THE HEAP LEACH SIMULATION SYSTEM

The basic test unit developed during this study is a cylindrical column operated with closed loop circulation; multiples of this basic unit were used during the experimental studies. The ore charge is retained in each column by a perforated plate to provide for percolation type leaching contact. A tank placed under the column holds the solution inventory required for a leaching test and also serves for collection of column effluent. A diagram illustrating the piping plan and elevations for a single unit, prepared for conventional downflow circulation, is shown in Fig. 1.

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18' - 6 "

FIG. 1. Column piping plan and elevations.

Column construction details

The basic structural component of the columns is a 2 ft dia, 5 ft long cylindrical section with flanged ends. Two sections are butted together and a 45° flanged cone attached to the bottom section. An assembled 10 ft column is equipped with three auxiliary liquid sampling ports mounted on one side of the unit and spaced at 2 ft intervals, and a solution level control box is mounted on the upper tube section.

The ore charge is retained by a perforated plate located between the lower flange and the cone, as shown in Fig. 2. The cylindrical sections and cone are constructed of

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DETAIL A - COLUMN TOP VIEW WITH OVERFLOW BOX 24" i.d.

(Reinforced epoxy fibre glass)

1/4 in bolt holes on a 26-3/8 in diameter bolt circle at 3 in centers.

30 holes required.

DETAIL B •• MOUNTING FLANGE (COLUMN AND CONE)

1/4 in bolt holes equally spaced.

1/4 in bolt holes equally spaced.

DETAIL D - FRONT VIEW OF CONE FLANGE 1. Overflow box inlet is a 1 in female threaded L-shaped PVC pipe fitting.

Attachment of a variable vertical extension is optional.

2. Gaskets are required on all mounting flanges.

3. Overflow outlet, sampling port outlet, and cone outlet flanges are 3/8 in thick.

FIG. 2. Detailed column design.

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fibreglass with a 3/8 in wall thickness². The perforated retainer plate is 1 in thick fibreglass with 1/4 in holes drilled at approximately 1 - 1/2 in spacing. A one-layer covering of neoprene matting and three to four layers of vinyl screening resting on top of the perforated base plate retain the ore fines. Figure 3 shows the appearance of the perforated base plate and the neoprene matting on the supporting flange.

Auxiliary equipment for each column unit consists of a 400 gal fibreglass reservoir3, a 1/4 hp gear drive portable mixer4, a totally enclosed centrifugal pump, and associated poly vinyl chloride (PVC) piping and ball values.

A steel framework supports the columns and allows ground level access to the base of the columns for discharging and handling leached residue. The framework is also equipped with a platform access to the top of the columns for ore loading (Fig. 4).

Heap leach simulation procedure

The heap leach simulation procedure consists of the following sequential steps:

(1) ore preparation, (2) ore loading, (3) preliminary wetting, (4) leaching, (5) residue washing, and (6) recovery and analysis of solution and residue. Details of these steps are discussed below.

Ore preparation

Each ore tested during the experimental studies was spread on a concrete pad and turned intermittently to air dry over a 3 to 7 day period. The ore was then screened on a Ty-Rock⁵ double-deck screen using a 4 in top screen and a 1/2 in or 1 in bottom screen. All plus 4 in ore was broken manually to pass the screen. The separate screen fractions were drummed for future use in preparing head samples for chemical analysis and feed samples for the leaching column. Feed and head samples were reconstituted by combining proper weight ratios of blended coarse and fine fractions.

A 300 lb reconstituted ore sample was used to prepare a head sample for chemical analysis6. This sample was crushed to minus 1/4 in and blended; then a 25 lb sample was split off. The 25 lb sample was roll-crushed to minus 10 mesh, reblended, and further split to provide analytical head samples.

2 1 in = 25.4 mm.

3 1 US gallon = 3.785 L.

4 1 hp = 0.746 kW.

5 Reference to specific trade names or manufacturers does not imply endorsement by the Bureau of Mines.

6 1 lb = 0.454 kg.

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FIG. 3. Ore retainer plates (scale in inches).

Ore loading

Blended ore charges were loaded into the columns using 5 gal buckets with a line attached to the bottom for dumping. Full buckets were raised to the upper access level by a hydraulic platform in groups of four to six buckets, lowered by rope to a point near the top of the ore level, and dumped. An average 8 ft ore charge weighed 2200 to 2600 lb.

Preliminary wetting

Tap water was measured into 4 ft by 4 ft reservoir mixing tanks, with exact amounts based on an operating volume of 50% solids. Water was pumped to the top centre of the ore beds at flow rates ranging from 1.0 to 11.0 gal/h with the column effluent valves initially closed. The water was allowed to soak through the ore beds by means of gravity flow to obtain air displacement by backfill. However, in some cases air pockets formed near the bottom of the beds; these were released by opening the effluent valves during the last portion of the wetting period. By this procedure,

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FIG. 4. Nine-column assembly used for test programme.

31 to 65 gal of water were used in a 4.5 to 70 h period. Following wetting, a 2 day period of downflow circulation was conducted to bring total water volume into a state of equilibrium with the ore charge. A constant head of approximately 12 in was maintained on top of the ore charge during circulation periods.

Leaching

As a continuation of wetting circulation, the leaching period was initiated by adding 93% H2SO4 to the reservoir mixing tank to a pH range of 1.0 to 1.2. When acid consuming components of the ore were neutralised, and the column effluent

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reached a pH of 2.0 or lower, oxidant in the form of NaC103 was added to induce an electromotive force (EMF) of —425 to —450 mV in the mixing tank. These pH and EMF conditions were maintained throughout the leaching period. Frequent atten-tion was required to maintain the desired pH and EMF during the first 5 to 10 days.

Thereafter, daily checks and adjustments at 4 to 5 day intervals were adequate.

Uranium solubilised during the test period was allowed to accumulate in the circulat-ing solution inventory. A 50 to 120 day leachcirculat-ing period was determined by a point of equilibrium in the U3O8 concentration found in reservoir or column effluent samples.

Test units were monitored for column effluent flow rates, pH, and EMF throughout the procedure. Sampling for U³O⁸ concentration was also conducted throughout the test period with particular attention given to contents of the column effluent stream and contents of the reservoir mixing tank. Samples were analysed at 4 to 8 h intervals during acid circulation, and then at 4 to 5 day intervals.

Additional information as to flow rates and U³O⁸ leaching characteristics was obtained by sampling side ports of the column units.

Residue washing

After the leaching period, the columns were allowed to drain for approximately 2 days, then water washed. Tap water was pumped into the freeboard space on top of the ore charges and allowed to percolate through the beds. This step was repeated until 6 to 10 volumes of 50 gal each had been added. After washing, the columns were allowed to drain for approximately 5 days. All of the wash effluent and drainings were collected with the pregnant solution, measured, and analysed for use in the metallurgical balance.

Recovery and analysis of solution and residue

Residue was recovered by disassembling and removing the cone base and retainer plate. A fork-lift was used to support the cone while the bolts securing the cone flange to the lower column flange were removed. The forks were lowered slowly. With some types of ore, residue plasticity provided for a clean exit of solids as the cone and retainer plate were lowered. Residues allowed to drain for extensive periods were not self-discharging and required additional effort for removal.

Discharged residues were spread on a plastic sheet, sectioned into a grid pattern, and sampled as required for moisture determination. Approximately one-fourth splits were taken from the grid pattern and air dried on a concrete pad. The splits were crushed and resplit as required to provide an analytical sample for U3O8. Screen assay tests were also conducted on the residues to determine mass and U3O8

distribution characteristics.

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TABLE I. ORES USED FOR HEAP LEACH SIMULATONS

Source Location Uranium company Head analysis,

U3O8% Golden Goose Mine

Bear Creek Mine Jackpile Mine Sunday Mine

Crooks Gap, WY Western Nuclear, Inc. 0.048 Bear Creek, WY Rocky Mountain Energy Company 0.029 Laguna, NM Anaconda Company 0.064 Uravan, CO Union Carbide Nuclear Company 0.034

Experimental data references and conclusions

The initial heap leach simulations were conducted on uranium ores as shown in Table I.

Detailed experimental data from the leach tests on these ores are described in two reports⁷ [1, 2].

Final phases in developing and proving the small scale heap leach testing technique were restricted to work with the Bear Creek ore. Two tests were conducted with 8 ft ore beds, one test was conducted with an 18 ft ore bed, and one field test was conducted by leaching an 18 ft heap located near Bear Creek Uranium Company mill in Wyoming.

Operating information and experimental data from the tests on the Bear Creek ore are presented in three reports⁷ [3-5].

All ore charges used in the Phase V studies were prepared and handled in the same manner as described earlier in this paper. The 18 ft ore bed was placed in a 20 ft column prepared by joining four of the 5 ft tube sections used in early studies.

Approximately 2500 short tons of Bear Creek ore were shaped into an 18 ft heap⁸. Essentially, the same procedure was used for leaching, washing, and residue exami-nation as described earlier in this paper, except for substitution of Bear Creek mill tailings pond water instead of tap water in all but one of the small column tests.

The following conclusions and comparisons for predicting the behaviour of a large scale heap leach from the 2 ft dia column leach results were developed during the Phase V and Phase VI studies:

(1) When solution depths above the ore bed are roughly equal, percolation rates will be similar for each scale of test.

7 Research carried out with the support of the Bureau of Mines under contract No. 0252022.

8 1 short ton = 0.907 t.

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J

Uf|Hg^(

IAEA-TC-453.5/17 177 (2) At essentially equal flow rates, leaching time for an 18 ft ore bed will be

approximately 20% greater than that required for an 8 ft bed.

(3) With leaching time equalized, uranium extraction results will be approximately the same for each scale of test.

(4) Reagent requirements for larger scale operations will be equal to or lower than those for the smaller scale test.

(5) Channelling or areas of insufficient acid wetting will either not occur or not cause problems in either scale of test.

(6) Lateral solution flow patterns cannot be predicted by small scale column testing because of confinement by the size of the column.