TEST PROCEDURES FOR THE METALLURGICAL EVALUATION
K. LANDSIEDEL, SCHROER Uranerzbergbau GmbH,
3. METALLURGICAL INVESTIGATIONS
In the early stages of test work, metallurgical investigations will be performed in the laboratory or on bench scale using representative ore samples from 50 to 200 kg. Several tests can be run at the same time in order to save time and funds. Parameters will be varied until optimum treatment conditions are reached. At the end of this test work, conceptual flow sheet and material balance of the process will be available ]3].
This step will cost between US $25 000 and US $100 000 and requires 3 to 12 man-months. A list of the different process steps and methods applied in the treat-ment of uranium ores is provided in Table II.
The final process will later be pilot tested, either entirely or for specific sec-tions. This is to confirm the previous laboratory results and to test the process, prior to plant engineering, in a larger and continuous operation.
A pilot test facility has a capacity of about 0.25-4.00 t/h and a total ore demand of 50-200 t or more. The test costs about US $ 0 . 5 - 2 million and requires
1.5—5 man-years.
3.1. Mechanical treatment
Mechanical treatment of ore comprises crushing, grinding, ore storage and mill-ing. Also preconcentration methods like flotation, gravimetric or electric separation,
IAEA-TC-453.5/4 103 TABLE I. COST AND ENERGY DISTRIBUTION FOR URANIUM MILLING
Cost centres Costs Power consumption
Comminution
Chemical storage, power supply Maintenance
and colorimetric or radiometric sorting are used during mechanical treatment [4].
Radiometric sorting is, however, the predominant method for preconcentration of uranium ore. Applicability of radiometric sorting can easily be tested in the labora-tory using hand held counters and specific small conveyors. Prerequisite is a high contrast ore, i.e. a good radiometric contrast between waste material and ore.
Tests on comminution can be performed in two steps. Crushing of at least a tonne size sample is recommended. This crushing test should be done by the manufacturer or supplier of the selected crushing equipment. Grinding tests require about 30 kg of material and can be performed by most test laboratories. This test determines the Bond Work Index which gives information about the energy require-ment of the grinding mill [5]. The index unit is kWh/t, usually between 6 and 25 for uranium ores. The index is calculated as follows:
44.5 • 1.1023
po.23 . Go.82 10 10
10 10
W,Bond
where P is the fines
F is the feed ore (/xm) G is fines/revolution (g)
104 LANDSIEDEL and SCHROER
TABLE II. METHODS FOR TREATMENT OF URANIUM ORES
Ore feed
3.2. Ore leaching tests
The leaching method to be selected depends on the chemical analysis of the ore.
For ores higher in carbonate than about 10% the alkaline leaching method is preferred, but in most cases leaching tests with both methods are conducted. Reagent consumption versus recovery rate is monitored through the test. The final decision upon the best leaching method is mainly based on the economics of the above ratio.
IAEA-TC-453.5/4 105 TABLE III. LEACHING CONDITIONS DURING STANDARD PROCESSES
Temperature
In acid leaching, acid consumption should be less than 200 kg acid per tonne ore, but other parameters like uranium grade, anticipated recovery, ore composition and structure must be taken into consideration [3].
During detailed leaching tests all relevant process parameters like retention time, temperature and reagent consumption must be investigated. (See Table III.) The applicability of pressure leaching or multiple stage leaching in Pachucas or vessels must also be considered.
Besides tests on conventional agitated leaching, tests on other methods like in situ leaching (ISL) or heap leaching are often of interest. Suitable tests on the permea-bility of the ore and core leaching are carried out in a special ISL test set-up. Core leaching results are reached from the viewpoint that cores from exploration drilling in general are vertically oriented to the bedding, while ISL is done parallel to the bedding (see Fig. 1). Therefore, the test results must be factored because normally the horizontal transmissivity is higher than the vertical.
The winning of uranium by heap leaching is of interest for dressing low grade ores [6]. For ores containing pyrite and sulphide, biologically assisted heap leaching can be investigated. The oxidation and acid built up by bacteria can result in high uranium recovery and in cost saving [7]. The duration of test work, however, is much longer than for standard laboratory leaching tests, because of the long retention time for ISL and heap leaching processes. Recoveries in the range of 75-85% can be reached in 100-300 d. However of great interest is the leaching effect during the first 30—50 d. From those test results the possible final recovery can be extrapolated by computer simulation.
3.3. Solid-liquid separation tests
The discharge from the leaching vessels contains the pregnant solution and the barren solids. Solids and liquids must be separated before the solution can be further processed for uranium extraction. Sedimentation and filtration tests are performed to
106 LANDSIEDEL and SCHROER
°2
1) = Leach solution vessel
s Pressurized core leaching device s Core
T ) 3 Graduated cylinder
= Multi channel recorder
» Pressure release valve
= Differential pressure manometer
FIG. 1. Test station for core leaching (ISLj and permeability tests.
determine the equipment requirements. In addition to these methods a sand slime separation can be carried out for ores easy to leach and so to separate.
3.3.1. Settling tests
The most common method for solid-liquid separation is countercurrent decanta-tion (CCD). Washing effect and required diameter of the thickener are determined in settling tests by studying the settling rate of the leached ore. The settling rate can be accelerated by addition of 50-200 g flocculants per tonne of solids. Sedimenta-tion tests on pulp with and without addiSedimenta-tion of flocculants can be carried out in simple graduated cylinders. After feeding the pulp into the cylinder, the position of the inter-face between solid and liquid is measured over time. With the test results the required unit area of the thickener and the underflow density can be calculated according to the Kynch method (see Fig. 2). For uranium ores the unit area is usually in the range of 0.1-1.0 m M - d "1 [8,9].
3.3.2. Filtration tests
Filtration is usually used for separation of solids and liquids in alkaline processes. A simple test with a hand filter plate 10—20 cm in dia can be carried out to determine the required filter area for belt filters. The suitability of several filter cloths should be investigated [10]. The specific filter capacity for bulk products usually ranges between 2 and 15 t-m"2-d"'. The test results can be used for the layout of several filter types.
IAEA-TC-453.5/4 107
Ho
30 t [mini
796ml
1 [min]
HCmm]
0 240=H0
1 197
2 170
3 150
5 115
10 85
15 7B
20 73
30 66= Hu
l>Cmin)Vw.p,,pCinl] . 11.5 796 M J ., 1.U SF,.d0..[g] H0[mmJ U ( 77.3 240
FIG. 2. Evaluation of settling tests.
3.3.3. Sand-slime separation tests
Uranium extraction from leached pulps with poor settling characteristics is facilitated by resin-in-pulp (RIP) methods. However, the coarse mainly barren frac-tion has to be separated from the pulp prior to their applicafrac-tion. While in earlier times of uranium extraction mechanical classifiers were used, today hydrocyclones are commonly used for classification.
The efficiency of hydrocyclones can be tested in the laboratory by determining the mass distribution in the split streams and the effective separation grain size. The minimum pulp feed should exceed 30 L of the representative pulp, with a grain size of about minus 0.5 mm [11].
3.4. Purification tests
For purification and upgrading the leach liquor, ion exchange (IX) or solvent extraction methods are usually applied. Direct precipitation of uranium from carbo-natic pregnant solution is possible, but at this time is not used commercially.
108 LANDSIEDEL and SCHROER
3.4.1. Jon exchange
The uranium extraction tests from acid and alkaline solutions can be carried out by IX [12]. Resins with good adsorption characteristics are available for a pH range from 1 to 14. For the test, different resins will be filled in glass columns with perfo-rated plates and the solution directed upflow or downflow through the resin bed.
Investigation of about 0.25 — 1 L of resin per specimen is needed for the test work.
Parameters like optimum flow rate, loading capacity and breakthrough time should be investigated.
Usually the flow rate is between 10 and 30 bed volume replacements (BV) per hour, the loading capacity between 20 to 60 g uranium per litre of resin. For the elu-tion of the resin, flow rates of about 10 BV/h will be used. With good eluants, 95%
of the adsorbed uranium should be stripped after 10 BV. The type of the eluant (10% H2SO4/(NH4)2SO4, NaCl) and the concentration have to be investigated.
3.4.2. Resin-in-pulp
The RIP adsorption method is usually only used for processes with low uranium grades and pulps with poor filtration or settling rates. In the tests unclarified solutions or fine suspensions are used as feed material. The test equipment can be an upstream column or multiple stage cells with air lift transport for the resin for continuous tests or simple agitated 0.2—1 L cells for batch tests [9]. After washing the resin with water the elution procedure is the same as for IX tests.
3.4.3. Solvent extraction (SX)
Solvent extraction is the most popular method for uranium extraction from acid solutions [13, 14]. First orientation tests can be carried out in simple stirred cells (100-250 mL volume). In earlier times, ether, tributyl phosphate (TBP) and alkyl-phosphoric acid were used as adsorption agents, now aliphatic amines are the most usual adsorbents, which are diluted with kerosene. Isodecanole is added as a modi-fier. For determination of the equilibrium curve (extraction isotherms), the pregnant aqueous and the barren organic phases are mixed in different ratios. The number of extraction stages can be calculated using the McCabe-Thiele method. Stripping of uranium out of the pregnant organic phase can be investigated by the same method with stripping solutions such as (NH4)2SO4. Important factors for the extraction and stripping are pH and ion concentration. Several litres of pregnant solution have to be available for these tests.
For the final design and layout of an SX process continuous tests with a multiple stage set-up have to be carried out. A volume of about 0.25 to 1.0 m3 must be available.
IAEA-TC-453.5/4 109
Raffinate/
Process
el I I Lii Water! I
C D
Neutralization pH 7-7.S
Filtration
BaCli Lim«
C O
Ba/RaSOi Coprecipitation pH » l l
Filtration
H J S O J
C D
pH-Adjustment pH -7
Precoat-Filtration
Solids Disposal Chemical Analysis
FIG. 3. Treatment of mill effluents.
3.5. Precipitation tests
The last process step in the uranium recovery cycle is the yellow cake precipita-tion. Precipitation tests can be done in the laboratory without special equipment. The uranium bearing solution is stirred in a beaker and the precipitation agent is added.
Parameters like temperature, retention time, type of precipitation agent, final pH and a number of steps must be investigated. After washing the yellow cake precipitate the product quality and impurities must be assayed and compared with product specifications. The type of precipitation reagent depends on the leach chemistry used.
For alkaline and carbonatic process solutions precipitation with sodium or mag-nesium hydroxide is the best way, for acid solutions precipitation with ammonia, hydrogen or hydrogen peroxide is preferred. Sometimes peroxide precipitation is mandatory to prevent co-precipitation of other metals. In this case, alkaline solutions have to be acidified.
3.6. Waste management
Strict regulations must be adhered to for disposal of the solid and liquid wastes derived from the mining and milling process. A pH adjustment and stabilization with lime is usually sufficient for disposal of the solid waste pulp. The treatment of liquid effluents is more complicated. Figure 3 shows one possible way of effluent treatment [15]. In the laboratory the first step of investigation is the addition of lime (bulk precipitation) and pH adjustment. After filtration barium chloride and lime are
1 10 LANDSIEDEL and SCHROER
added for Ba-Ra-sulphate co-precipitation. After another filtration step remaining radium and concentrations of other relevant ions are determined in the filtrate.
Radium can be analysed by the emanation method or by direct alpha spectroscopy.
In most IAEA Member States the Ra concentration in effluents should not exceed 0.7 to 3 pCi/L1.
Besides the Ba precipitation, adsorption tests with Ra specific complexers [16]
can be carried out by the method described in Section 3.4.1.
4. INFRASTRUCTURE
During planning and evaluation of the metallurgical tests specific conditions in the country, such as climate, infrastructure and availability of reagents must be taken into account. For example the availability and quality of process water must be inves-tigated. In addition the influence of ambient temperature, barometric pressure and wind velocity are necessary for process building design. The availability of operators and their skill levels influence the selection of machines and processes and the grade of automation.