaction products which can clog-up open pores. Heap effluents are generally very dilute with respect to dissolved metals requiring elaborate concentration, purification, and recovery techniques. Environmental impact also needs to be considered with respect to generation of toxic chemical solutions and solution-seepage. The fate of spent heaps is another area of concern. Spent ore from used-up heaps can be removed and properly disposed in a waste pile. Another way is the reuse of spent heap to stack fresh materials on top.

Biooxidation mechanisms in the heap bioleaching of copper, uranium, zinc, nickel, and gold are illustrated in Chapters 6–8Chapter 6Chapter 7Chapter 8, Bioleaching of Copper and Uranium, Bioleaching of Zinc, Nickel, and Cobalt, and Biotechnology for Gold Mining, Extraction, and Waste Control, respectively. Also, examples heap bioleach operations around the world for copper, uranium, gold, and multimetal ores are given in Chapters 6–8 and 11Chapter 6Chapter 7Chapter 8Chapter 11, Bioleaching of Copper and Uranium, Bioleaching of Zinc, Nickel, and Cobalt, and Biotechnology for Gold Mining, Extraction, and Waste Control, and Extended Applications of Metals Biotechnology, respectively, along with different types of heap bioleaching such as Geocoat, Geoleach, and high temperature and ambient temperature heap leaching processes.

Heap microbiology

Heap bioreactors provide a widely heterogeneous environment for microbial growth. Types of microbial colonies change with change in heap environments with time. Generally, the indigenous organisms attach to ores particles in the heap and grow as biofilms. Extreme conditions often prevailing in heaps influence their microbiology. Microbial consortia existing in pilot and industrial heaps composed of several species having widely different pH and temperature optima and can be generally categorized in terms of mesophiles (24°C–40°C), moderate thermophiles (40°C–60°C) and extreme thermophiles (60°C–80°C). Heaps are thus subject to greater biodiversity with variations in dominant microbial species during different stages of heap operation [11]. Differences between microorganisms that are competitive in a heap environment compared to those in a stirred tank bioreactor can be expected. In stirred tank bioleaching, existing turbulent shear conditions can disrupt bacterial attachment and cell wall integrity, unlike often the case with heap environments.

Acidophilic microorganisms identified in heaps include

•

A. ferrooxidans, A. thiooxidans, Acidiphilium cryptum (chalcopyrite overburden) and

•

Acidithiobacillus spp., Leptospirillum ferrooxidans, Acidophilum spp., Ferrimicrobium spp., (copper oxides/sulfides)

•

Sulfobacillus spp., Ferromicrobium spp., A. ferrooxidans, A. thiooxidans (chalcocite).

Due to sulfide mineral oxidation with in heaps, greater temperature biodiversity can be expected [12]. Variations in pH, temperature, aeration, and liquid compositing with time occurring within a heap have a significant effect on the microbial activity and populations. An understanding of microbial flora along with the physico-chemical profiles as a function of time in a heap would be beneficial in controlling biooxidation rates and to take remedial measures whenever microbial activity is getting inhibited. Also, adaptation of leaching microorganisms to heap environmental conditions could effectively enhance metal extraction rates. The development of new molecular biological techniques such as polymerase chain reaction (PCR), real-time quantitative PCR, denaturing gradient gel electrophoresis (DGGE), and fluorescent in situ hybridization have helped to detect and quantify microbial populations to establish heap biodiversity and to monitor changes in microbial consortia without the necessity to culture “in situ” organisms. A recent development is the rapid determination of active biomass in leach solutions based on ATP concentrations to quantify microbial activity in a simplistic fashion.

Microbiology of heap bioleach systems is generally assessed based on the analysis of liquid samples from pregnant leach liquors and raffinates. In order to gain practical insight into actual microbial activity, it is also essential to enumerate and study activity of particle-attached microorganisms.

A few examples of detection of microorganisms in some industrial heaps are illustrated in Table 5.1.

Table 5.1. A Few Examples of Detection of Microorganisms in Some Industrial Heaps [13–16]

Zijinshan copper mine heap, China (secondary sulfides)	Leptospirillum, Ferroplasma, A. ferrooxidans, Sulfobacillus thermotolerans, A. caldus	16S rRNA gene clones, real-time quantitative PCR
Talvivaara Black schist	A. ferrooxidans, Leptospirillum ferrooxidans, A. caldus, Sulfobacillus sp. Thermophilic archaea	PCR-DGGE
Myanmar, Ivanhoe, Chalcocite heap	Leptospirillum ferriphilum, A. caldus Sulfobacillus sp,. Ferroplasma sp.	PCR-DGGE, Culturing techniques
Escondida, Chile, Run-of mine copper ore	A. ferrooxidans, L. ferriphilum, Sulfobacillus sp., Ferroplasma acidiphilum	PCR, DGGE

Representative liquid and solid samples from the heap systems need to be collected for practically relevant assessment of role of microbiology in heap leach kinetics.

Heap bioleaching processes are generally dependent on activities of indigenous microbial colonies. However, to achieve desirable metal extraction within shorter time periods, it becomes essential to promote microbial colonization of desirable organisms with in all cross sections of the heap. The necessity for microbial inoculation from outside thus needs to be considered. An example is heap bioleaching of chalcopyrite which needs to be operated at high temperatures to augment copper dissolution for which organisms that grow and are active over a range of temperatures from ambient to 60°C–80°C are required. Due to the lack of sufficient thermophilic cultures under indigenous environments, it becomes essential to undertake inoculation of desirable microorganisms. Some reported examples of inoculation strategies are indicated below:

•

Microbial growth enhanced in ponds through nutrients addition and used for heap irrigation [17].

•

Culture mixing with ore and ore attached organisms distributed through heap [18].

•

Adding ferric iron-rich solution containing iron-oxidizing bacteria from a biological contactor [19].

•

Adapting organisms to the ore and scaling up the desired volume of inoculum for heap inoculation [20].

•

Ultra-small organisms through gaseous suspension delivered through aeration lines [21].

•

StickiBugs process involving strains rendered temporarily nonadhesive [22].

•

Initial inoculation with mesophiles and as the heap temperature increases, moderate thermophiles and when thermophilic conditions are attained, appropriate thermophiles added (for high temperature heaps) [23].

•

Increasing bioleaching rates of sulfide ores or concentrates through continuous inoculation with leachant containing isolated microorganisms with or without native organisms [24].

Rather than waiting for indigenous microorganisms to grow “in-situ” with in a heap, it would be prudent and timely to inoculate a new heap with a chosen microbial consortia at appropriate time intervals [12].

Would it be possible at all to produce an ideal and optimal microbial consortium suited to a given ore, mineral, or a bioleaching process?

Two approaches have been suggested [12]:

•

With the reference point as rate of mineral oxidation by a pure culture of one or more chemolithotrophs (e.g., Leptospirillum), additional mixed acidophilic cultures which possess complimentary capabilities as sulfur oxidation or heterotrophic growth are compared.

•

Use of an inoculum containing a wider variety of various species of acidophiles on the basis that the most suitable among them to a particular mineral (concentrate) will survive, while the “unfit ones” will be eliminated in the competition.

Preadaptation of leaching microorganisms to increased toxic metal levels and ore or concentrate substrates would be a simpler approach to enhance mineral dissolution kinetics and genetic improvement. Microbial tolerance to high metal and salt concentrations has been studied. Metal toxicity and development of toxic metal tolerant strains of A. ferrooxidans are discussed in Chapter 4, Bioleaching Mechanisms. In heap bioleaching processes, large amounts of undesirable gangue mineral constituents as well as prolonged exposure to recycled solutions containing various cations and anions are encountered. Toxicity of leaching microorganisms to chloride, sulfate, nitrate, and fluoride as well as metal cations such as copper, zinc, nickel, and aluminum (all present together) needs to be ascertained.

Another aspect is the role and activity of mineral attached bacteria compared to unattached planktonic organisms. Impact of bacterial attachment, biofilms, temperature variations, and irrigation rates on microbial activity in heaps needs to be understood.

Dump Leaching

Dump leaching was initiated during the late 1960s. One of the well-known dump leaching operations is located in Bingham Canyon, Utah (The United States) of the Kennecott Copper mines. The largest of the dumps at this site contained about 4 billion tons of low-grade copper ore. A more recent deliberately built-up dump is the Bala Ley plant, Chuquicamata, (Codelco) in Chile consisting of run-of mine ore piled to 350 m height. The dumps are subjected to preconditioning cycles, irrigation, rest periods, and washing stages extending to over a year. Microorganisms indigenously present in the dump dissolve the copper sulfide minerals, and the copper-laden effluents are removed from dump bottom [25].

Dump leaching involves recovery of metal values from lean (submarginal grade) and waste ores generated from open-cast mining operations. Such waste ores were often piled haphazardly near the mine site since ancient times. The uncrushed, fractured ore materials are lifted by trucks or loaders and dumped to form truncated cones at suitable sites in the vicinity. Generally, steep-sided valleys or hill sides are chosen to ensure solution percolation and collection. Schematic illustration of a dump design on hill side or steep valley is shown in Fig. 5.3. Dumps may consist of up to 5 m thick alternating layers of coarse lumps and fine-grained rock materials and can be 200 m tall, 80 m wide at top, 250 m wide at bottom containing as much as 500,000 tons or more of ore. To enhance the surface area to volume ratio for improved aeration, finger dump having much greater length than the height or width can be used. At Butte, Montana, a finger dump, 800 m long, 35 m high, and 200 m wide was constructed [26].

Figure. 5.3. Dump cross-section across a valley or steep valley.

The permeability of the fragmented rock materials in the dump is important for efficient percolation through the bed cross-section. Solution irrigation management is generally similar to both dumps and heaps. Shallow ponds covering the upper surfaces can be used. At the Silver Bell mines, Arizona, square ponds with 18 m × 0.6 m dimensions were used. The leach solutions can also be sprayed on upper surfaces using sprinklers or perforated pipes. At Anaconda Butte mine in Montana, liquor injection method providing vertical holes on centers and interactions from the dump surface was practiced [26]. Adequate aeration can be provided by compressed air passed under pressure though PVC lines. Temperature within dumps needs to be ascertained. Microflora inhabiting dumps participate in the mineral dissolution and exhibit varying activities with temperature changes. While the ambient temperatures may be very low (−5°C), the temperatures within deep regions of the dumps and in solutions could be higher. For example, in certain regions of the dump at Bingham Canyon, temperatures as high as 60°C–80°C have been measured. Thermophilic organisms are favored within dumps. The microflora of commercial dumps is often complex and heterogeneous promoting of synergistic growth and activity among aerobes, anaerobes, mesophiles, and thermophiles [26].

Typical microbial presence in a dump is given below:

Acidithiobacillus	A. ferrooxidans, A. thiooxidans, Thiobacillus thioparus
Iron-oxidizing bacteria	Gallionella, Leptothrix
Sulfate-reducing bacteria	Desulfovibrio spp.
Heterotrophic bacteria	Bacillus spp., Pseudomonas spp.
Fungi, algae	Cladosporium, Penicillium, Aspergillus, Ulothrix

Vlaikov vrah mine in Bulgaria contained about 30 million tons of waste low-grade copper sulfide and mixed ores. Several ore dumps exist at the mine sites. The largest dump was formed on a moderately steep hill without ground preparation during 1960–79. From the open cast mines, the ore materials were dumped to the top through trucks, as a series of thin sloping layers and contained all sizes (1–2 m diameter boulders to 500 mm and fines). Average copper content was 0.10%–0.15%, in the form of chalcopyrite, covellite, and chalcocite along with pyrite. Leaching was started in 1972 using solution containing acidophilic chemolithotrophs in sulfuric acid pumped to the top. Effluents from the bottom were sent to for copper cementation with iron. Spent solutions with make-up water were reintroduced to the top of the dump by spraying and flooding using sprinklers and ponds. Rest periods in between were employed. In the dump sections containing chalcopyrite, the annual recovery was as low as 3%. Cumulative copper extraction after 10–12 years was less than 20%. A test dump containing 100,000 tons of mixed ore with 0.15% Cu was leached for 10 years and problems analyzed with respect to percolation of solution, formation of clayey layers, and stagnated zones. Analysis of the microflora revealed the presence of acidophilic chemolithotrophs in the dump and recirculation solutions [27].

In Situ Leaching

Decreasing grades of near-surface mineral deposits have resulted in increase in mining costs due to the necessity of processing and transport of large volumes of waste rocky materials per unit of recovered products. Both open cast and underground mining and extraction of valuable metals from lean-grade ore deposits have become economically and technically unviable under the circumstances. ISL has therefore received renewed attention as a cost-effective mining and metal extraction technology. In situ mining is defined as the removal of valuable metal components of a mineral deposit without physical extraction of the rock. Minerals are leached from rocks through permeation of lixiviant solutions and pumped back to the surface [28].

Uranium recovery from permeable (porous) sand stone deposits has been estimated in the range of 60%–90%. Comparatively, copper recovery from porphyry deposits using ISL is on a much lower scale. San Manuel “in situ” mining in Arizona reported copper recoveries in the range of 50%–60% over a period of 5 years. Copper was recovered through this method at ASARCO’s Silver Bell Copper mine in Arizona from fragmented ores. Even recoveries as small as 20%–25% is believed to be economical. ISL accounting for over 40% of world uranium production was developed in the United States, Canada, and the Soviet Union during the early 1960s. This method for uranium extraction is effective at depths of 200 feet or more, even for low-grade deposits (0.1% U3O8). In the 1960s, uranium extraction by bioleaching was carried out by spraying stope walls with acid mine drainage solutions. In situ irrigation of fractured underground ore deposits was also carried out. ISL recovery for copper has been limited to a few operations in the United States. Previously mined sites (as different from virgin ore deposits) had been used for in situ copper recovery at ASARCO’s Silver Bell mine and BHP’S San Manuel Copper mine, both in Arizona. Magma Copper carried out demonstration of ISL leaching for copper during the 1990s. In situ bioleaching of copper from sulfide deposits (pyrite, pyrrhotite, and chalcopyrite) was piloted at San Valentino di Predoi mine in Italy [28].

ISL has been used in combination with open pit or underground excavation and can facilitate low-cost recovery of copper from untapped low-grade ores. Areas such as underground workings and pits can be targeted for copper extraction. It could be a viable option especially in regions below the water table in which the mineralization is easily accessible and having natural permeability. Permeability and porosity can also be enhanced wherever necessary through selective fracturing. Stope bioleaching and underground in situ uranium leaching are illustrated with examples in Chapter 6, Bioleaching of Copper and Uranium. Diagrammatic representations of ISL of fractured underground deposit and that of old mine workings are shown in Figs. 5.4 and 5.5.

Figure. 5.4. ISL of fractured underground deposit.

Figure. 5.5. ISL of old mine workings.

ISL is based on several interdisciplinary disciplines such as geology, geomicrobiology, solution chemistry, process chemical engineering, rock mechanics, and metallurgical chemistry. Appropriate lixiviant solutions are pumped through permeable rock media in such a manner that desirable mineral components are solubilized. The pregnant solution is then collected and pumped to the surface and treated for metal recovery.

Leaching of metals can be done in underground stopes, promoted by bacterial oxidation processes. Permeability of the ore matrix is an essential criterion. Newer methods of fracturing the rocks through blasting would facilitate lixiviant permeation through the ore body. The following procedural aspects for ISL need to be considered for process optimization.

•

Rubblization and solution wetting: Different rock fracturing methods to increase permeability. Fracture direction and dimensions need be controlled. For uniform mineral leaching, proper passage of lixiviant solution through mineral-enriched zones of contact needs to be ensured. Solution flow patterns are dictated by physical nature of fractures. Uniformly distributed and interconnected fractures become essential to ensure effective mineral–solution contacts.

•

Confinement of leaching solutions and dissolved metals to the mining area is another challenge.

•

ISL chemistry and microbiology need to be understood.

Geological and topographical features relevant to in situ mining include location, size, and form as well as mineralogical compositions and lithology of the ore and gangue. Besides, the rock mechanical properties of the host rock and ore minerals are also important. The location is of practical significance as the leach solutions usually circulate in a downward direction and collect in the deepest accessible regions, and need to be pumped to surface. Increasing depths leads to enhanced pumping costs. For uniform contact with leach solutions, the ore body needs to be regular and compact. Veined ore bodies with extended strikes and depths may require multiple solution injection points. Mineralogical characteristics of the ore and gangue constituents will determine dissolution rates and choice of lixiviants [26].

Based on the position in relation to the water table, solution mining systems have been classified into three types [29].

1.

Deposits above the water table or exposed in the bottom of open pits.

2.

Deposits below the natural water table, but not so deep that conventional underground techniques are uneconomical.

3.

Deposits located so far below water table that conventional mining methods are generally uneconomical.

Many of the commercial in situ bioleaching trials involved the utilization of ore bodies which were initially exploited by conventional mining and subsequently considered depleted. For example, at Derg tyarskii mines in the former Soviet Union, leach solutions were prepared in microbial regeneration tanks and pumped to injection wells extending to mined-out stopes, drifts, and adits. Pregnant solutions were pumped up to the surface for metal recovery. Examples of ISL of virgin ore bodies are also known [26].