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Smith Ranch: North America's newest ISL uranium mine.

During the 1980s, In-Situ Leaching (ISL) emerged as an economically and environmentally preferred means for extracting uranium ores in the US. In 1989, Rio Algom acquired the Bill Smith properties in Converse County, Wyoming. In-place uranium reserves now exceed 19,600 t [U.sub.3][O.sub.8], with an average grade of 0.10% [U.sub.3][O.sub.8]. Following a review of prior work and substantial additional geological, licensing and engineering studies, Rio Algom Mining Corp. (RAMC), the wholly-owned indirect US subsidiary, has constructed a commercial ISL mine, the Smith Ranch Project, with planned annual capacity of 907 t [U.sub.3][O.sub.8] from next year.

Development began in May 1996 with an approved capital budget of US$44 million. Leaching operations began on June 20 this year and 1997 production is expected to be 180 t. Early in 1997, RAMC began an 18-month, US$4.1 million programme at Reynolds Ranch, immediately north of the Smith Ranch area, to confirm and expand resources of 3,720 t [U.sub.3][O.sub.8], and conduct environmental baseline studies for possible production beginning in the year 2000.

RAMC already produces uranium by solution mining in the Ambrosia Lake area north of Grants, New Mexico, where it owns nine closed underground mines and a large closed uranium mill. The process involves circulating water through the old workings and then through an ion exchange plant. RAMC produced 83.9 t of uranium in 1996 from mine water recovery activities in New Mexico and plans to increase production from the current rate to some 160 t [U.sub.3][O.sub.8] in 2000.

Regional geology

Smith Ranch is located in the southern portion of the Powder River Basin near Douglas and Glenrock, Wyoming. All the important uranium deposits in the basin are in Tertiary strata. Within the permit boundary the host sandstones for uranium mineralisation are the arkosic sandstone units of the upper Paleocene Fort Union formation and lower sandstone units of the Eocene Wasatch formation.

The Wasatch formation is the youngest bedrock unit throughout the permit area with thickness ranging from 61-91 m in the northern and southern portions of the permit area, and up to 152 m in the central area. The Fort Union formation is over 305 m thick. However, only the upper 183-213 m contains the arkosic sandstone units with associated uranium mineralisation.

RAMC has arbitrarily named the major sandstone and shale units within the permit area. Sandstone units from youngest to oldest are E, W, U, S, Q, O, M and K. Actual contact between the Fort Union and Wasatch formation is defined as the base of the School Coal Seam or the correlatable lignite zone present throughout the permit area. In general, the contact would be at the top of W sandstone unit when present.

Resources for the permit area are primarily in the Paleocene Fort Union formation. The O, M and K units account for the bulk of resources with Q, S and U units locally having significant leachable reserves. Thickness of these sandstone units ranges from 3-60 m with the O sandstone the thickest and most persistent. The ore occurs as typical Wyoming roll fronts, generally north facing C shaped features. The sandstone units, depending upon thickness, interbedded shales and high lime zones, can contain one to 20 mineral fronts with the O unit being the most complex.
Figure 1: Smith Ranch lSL pilot summary

                           Q-sand              O-sand

Leaching period            Oct 1981-Nov 1984   Aug 1984-Jan 1991
Restoration period         Nov 1984-May 1986   -
Restoration certified      Aug 1987            -

Pilot flow rate            378 litres/min      568 litres/min
5-spot pattern size        30m x 30m           36 m x 36 m
Ore depth                  152 m               229 m

In-place reserves          61,235 kg           103,480 kg
Production                 35,380 kg           96,615 kg
Recovery                   58%                 93%

Fluid processed
Pore volume                11,730 [m.sup.3]    45,800 [m.sup.3]
Pore volume - production   42                  45
Pore volume- restoration   20                  -
Days/pore volume           22                  56

Laboratory scale tests by the previous owner showed the amenability of the main ore deposits to a mild alkaline lixiviate. This also was confirmed by two successful field pilot programmes.

Pilot testing

Pilot operations were conducted to obtain plant and wellfield information for economic analysis and to satisfy Wyoming Department of Environmental Quality requirements for licensing. Insofar as possible, they simulated commercial operations. Two tests were run, one each in the Q sand unit of Sec. 36 and the O sand unit of Sec. 26. The Q test operated from October of 1981 through November 1984 with restoration continuing to May 1986. Aquifer stability and restoration of the Q test was accepted by the state in August 1987. The O test began in August 1984 and continued through 1990. Results of this pilot testing are summarised in figure 1.

The Q unit is one of the thinnest ore-bearing units in the Smith Ranch area. It sometimes thins to 1 m or less and the entire sand interval can be mineralised. Within the test area, it ranges from 3 to 15 m in thickness. O is, in general, the thickest zone within Smith Ranch. It is formed from the coalescing of several individual sand units. In the test area, it is 75-90 m thick with numerous interbedded discontinuous mud-stone units. Uranium mineralisation in the test area occurs in the lower one-third of the sand unit.

Both wellfields were arranged as five-spot patterns - four injection wells arranged in a square with the recovery or extraction well in the centre. All wells, including monitor wells were connected to a wellfield header hose where injection and recovery flow meters, pressure meters, injection and recovery flow controls, oxygen mixers and sample ports were located. Each monitor well was equipped with a pump for ease of sampling at the header house. The well fields were five-spots and the recovery plant was a standard anionic resin ion-exchange (IX) system. Since both tests operated simultaneously for some period, two IX systems were installed.

Injection and recovery pipelines to each well were buried 1.5 m deep. Pregnant lixiviate from each recovery well was co-mingled at the header house and pumped directly to the plant. Injection lixiviate was pumped to the header house and distributed to individual injection wells. Lines between the header house and plant were also buried 1.5 m deep.

The average uranium concentration for the Q test, from inception until its average dropped to 20 mg/litre (a generally accepted cutoff point to begin restoration), was very close to 90 mg/litre. The Q wellfield averaged about 70 mg/litre over its life. The Q sand recovery wells produced at 0.95 to 1.25 litre/s while the O wells produced between 1.25 and 1.89 litres/s. The difference was due to differing license conditions for each test. Aquifer drawdown was not a problem. Balanced injection was maintained within 0.65 litre/s.

Lixiviate chemistry was a standard bicarbonate and carbon dioxide leach with oxygen. Early in the Q test sequence, hydrogen peroxide was used as a downhole oxygen source. Later, gaseous oxygen was used, decreasing costs accordingly. All ISL domestic [US] operations now use gaseous oxygen as the oxidant. As the final phase of the pilot programme, several cores were obtained from the two wellfields during 1990. Analysis of the cores provided direct information regarding the physical and geochemical state of the ore bodies following prolonged leaching.

Two cores were cut within the Q pilot and five within the O sand site. Samples were analysed for residual uranium and subjected to petrographic study. More than 90% of the uranium mineral was removed from the clean sands within the Q pilot. The only significant residual minerals were intimately associated with impermeable clays, shales and organic debris. Residual ore grades (in place tails) of less than 0.005% were common. Results from the O pilot confirmed this high efficiency leaching. Residual mineral was primarily associated with the impermeable zones. This illustrates the key fact that only reserves accessible to the lixiviate can be mined in an ISL operation. Reserves associated with shales and impermeable cemented zones are not mineable in-situ reserves.

Commercial operations

Smith Ranch is extracting uranium from sandstones at depths ranging from 137 to 305 m. Once extracted, the uranium is recovered by ion exchange. Periodically, the ion exchange resin becomes saturated with uranium. Uranium is removed from the resin by contact with a salt water solution. The ion exchange resin, stripped of uranium, is returned to recover additional uranium. The eluted uranium is precipitated, washed to remove impurities, dried and packaged for shipment.

The mechanics of lSL mining are relatively straightforward. As the lixiviate moves through the aquifer contacting the ore, the oxygen reacts and oxidises the uranium to the +6 valence state. The oxidised uranium then complexes with the carbon dioxide and water to form a soluble uranyl dicarbonate ion [[U[O.sub.2](C[O.sub.3])2].sup.-2]. Pregnant lixiviate flows to a recovery well, is pumped to surface by submersible pumps and then piped to a surface recovery plant. At the plant, the uranium is removed from the fluid by ion exchange. The barren fluid is refortified with carbon dioxide and oxygen and reinjected to extract additional uranium.

ISL mining selectively removes uranium from the orebody. No tailings are generated by the process, thus eliminating a major concern associated with conventional uranium mining. When installing an ISL wellfield, only limited surface disturbance occurs. Much of this will be reseeded and reclaimed during the operating life of the well field. The final product of the recovery plant is vacuum-dried yellow-cake, which minimises the potential for airborne uranium particulates.

After each mining phase of the project, reclamation in that area will be undertaken. After completion of groundwater restoration, which will be approved by the Wyoming Department of Environmental Quality and the Nuclear Regulatory Commission, all cased wells will be permanently plugged and capped. The casing will be cut off below plow depth and the site revegetated. Similarly, all other surface disturbances will be reclaimed and state-approved grass seed mixtures will be used to re-establish vegetation.

The wellfield areas are divided into mining units for scheduling development and establishing baseline data, monitoring requirements and restoration criteria. Each mining unit consists of an 8 to 24 ha reserve block. Approximately 15 such units will be developed. Two to three mining units are in production at any one time with additional units in various stages of development and restoration. A mining unit is dedicated to only one production zone and typically will have a flow rate in the 189 litres/s range. Aquifer restoration of a mining unit begins as soon as practicable after mining in the unit is complete.

In each mine unit, more lixiviate will be produced than injected. This creates a localised hydrological cone of depression or pressure sink. This pressure gradient provides containment of the lixiviate by causing natural groundwater movement from the surrounding area toward the mine unit. It is expected that the over production or bleed rates will be a nominal 0.5% of the production rate for the Q mining unit and a nominal 1.5% for the O unit.

Production zone monitor wells are located approximately 150 m beyond the mining unit perimeter with a maximum spacing of 150 m between wells. Monitor wells are also completed in the aquifers directly overlying and underlying the production zone. Such monitor wells are uniformly distributed across the mining unit area with one overlying and one underlying monitor well for each 1.6 ha of wellfield.

Each injection and recovery well is connected to the respective injection or recovery manifold in a header building. The manifolds route leaching solutions to pipelines that carry them to and from the IX facility. Flow meters, control valves and pressure gauges in the individual well lines monitor and control the individual well flow rates. Wellfield piping is high density polyethylene pipe, PVC and steel. The individual well lines and the trunk lines to the recovery plant are buried to prevent freezing. Field header buildings and buried lines are proven to protect pipelines. The pilot programmes employed this method and operated continuously through the winters without freeze-ups or other significant weather related problems.

Well completion

Monitor, production and injection wells are drilled to the top of the target completion interval with a truck-mounted rotary drill rig using native mud and a small amount of commercial viscosity control additive. The wells are cased and cemented to isolate the completion interval from all overlying aquifers. Cement is placed by pumping it down the casing and forcing it out the bottom of the casing and back up the casing-drill hole annulus.

The well casing is Schedule 40 PVC, which is available in 6 m joints. Typical casing has a 127 mm nominal diameter with a minimum wall thickness of 6.55 mm and a pressure rating of 1,480 kPa. Three casing centralisers located approximately 9, 27 and 46 m above the casing shoe are placed on the casing to ensure it is Centred in the drill hole and than an effective cement seal results.

The cement volume for each well is 110% of the calculated volume required to fill the annulus and return cement to the surface. The excess is to ensure that cement returns to the surface. Occasionally the drilling may result in a larger annulus volume than anticipated and cement may not return to the surface. In this situation the upper portion of the annulus is cemented from the surface.

After the cement has cured, the plug is drilled out and the well completed. The well is then air lifted to remove any remaining drilling mud and cuttings. A small submersible pump is used for final clean-up and sampling. If sand production or hole instability problems are expected, wire wrapped screen or a similar device may be installed across the completion interval.

After a well is completed and before it is operational a Mechanical Integrity Test (MIT) of the well casing will be conducted. In the MIT, the bottom of the casing adjacent to or below the confining layer is sealed with a downhole packer, or other suitable device. The top of the casing is then sealed and a pressure gauge is installed inside the casing. The pressure in the sealed casing is increased to a minimum of 20% above the maximum anticipated operating pressure and the well is closed, and all fittings are checked for leaks. After the pressure is stabilised, pressure readings are recorded at two minute intervals for ten minutes.

If a well casing does not meet the MIT, it is repaired and retested. If a well defect occurs at depth, the well may be plugged back and recompleted for use in a shallower zone provided it passes a subsequent MIT. If an acceptable MIT cannot be obtained after repairs, the well is plugged. A new well casing integrity test is conducted after any well repair using a downhole drill bit or under reaming tool.

Drilling is focussed on the first three production areas, each capable of producing over 450 t [U.sub.3][O.sub.8]. Final delineation of the first area was completed in December 1995, and as a result both the monitor and operating well locations were finalised in early 1996. Drilling and installation of monitor wells began in June followed by installation of operating wells beginning in October. This latter task continued into the middle of this year. At year end 1996, 140 of the nearly 200 wells required for the initial startup had been drilled and cased. This wellfield should produce some 450 t [U.sub.3][O.sub.8].

Final delineation of the second wellfield was completed in early 1997. Monitor and operating wells for this area are being installed to facilitate startup of full production on May 1, 1998. The production rate of 907 t/y [U.sub.3][O.sub.8] also requires the development and startup of the third wellfield in 1998. Delineation drilling and wellfield installation efforts continue on schedule with 15 drilling rigs.

Processing plant

There are two IX recovery plants and a Central Processing Plant (CPP). Facility #1 is next to the CPP while the second is a satellite unit (IX Facility #2), both equipped with resin loading and bleed treatment circuits. Each facility can process 189 litres/s of lixiviate. IX resin is transferred by pipeline between Facility #1 and the CPP. Truck trailers are used for IX Facility #2. The CPP elutes resin from both IX Facilities. The precipitation, product filtering, drying and packaging circuits process up to 2.54 t/d [U.sub.3][O.sub.8] (907 t/y).

Resin loading-The resin loading circuit in each IX Facility consists of six pressurised vessels, each containing 14.2 [m.sup.3] of anionic IX resin. These vessels are configured as three parallel trains for two-stage downflow loading. Booster pumps are located upstream and downstream of the trains.

As the pregnant lixiviate enters the IX Facility, the upstream booster pumps pressurise the fluid to 791 kPa. The dissolved uranium in the lixiviate is chemically adsorbed onto IX resin. Any sand or silt entrained in the pregnant lixiviate is trapped by the resin bed like a traditional sand filter. The barren lixiviate exiting the second stage normally contains less than 2 mg/litre of uranium. This fluid will be pressurised to 791 kPa by booster pumps and returned to the wellfield for reinjection.

The lixiviate is native groundwater, carbon dioxide and oxygen. Carbon dioxide is added in the IX Facility, both upstream and downstream of the resin vessels. Oxygen is added to the barren lixiviate at the wellfield header prior to the injection manifold. The lixiviate concentration of carbon dioxide will be maintained at approximately 2,000 mg/litre while the oxygen concentration will approximate 500 mg/litre.

Bleed treatment- To control the movement of lixiviate within the ore zone, a fraction of the barren lixiviate is continuously removed. More fluid is produced than injected. This bleed, or blow-down, creates a hydrologic cone of depression within the ore zone causing natural groundwater from the surrounding area to flow toward it. This negative pressure gradient holds or contains the lixiviate within the desired ore bearing region and prevents the unwanted excursion of lixiviate away from the ore. It also minimises the lixiviate dilution by uncontrolled fluid movement. Bleed rates approximate 0.5% of the production rate for Q sand mining units and 1.5% for O units.

The bleed fluid is treated to remove radium mobilised by the ISL mining process and residual uranium normally contained in the barren leach solution. Uranium removal is accomplished by additional IX treatment in a single train of two-stage downflow vessels. Radium removal is effected with conventional barium/radium sulphate co-precipitation. A filter press removes the barium/radium sulphate precipitant.

Elution circuit- When resin in a first stage IX vessel is loaded with uranium, the vessel is isolated from the normal process flow. The resin is transferred in 14.2 [m.sup.3] lots to the CPP. In IX Facility #1, the transfer is hydraulic using dedicated transfer piping. For Satellite IX Facility #2, a bulk tank trailer is used. At the CPP, the resin passes over vibrating screens with wash water to remove entrained sand particles and other fine trash. It is gravity fed into pressurised downflow elution vessels for uranium recovery and resin regeneration.

In the elution vessel, the resin is contacted with an elute containing about 90 g/litre sodium chloride and 20 g/litre sodium carbonate (soda ash) which regenerates the resin. The eluted resin is rinsed with fresh water and returned to an IX vessel for reuse.

Using a three-stage elution circuit, 170 m3 of eluate contact 14.2 [m.sup.3] of resin to create 57 [m.sup.3] of rich eluate which contains 10-20 g/litre [U.sub.3][O.sub.8]. The fresh eluate, 57 [m.sup.3] per elution, is prepared by mixing quantities of saturated sodium chloride solution, saturated sodium carbonate solution and water.

Precipitation circuit- In the elution circuit, the uranyl dicarbonate ions are removed from the loaded resin and converted to uranyl tricarbonate by a small volume of strong sodium chloride/soda ash solution. The resulting rich eluate contains sufficient uranium for economic precipitation.

Sulphuric acid is added to the rich eluate to break the uranyl carbonate complex that liberates carbon dioxide and frees uranyl ions. The acidic, uranium rich fluid is pumped to agitated tanks where hydrogen peroxide is added (0.2 kg [H.sub.2][O.sub.2]/kg [U.sub.3][O.sub.8]) in a continuous circuit to form an insoluble uranyl peroxide compound. Ammonia is then added to raise the pH to near neutral for digestion.

The uranium precipitate (slurry) gravity flows to a 11.6 m diameter thickener. The uranium depleted supernate solution overflows the thickener to surge tanks for disposal via a deep injection well.

After precipitation, the settled yellowcake is washed, filtered, dried and packaged in a controlled area.

For control and monitoring purposes, the instrumentation philosophy provides for two separate control systems. Each system is fitted to accommodate the steady state (wellfield/resin loading circuit and precipitation) or batch flow nature (bleed treatment, elution and product filtering, drying and packaging) characteristic of the process flow streams.

Since the wellfield/resin loading circuit operates at a steady state, small deviations from the normal operating flow rates and pressure profiles ([Angstrom 10% or greater) indicate major operating upsets. An automatic Emergency Shut Down (ESD) system consisting of pressure and flow rate switches is provided for this circuit. If an automatic shut down occurs, an alarm notifies the operator of the situation. Once the major upset is identified and corrective action taken, only then can the circuit be manually restarted. This type of control system provides the best protection against major spills. Backup for the automatic ESD system is provided by local displays of the same flow rates and pressures that the ESD system monitors.

The batch mode circuits are controlled by PLCs that automatically open and close the appropriate valves once the processes are manually initiated. The PLC provides closed loop feedback control of the flow rates in the elution and precipitation circuits. All automatic valves have manual control override. Local indication of pressures, levels, flow rates, pH and temperature are provided for complete manual control of these circuits as required.

The environmental impact of ISL mining and yellowcake processing are minimal. No tailings are created. Nearly all radioactive daughter products remain underground. Airborne emissions from yellowcake drying are kept at an absolute minimum by a vacuum drying system. Only radon gas is mobilised during the process and is readily controlled by conventional scrubber technology.

lSL mining of uranium stands today as an outstanding example of a multi-disciplinary technology. The skills of geologists, hydrologists, reservoir engineers, chemical engineers, geochemists and mechanical engineers are interwoven into every successful project. These technical skills coupled with strong management and environmentally aware organisations are the real keys to ISL.

RAMC has sales contracts signed and pending for future deliveries of production totalling approximately 2,990 t [U.sub.3][O.sub.8] to the year 2003, representing approximately 67% of Smith Ranch's first five years of production. Approximately 60% of these are matched sale contracts, requiring delivery of an equivalent quantity of Russian origin uranium.

Dennis E. Stover, Vice president, engineering and project development, Rio Algom Mining Corp.
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Title Annotation:Mining North America; in-situ leaching
Author:Stover, Dennis E.
Publication:Mining Magazine
Date:Oct 1, 1997
Previous Article:The KMS commitment to mining.
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