SHABBIR, NAEEM-UL-ZAMAN Pakistan Atomic Energy Commission,

University Town, Peshawar,

Pakistan

Abstract

PROCESS DEVELOPMENT STUDY ON URANIUM-BEARING CARBONATITE.

Mineralogical and chemical studies on uranium-bearing carbonatites showed the presence of apatite and calcite as major constituents. The presence of uranium in pyrochlore structure was suggested by X-ray diffraction studies. The average uranium content was of the order of 200 ppm. The standard amenability tests indicated the refractory behaviour of the ore with respect to carbonate leaching. A 78% leaching recovery was achieved employing elevated temperature and pressure. The physical upgrading of the ore was possible using conventional gravity separation methods.

INTRODUCTION

The discovery in Pakistan of uranium in carbonatite host rock was reported. Radiometric investigations revealed the presence of low-grade mineral over a considerable area. The low-grade nature of the ore necessitated an early and thorough study from the metallurgical point of view.

The first step towards such studies was to find out the type and composition of the ore and to perform amenability tests.

This paper deals with the laboratory and preliminary pilot-plant investigations. The results obtained are to be used in the development of a process now diagram for the recovery of uranium from this ore.

DESCRIPTION OF THE ORE

Microscopic examination of ore samples showed that it consists predominantly of calcite, apatite and biotite. Sodic hornblende is a subordinate constituent of the sample; magnetite and hydrous iron oxides are present in minor quantities. The radioactive mineral grains appear to be of Metamict origin. The internal structure of the grains has suffered from radiation damage.

The change of colour from yellow to yellow-brown and dark brown is indicative of the degree of radiation damage to the mineral. The X-ray patterns resemble those of the pyrochlore and betafide series [ 1 ]. Electron probe analysis of the hand-picked pyrochlore grains showed the presence of about 35% uranium, 22% niobium and 7% tantalum.

Spectrographic analysis of the ore showed that Ag, As, Au, В, Ba, Bi, Cd, Ce, Co, Cr, Cs, Ga, Ge, Hg, In, Ir, La, Li, Mo, Ni, Os, Pd, Pt, Rb, Re, Rh, Ru, Sb, Se, Sn, Sr, Ta, Th, Tl, U, W and Zn are not detectable.

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The elements detected by spectrographic analysis, together with their probable contents in per cent, are summarized as follows:

Ca > 10%

The chemical analysis of a typical sample is (assay per cent):

u

o

The standard amenability tests were made, using the following procedure. The ore was ground to -65 mesh and leached at 50% solids in a sodium carbonate and bicarbonate solution for 24 hours.

Leach solutions containing (a) 60 g sodium carbonate and 20 g sodium bicarbonate; and (b) 100 g sodium carbonate and 50 g sodium bicarbonate were used. The influence of oxidant was studied with the addition of 10 lb of potassium permanganate per ton of ore.1 The leaching temperatures were ambient and 80°C.

The leached slurry was filtered and the cake washed with two carbonate washes of 100 ml each and two water washes of 50 ml each. A carbonate solution containing 20 g sodium

carbonate per litre was used for washing when the leach solution contained 60 g sodium carbonate and 20 g sodium bicarbonate per litre. When the leach solution assayed 100 g sodium carbonate and 50 g sodium bicarbonate, the wash solution contained 50 and 25 g of sodium carbonate and bicarbonate respectively. Each wash was with half the volume of leach solution. The slurry was readily filtered. The data are summarized in Table I.

The data indicate that the ore responded poorly to the carbonate leaching. At atmospheric pressure the increase in carbonate and bicarbonate concentration gave a small increase in uranium extraction. The extraction is also increased to some extent by the addition of oxidant and an increase in temperature. A maximum of about 25% of the uranium was extracted by the standard amenability testing procedure. For the maximum extraction the leaching temperature was 80°C, and 10 lb of potassium permanganate per ton of ore was used. The carbonate and bicarbonate concentrations were about 60 and 25 g/1 respectively. The leaching was done at 50% solids.

For 24 hours the bicarbonate decomposition was about 30 lb per ton of ore. This is quite high for the low extraction of uranium.

Pressure leaching was done in a one-litre stainless steel vertical-type autoclave. The autoclave was equipped with a top-mounted, turbine-type agitator. The agitator was powered by an electric motor via a reduction pulley mechanism. The pressure was regulated manually and maintained using an oxygen gas cylinder.

1 Tons are short tons throughout the paper: 1 short ton = 2000 lb = 907.18 kg = 0.90718 tonnes.

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TABLE I. AMENABILITY TESTING DATA OF URANIUM ORE

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TABLE II. PRESSURE LEACHING DATA OF URANIUM ORE

Test

Two hundred grams of the ore ground in a disc mill to -65 mesh was leached at 50% solids for eight hours at 120°C in a carbonate solution containing 100 g sodium carbonate and 50 g sodium bicarbonate per litre. A gauge pressure of 90 lbf/in² or 50 lbf/in² was maintained with oxygen. The leached slurry was filtered. The cake was washed twice with carbonate solution containing 50 g sodium carbonate and 25 g sodium bicarbonate per litre using 100 ml of solution for each wash. The carbonate washing was followed by two water washes with 50 ml of water for each wash. The results given in Table II indicate that over 78% of the uranium was extracted.

The per cent extraction was pressure and particle-size dependent. The extraction of uranium was not improved by supplemental oxidation with potassium permanganate.

PHYSICAL UPGRADING OF ORE

Uraniferous multiple oxide minerals tend to be metallurgically refractory but can possibly be concentrated rather easily, making the concentrate potentially of economic interest. The investigations made by Dodd [2] on this mineral are suggestive of a uraniferous multiple oxide, thereby directing attention towards physical beneficiation studies. The view is supported by the heavy-media separation tests.

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TABLE III. PARTIAL CHEMICAL ANALYSIS OF VARIOUS FRACTIONS OBTAINED BY HEAVY-MEDIA SEPARATION

A sample was screened through 325 mesh and the +325 mesh was then separated into light and heavy fractions with tetrabromoethane (specific gravity 2.94). The results of the chemical analysis of the three products are shown in Table III. This analysis shows that approximately a seven-fold concentration of U3O8 has been achieved in the heavy mineral fraction, while maintaining a 77% recovery.

A batch test employing a gravity separation technique using a Wilfly shaking table proved effective.

To obtain technical data for the physical upgrading of the ore a pilot concentration unit was designed which consists of the following:

Crusher

Fine ore storage bin Conveyor belt

Continuous discharge wet grinding mill Vibrating screen

Wilfly shaking table Thickeners

Storage tanks for concentrate, middling and slimes.

The flow diagram of the pilot concentration unit operating in the PAEC Laboratories is shown in Fig. 1.

The ore is crushed in the jaw crusher to -h in size and fed manually to a 5 ton hopper. A 9 in wide feeder belt operating at a speed of 2 ft/min takes the charge from the hopper through an adjustable gate and discharges it into the grinding mill. The 24 in dia. by 48 in long grinding mill is operated at 60 rev/min powered by a 20 h.p. motor. The grinding media (1 in dia. rods) and mill lining are of high manganese steel. The grain size is controlled by the charge of the grinding media and the solid-liquid ratio.

The grinding mill discharge falls onto a vibrating screen fitted with a 30 mesh wire gauze. The oversize particles are recycled and the rest pumped to the table through a 1 in cyclone separator.

The slimes are separated with overflow of the cyclone and a slurry of the required density is obtained as underflow by varying the vortex opening. The liquid-solid ratio on the table is controlled by the addition of wash water. Separation on a concentration table depends on the density difference and on the grain size of the mineral and the gangue. The screen analysis of the ground ore (Table IV) shows that the grinding characteristics of the mineral are nearly the same as those of the ore and the distribution of uranium is uniform in each fraction.

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TABLE IV. SCREEN ANALYSIS AND URANIUM DISTRIBUTION SCREEN

The following are the conditions of a typical test run:

Ore grade

The discharge was split into three fractions and the results obtained are as follows:

wt% U3O8

These results indicate a 70% recovery for a concentration ratio of six and a relatively high loss of uranium in slimes.

The recovery figure and the concentration ratio can be increased by employing multistage concentration. The high loss of uranium in slimes is due to the fine grinding of a high fraction of feed. It is anticipated that more rigorous control of the grinding conditions would minimize both the formation of slimes and the loss of uranium in slimes.

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