Outcomes of performed investigations 1. Laboratory tests

2.2.1.1. Determination of sorptive properties

The most complete tests have been performed for gray and yellow sands from the Kanzhugan uranium deposit using the method described in 2.1.1.1. When concentrations of initial solution and filtrate are known, sorptive coefficients are easily calculated. Results are shown in Table 4 and in Figure 4. The graphs show that the sorptive coefficients of sulphate-ion follow an exponential curve. They decrease with decreasing concentration of sulphate-ion in solution. During this year, similar investigations were carried out for three uranium deposits Kanzhugan, Karamurun and Uvanas under the framework of the project “Assessment of influence of IS-leaching polygons on environment”. Sorptive properties were studied for the most permeable parts of the ore horizons that are likely to have the most impact on acquifers.

Table IV. Sorption coefficients calculated for SO₄, FE, AL, PB-210 in yellow and gray sand

Experimen t number

Solution or dilution

Sulphate-ion Iron Aluminum Lead-210

Initial

2.2.1.2. Oxidation and recovery processes

Duringthe first stage of the experiment (Fig. 5), uranium is leached out of rocks with the first portion of the filtrate and reaches about 15% of total reserve at a liquid and solid ratio (L/S) of 0.2. The concentration of uranium is 110 mg/l and the concentration of sulphate-ion is up to 5g/l in filtrate. Then concentrations of uranium and sulphate-ion decrease to 0 when the ratio L/S is 0.5 after displacing 2-3 porous volumes. Other chemical compounds and elements such as iron, aluminum, and silicon oxide are not dissolved and are not present in the filtrate.

FIG. 4. Sorption capacity of yellow and gray sands for S0₄^-2, Al, Fe, Pb-210 depending on their content in solution.

LEGEND

________________ yellow oxidized sand; --- gray unoxidized sand.

After achieving a L/S of 0.87 ratios and stabilizing the state of the infiltration, the second stage of the experiment (uranium extracted by sulphuric acid) is carried out at a 2 m/d flow velocity. This stage can be divided into 4 phases:

— Effect of “alkaline” leaching at L/S ratio 0.87-1.3;

— Active acidic leaching at L/S ratio 1.3-2.0;

— Bypassing acidic solution without noticeable extraction of essential and accessory components at L/S ratio 2.0-3.0 and

— Bypassing acidic solution with noticeable extraction of accessory components at L/S ratio 3.0-3.6.

During the first phase (a) pH decreases from 8.0 to 6.7 and uranium reaches 110 mg/l in filtrate, sulphate-ion increases up to 4 g/l from the reaction of sulphuric acid on rock, and Eh increases insignificantly from 190 mV to 210 mV. At this time the concentration of sulphuric acid is zero, and both iron and aluminum are absent in the filtrate. Other parameters such as Eh, pH, SO4-2 reach a plateau as these parameters do not change during this phase. In this phase, 21% of the total amount of uranium is leached from the sample.

The second phase begins with an abrupt decrease of pH from 6.7 to 6.0 and Eh to 50-40 mV, this is caused by the passage of trivalent (up to 290 mg/l) and divalent (2 mg/l) iron into filtrate. Further decreases in pH and increases in Eh are accompanied by a simultaneous increase in concentration of uranium, Th-230, Pb-210, trivalent and divalent iron, aluminum and sulphate, and total acidity (Figures 5, 6, 7, 8), as well as generation of polysilicic acid (xSiO2*nH2O). At the same time, the pressure increases abruptly in the column indicating plugging from the generation of colloidal hydrous ferric and aluminum oxides and silicic acid.

This leads to an increase of solution viscosity and a decrease in the infiltration coefficient. The blocking process continues to pH 2.0 and then all colloids are dissolved and the filtration coefficient increases and the pressure falls accordingly. At the L/S ratio 1.55, the concentration of uranium and all other elements reach maximum values in the filtrate (U–

300 mg/l, Fe⁺³–100 mg/l, SiO2–350 mg/l, Al–700 mg/l, sulfation-iron–12 g/l, acidity–9 g/l, Eh–480 mV). Maximum values of aluminum and sulphate-ion appear at the L/S = 1.55, and the maximum concentrations of iron, thorium-230 and lead-210, accompanied by an increase in the concentration of silicic acid, appear a little later at (L/S = 1.6-1.7).

At L/S ratio 2.0, the process of active leaching ends, and the third phase starts. It corresponds to low steady values of all leached elements (U, Th, and Fe, except for Pb) and high steady values of Al, SiO2, SO4, and H2SO4. It indicates that the basic leaching process can be considered as complete when the L/S ratio reaches 2, although Pb-210 continues to wash out.

In the fourth phase, higher extractions of silicic acid and aluminum are noticed when the acidity reaches the initial solution concentration.

The third stage (–washout by natural groundwater–) begins at L/S ratio 3.6. Almost immediately the pH increases from 1.2 to 2.0 after displacement of one pore volume of sulphuric acid by groundwater (Fig. 6). The pH then increases gradually to 3.3. It is notable that values of sulphuric acidity and sulphate-ion decreases abruptly to safe level (from 10 g/l to 3.6 g/l. Just after washing out the first portions of pore solution with groundwater, the concentration of almost all elements in the filtrate almost reach groundwater levels. Silicic acid decreases from 350 mg/l to 150 mg/l and then remains steady at 100-120 mg/l for a long time. By comparison, the value of silicic acid is 18 mg/l in initial groundwater (Fig. 8).

0 2 4 6 8 10 12

0 1 2 3 4 5 6 7 8

Full change of one porous volume becomes at L/S=0,187 Ratio: liquid to solid FIG 5. Change of pH, U, ²³⁰Th, ²¹⁰Pb in process of uranium leaching and "recovery" of affected sand media.

U x50 mg/l Th230 x1000 Bk/l ɪɇ Pb210 x100 Bk/l

Distilled

water 10g/l H2SO4 Kanzhugan groundwater: pH=7.75, SO₄^-2=360 mg/l, SiO₂=17mg/l

L/S

189

0 2 4 6 8 10 12

0 1 2 3 4 5 6 7 8

Full change of one porous volume becomes at L/S=0,187 Ratio: liquid to solid

FIG. 6. Change of pH, Eh, H₂SO₄, SO₄^-2 in process of uranium leaching and "recovery" of affected sand media.

H2SO4g/l SO4 g/l ɪɇ Eh x 100 mV

Distilled water

10g/l H2SO4 Kanzhugan groundwater: pH=7.75, SO4^-2=360 mg/l, SiO₂=17mg/l

L/S

190

0 1 2 3 4 5 6 7 8 9 10

0 1 2 3 4 5 6 7 8

Full change of one porous volume becomes at L/S=0,187 Ratio: liquid to solid

FIG. 7. Change of pH, Fe⁺³, Fe⁺², pressure in process of uranium leaching and "recovery" of affected sand media.

Fe +3 ɯ50 mg/l Fe +2 ɯ20 mg/l ɪɇ Pressure x2 bar

Distilled water

10g/l H2SO4 Kanzhugan groundwater: pH=7.75, SO₄^-2=360 mg/l, SiO₂=17mg/l

L/S

191

0 1 2 3 4 5 6 7 8 9

0 1 2 3 4 5 6 7 8

Full change of one porous volume becomes at L/S=0,187 Ratio: liquid to solid FIG. 8. Change of pH, SiO₂, Al in process of uranium leaching and "recovery" of affected sand media.

SiO2 x100 mg/l Al x100 mg/l ɪɇ

Distilled water

10g/l H2SO4 Kanzhugan groundwater: pH=7.75, SO4-2=360 mg/l, SiO2=17mg/l

L/S

192