LEACHING AND WASHING 1. Experiments in 1 m columns

A large number of leaching experiments with 11 kg samples were carried out in 1 m by 10 cm glass columns with a standard representative sample of ore containing 0.239% U3O8. They showed the optimum leaching conditions to be as follows: particle size up to 1.5 in, sulphuric acid con-centration between 25 and 50 g/1, and recirculation of the acid at a flow rate of about 100 1/h • m2

for up to 1000 hours. Oxidants had no detectable effect. The efficiencies were a few per cent lower at 0°C. Using recirculation, the optimum leaching efficiencies were about 80 to 85% and the uranium concentrations in the liquor were between 7 and 11 g/1. Leaching for 100 hours instead of 1000 hours reduced the efficiency by about 2%. Impurity contents of the leach liquor were relatively low. The volume of leach liquor in the recirculating system could be reduced to the minimum practicable without affecting efficiency but it was important to maintain the free acid above 25 g/1.

Washing with water was as effective as washing with dilute acid. Recirculation during washing would not, of course, remove the uranium efficiently and a very large volume of wash water was used in a single pass in these early experiments to recover the uranium completely. It was noticed that an interruption of washing led to an increase in the uranium concentration in the next portion of wash liquor. This led to the idea that a longer time was needed for the rich liquor coating or impregnating the ore particles to mix with or diffuse into the wash water. A reduced flow rate or intervals with no flow should therefore be beneficial.

2.2. Experiments on the 500 kg and 900 kg scale

In the 500 kg experiments the ore was leached and washed in wooden boxes of 60 cm square section by 1 m in height. It was assumed that the increased area would eliminate any 'wall effect' of a narrow column. Irrigation was by means of a perforated flexible PVC tube laid over the surface of the ore.

The first five experiments were with the optimized leaching conditions established earlier, but for only 110 hours. A large volume of wash water was used, and the combined leach and wash liquor had a final uranium concentration of 1 to 2 g/1. The only variable was the flow rate. In the five successive experiments, flows of 166, 166, 139, 111 and 111 1/h-m2 gave leaching efficiencies of 72.7, 79.7, 78.8, 78.8 and 78.5% respectively, based upon analysis of the solid residues. Except for the first anomalous result, the leaching efficiencies were quite close to those obtained in the glass columns after only 100 hours.

A representative 11 kg sample of the residue from the last experiment was leached for an additional 36 hours in a glass column under comparable conditions. The additional 3.4%

efficiency (for a total of 81.9%) was presumably due to redistribution of the ore particles and to the difference in scale. Conventional acid leaching of the twice leached residue, after grinding to below 100 mesh, only gave an additional 0.5% yield.

The next six 500 kg experiments allowed a comparison to be made of three recirculation leaching flow rates with and without periods of interruption, but all continued for about

1000 hours, as shown in Table I. Optimum conditions were used, with temperature fluctuations between 10°C and 20°C. This table shows clearly that the highest leaching efficiency is obtained by using the highest flow rate with a continuous flow, as in experiment 6. The loss in efficiency

IAEA-AG/33-13 TABLE I. LEACHING RESULTS ON 500 kg SCALE

109

A - Leach liquors flowing continuously for 1000 hours.

В - Four cycles of 2 days' leaching and 6 days without leaching.

TABLE II. WASHING RESULTS ON 500 kg SCALE Experiment

" Per cent of dissolved uranium recovered.

Concentration in mixed leach and wash liquors.

c Concentration in last liquor to leave column.

A - 0.5 h wash, 7.5 h no flow, 8 d total.

when the flow is interrupted for long periods is not justified by the reduced cost of pumping.

It is interesting to note that all the efficiencies with interruption of flow are intermediate between all those with no interruption at 110 hours and 1000 hours. This corresponds with the fact that the periods of actual flow totalled about 250 hours. Acid consumptions were all similar, at about 18 kg/t, which compares favourably with about 60 kg/t for conventional leaching of this ore.

1 1 0 AJURIA-GARZA and JAMRACK

The washing results of these six experiments are shown in Table II. This table demonstrates clearly the superiority of washing with a high flow rate for successive short periods of time with long interruptions, as in experiment 6. All results with flow interruption are better than all those without. The fairly high uranium concentration (2.8 g/1) in the combined leach and wash liquors of experiment 6, combined with the high wash efficiency, provided grounds for the belief that a countercurrent washing process with flow interruption might be highly effective.

The 900 kg experiments were carried out in rubber-lined steel tanks at flow rates of 400 1/h-m2. In one experiment, 816 hours of continuous leaching gave a leaching efficiency of 83.2%. In the other experiment the ore was leached for 1008 hours, but with interruptions, in regular cycles lasting 75% of the time. Leach solution was actually flowing for 264 hours giving an efficiency of 78.7%. This difference again confirmed the results that continuous flow was justified during leaching (but not during washing).

2.3. Experiments on the multi-ton scale

Five large-scale experiments were performed to study the leaching process and the mechanical construction of the heaps. The dissolved uranium was always washed out with an excess of water and the washing process itself was not optimized. The object of the first two experiments was mainly to determine the best method of constructing large heaps and therefore the leaching results were not typical.

The third experiment, with 10 t of ore, gave a good leaching efficiency of 71 %, considering the very short leaching time of 64 hours.

The fourth experiment, with 28.5 t of ore and a grade of 0.204%, gave quite a high efficiency of 78% in 492 hours, even though a high proportion of the ore leached was in the shape of a regular slope. The method of construction, using a concrete base (and walls on the pilot scale) with PVC sheet bonded to it by bitumen, was cheap and very satisfactory. The temperature range of 1 -30°C was typical of the El Nopal location.

The fifth experiment, with 72.5 t of ore in a tank, was leached for 1520 hours with leach liquor «circulated for 25% of the time, the flow rate being increased in stages from 12 to 100 1/h-m2. The gradient of the graph of efficiency against time increased at each stage, showing that the highest flow rate was best. This was assumed to be equivalent to 380 hours continuous leaching, and the time was then increased by further continuous leaching at 100 1/h-m2 to a total of 1000 hours.

During leaching of the 72.5 t heap, several arrangements of perforated hose and sprays were used to obtain good liquor distribution above the heap and, for 1000 hours, it is believed that any loss of efficiency owing to maldistribution of the liquor would be small.

A possibly more serious defect was noticed towards the end of leaching, by digging into the top surface of the heap. A single channel appeared to have been created which took a high proportion of the liquor flow. This maldistribution of liquor was confirmed by later studies in large wooden boxes. It is believed that the problem can be alleviated by removing the fines and treating them in a conventional leaching plant to be built later.

2.4. Complex leaching and countercurrent washing experiment

Complex experiments involving a number of columns were needed to simulate a four-stage, countercurrent washing system. Table II shows that washing is most efficient with a number of cycles, each consisting of a short washing period followed by a long period without flow.

Fifteen 6 m high columns, 10.6 cm in diameter, were erected and each was filled to a height of 4.5 m with 55 kg of typical crushed ore of grade 0.232%.

IAEA-AG/33-13 111 All 15 columns were leached together, at 70 I/h-m2, for 672 hours. By analysis of the washed leached ore residues the leaching efficiencies varied from 76.1% to 87.2%, with an average of 82.9%, and the uranium concentration of the leach liquor was 8.43 g/1.

A true four-stage countercurrent washing system was used in this experiment, gradually building up to equilibrium. The various washing routes in sequence were as follows:

Columns 1,2, 3 , 4 Columns 9, 10, 11, 12 Columns 2, 3, 4, 5 Columns 10, 11,12, 13 Columns 3, 4, 5, 6 Columns 11, 12, 13, 14 Columns4, 5, 6, 7 Columns 12, 13, 14, IS Columns 5, 6, 7, 8 Columns 13, 14, 15' Columns 6, 7, 8, 9 Columns 14, 15 Columns 7, 8, 9, 10 Column 15 Columns 8, 9, 10, 11

End of system only

Thus, from 15 columns, 12 solutions were available which had passed through the full set of 4 columns. Each of these was mixed with one fifteenth of the total volume of leach liquor from the fifteen columns, to represent the final product liquor from one column (although actually each liquor represented part of the uranium from four identical columns).

Each of the wash solutions had been applied to each column in its route in the form of 36 portions. Each portion was applied over \ hour to the first column in its route and the liquor passed immediately to the next column and so on, down the series of four columns. Each half-hour period of now at 100 1/h-m2 was followed by 7.5 hours without flow. Thus each washing route required 288 hours (i.e. 12 days). As expected, the concentrations of uranium leaving the columns were rather uniform. These were, in sequence, 8.49, 9.33, 11.66, 9.83, 10.33, 10.66, 10.33, 10.83, 10.70, 9.33, 8.08 and 7.95 g/1. The twelfth liquor should be regarded as fairly typical of the equilibrium condition.

From all fifteen liquors, 84.4% of the total uranium in the system was recovered at 8.44 g/1, which is nominally slightly more than the amount leached, so the washing efficiency is presumably near to 100%. The countercurrent washing system was therefore fully justified.

3. AMINE EXTRACTION