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5EARTH SCIENCES
MODELING OF ACIDIFICATION EFFICIENCY ON URANIUM PRODUCTION
Imanbay M.
Master's Degree, Faculty of Mechanics and Mathematics Al-Farabi Kazakh National University, Almaty https://doi.org/10.5281/zenodo.7257039
Abstract
The uranium is the fuel most widely used for nuclear energy as its atoms split apart relatively easily. Though its common metal found in many rocks all over the world, for producing the nuclear energy its specific type U-235 is used that makes up less than one percent of the uranium in the world. Kazakhstan and Canada are the world leaders of exporting of that type of uranium used for nuclear energy providing. Uranium in Kazakhstan is produced by the method of in-situ leaching.
In-situ leaching (ISL) mining technology was developed in the 1960s for recovering uranium from a rollfront type deposits. ISL (in situ leaching) or ISR (in situ recovery) is a production technic which allow to produce metals thanks to a leaching process performed directly inside geological reservoir on the mineralization. The metal change from solid phase to liquid phase due to a leaching solution injected. The ISL method allow to strongly limit the pollution in surface at the difference of classic mining extraction that involved the creation of radioactive rock stockpiles or radioactive tailings.
The balance between the volumes of injected and extractive liquids is very important for the ISL process and must always be observed, but it is not always possible to observe this rule during the extraction process, so there may be cases of overproduction /excessive injection on the scale of a block or cell. Thus, this leads to the need to model hydrodynamic behavior for a better understanding of the acidification and extraction process.
Keywords: in-situ leaching, uranium extraction, hydrodynamic model, key performance indicators, aquifer effect
Introduction
Situ Leaching (ISL) is a mining method that consists of extracting a mineral substance by dissolving it in the geological layer that contains it, is a mining method which consists of:
- establishing a circulation of a solution in the mass of the deposit capable of selectively selectively dissolving the mineral to be mined,
- pumping the mineralised solution into surface processing facilities, where the valuable mineral is separated and concentrated into a marketable product, - recycling the solution back to the deposit after repackaging.
This method of mining is a special case of solution mining or solution mining methods, initially applied to soluble minerals, salt, coal and to soluble minerals, salt, sulphur, or potash. These methods move substantial volumes of ore by These methods move substantial volumes of ore by solution mining, and create large voids in the
Depending on the chemistry of the leaching solution, the ISL method is said to be alkaline or acidic. This leach solution is usually injected under pressure into the mineralised horizon through a network of wells by a network of injector wells reaching the permeable horizon and allowing the solution to. This leaching solution is generally injected under pressure into the mineralised horizon by a network of injector wells reaching this permeable horizon and allowing the solution to come into contact with the mineral to be dissolved. A second network of producer wells, inserted into the previous one, pumps the mineralized solution to the plant
installed on the surface for a classic treatment of extraction of a mineral in liquid phase. After treatment, the leaching power of the solution is regenerated before it is reinjected into the injector well network, and so on.
- This so-called non-conventional mining method therefore saves the extraction of the ore and its transport, mechanical treatment, crushing and grinding, storage of solid effluents site modifications, which are characteristic of conventional mining methods.
Modeling the trasport reactive
Reactive transport modeling is based on the coupling between a transport model and a speciation model describing fluid/rock chemical interactions. It is an approach phenomenological based on a model for understanding the geological environment describing:
- the hydrodynamic properties of the environment, the hydrogeological context as well as the terms sources linked, in our case, to injection and production facilities: these parameters constrain a hydrodynamic model.
- a geochemical model describing the fluid / rock interactions of interest for the object of the study. This model contains the characterization and quantification of the reactive phases, as well as the initial composition of the circulating solutions (water from the aquifer before acidification and solutions injection). These data will feed the speciation model simulating the state of the solutions at equilibrium or under kinetic stress.
Reactive transport modelling for ISR
The exploitation of uranium by ISR is an obvious field of application: the recovery of uranium is indeed conditioned to its dissolution by oxidative attack in an
acid medium, and to its transport in solution to the production well screens.
The advantages of 3D reactive transport simulations are multiple:
-the spatialization of flows. Setting a specific flow rate for each well represents the heterogeneity of the flow within the aquifer (from one cell to another in particular). This heterogeneity is linked to the natural variation in flow rates from one well to another due to the clogging/RVR cycles and operating scheme (single or double production stage, management of flows to favor the most productive zones, addition/stopping of works).
-the possibility of simulating the behavior of all the chemical elements defined inthe model, in addition to the uranium concentration. For example: the evolution of the pH, the Fe(III) concentrations, the residual solid uranium. Input data
3D geological model (KATCO/GEOS)
• Stratigraphy
• Lithology : clay, sand
• Oxidized zone
• Orebodies at 0.01m% in permeable sands
3D grade simulation (DGS/DRR)
• 100 grade realizations inside 3D orebodies envelopes
• Constant ore
Block model made with Isatis using envelops
• 5mx5mx1m grid
• Lithofacies
- 2 classes : clay-sand
- Carbonate : reduced and not permeable
• • Redox
- classes : oxidized - mineralization -reduced
Workflow Flowrates: Determine future flowrate by wells
Options :
Production well: Global : specify the options of initial, min, max, decrease coef, and exponential or linear curve
Injection well: Balance: for injectors only, to have a global balanced flowrate between injector and producers.
RESULT
The analysis of production data is very complex due to the variability of operating parameters, either over time or spatially, between cells and process units. Analysis of flow rate, acidity, uranium production, and iron concentration data from Muyunkum (MSK) does not show clear correlations to provide indicators that can be used by the operator to optimise production.
The MSK units were selected in consultation with DEM as candidate tests to understand the impact of two key parameters for in situ uranium production: acidity and production: leaching solution acidity and operating rates. The acidity changes tests aim to relate the acidity, iron and uranium concentration parameters to the uranium concentrations in the production juices.
DESCRIPTION OF THE SCENARIOS
During underground leaching of uranium, there are three main stages of mining:
acidification of the ore-bearing formation, leaching (or active leaching) and pre-leaching (or washing). There is no clear boundary between these stages. On the one hand, in the process of saturation (acidification) with a solvent of an ore-bearing formation, intensive leaching occurs simultaneously, on the other hand, saturation proceeds for quite a long time, during which a significant part of the extracted metal is extracted from the subsoil. Stages are allocated for better control and management of the underground leaching process. To separate the leaching at the stage, the ratio of L/S is used: acidification - L/S from 0 to 0.25; leaching - L/S from 0.25 to 1.5; additional leaching of L/S - from 1.5 to 2.5 - 3.
With this in mind, we have created a plan for acidification:
First phase of acidification: Objectif to reach as fast as possible the peak of U, to invade as fast as possible the full mineralization (indicator AIP).
- Should we put 15 / 20 / 25 g/l ?
- How long should we put this high acidification ? L/S =0.2 - 0.3 - 0.35 - 0.4 ?
With any acidification method, the acid concentration in the leaching solutions in the initial period should not be lower than 20-25 g/l, this will reduce the secondary enrichment zones and the loss of uranium, reduce colmatation phenomena, accelerate the acidification of the block.
Second phase of acidification: Objectif : maintain a good pH < 1.8, continue to invade the reservoir (indicator AIP), target is 50% of recovery.
- Should we put 6 / 8 / 10 / 12 g/l ?
- How long should we put this high acidification ? L/S =1 - 1.5 - 2?
Third phase of acidification: Objectif maintain pH < 2, continue to invade the reservoir (indicator AIP). Continue acidification until there is a effect on it, after we can stop the acid (this phase will mainly depend on the recovery).
From what I did before, the acid has a consequent impact until recovery 70% , to confirm ?
Figure 1. Evolution of acid values
In our work historical data will not be considered. Only the geological model will be taken as an input. Then an optimize and ideal method of operation is used.
- 10m3/h for producer during all life of block.
- Balance flowrate for injector at cell scale (it means that injector connected to several cell will have a higher flowrate than the injector connected to one cell only).
Scenariol
Initially, 3 scenarios of acidification will be performed on 9 blocks chosen on Kanzhugan during 2 years of production. Preferentially blocks where the history matching has succeeded will be used. Some news blocks were also used, in order to perform this methodology in case of convincing result.
- Scenario 1: 20g/l until L/S =0.2, then 7g/l until L/S = 1.5 and 5g/l until the end.
20
.15
20 g/l
7 g/l 5 g/l
0 0,5 1 1,5 2 2,5
US
Figure 2. The specified acid value is applied to the MSK block. Scenario 1.
Scenario 2: 20g/l until L/S =0.2, then 14g/l until L/S = 1.5 and 10g/l until the end.
Scenario 2
3,5
25
20
15
10
20 g/l
14 g/l
7 g/l
0,5
1,5
L/S
2,5
3,5
3g/l.
Figure 5. The specified acid value is applied to the MSK block. Scenario 2. - Scenario 3 : inspired from planning tool: 20g/l until L/S = 0.2, then exponentially decrease acidification to
25
20
3D15
i TJ
0 10 <
Scenario3
20 g/l
1 S 3 g/l
0,5
1,5
L/S
2,5
3,5
Figure 6 The specified acid value is applied to the MSK block. Scenario 3.
The onset of acidification is the same for all production units and occurs in the form of an acid attack. Once this acid exposure has been completed, the amount of acid injected depends on the tonnage of the metal in the area to be leached.
It should be noted that the production units are put into operation at different times, depending on the production planning. There are therefore two types of production units :
Blocks launched in isolation and requiring complete acidification of the territory ;
The blocks launched next to the blocks already in production and therefore preacidified. The proximity of the blocks already in production will also affect the rate of obtaining the first concentrations of uranium.
MODELING OF URANIUM RECOVERY IN REACTIVE TRANSPORT ON THE MSK36 BLOCK
The workflow has been applied to specific 9 blocks from Kanjugan, which shows specific results from only two blocks. Each block was covered with a 3D geological model. For reasons of computing speed and memory size, the simulation was carried out block by block. An auxiliary grid measuring 5m x 5m x 1m is extracted from a 3D geological model.
Block Model:
The MSK36 block consists of 13 cells, 1 of which is two-level (MSK36_02_03_B), which initially represents 14 production wells and 49 injection wells. During the operation, several wells were added, which among them are wells to replace the damaged well and to accelerate production in cells with the richest solution
URANIUM PRODUCTION
100 150 260 260 3tto 350 460 460 560 560 «bo ¿>60 7do 750 Time (d)
Figure 7. Modeling of all scenarios of distribution of uranium content and pH curves at the production reservoir
for MSK36.
The concentration of uranium and pH over two years of production are shown in Figure 7. The curves shown in the figure are modeled in HYTEC and shown in the ParaView program. Here are the results of all the scenarios that came out of them: red line is scenariol, blue line is scenario2 and green is scenario3.
For clarity, lines starting from 0 indicate the concentration of uranium, and those lines starting from 8 (Y axis on the right side), this is considered the pH of the line.
Looking at figure 13 we can say some things:
For scenario 1, In the first phase of acidification, the concentration of uranium is constantly increasing, and reaches up to 50 mg/ l. In the second phase of acidification, the concentration of uranium reaches its maximum peak:
- Peak =170 mg/l
- Time to reach peak = 90 day
For scenario 2, In the first phase of acidification, the concentration of uranium is actively increasing. This is followed by a rapid drop in concentration, starting from 100 days of production. This effect is most likely caused by too rapid consumption of mineralized volume and uranium reserves during the first life of the unit. The expected result for this study is was to show that with a more aggressive method of acidification the production will be faster. In term of a global strategy, it
means that you may be able to have less blocks in activity with higher acidification. But as shown the acidification has its limit, in term of efficiency on the reservoir and also in term of cost.
For scenario 3, The results of this scenario are fully expected to be comparable with those two scenarios, the peak point is 150 mg/l, and the time to reach the peak concentration of uranium is 85 days, after that the concentration of uranium gradually drops. Higher than 100 mg/l lasts about 60 days.
In this scenario, the pH value is constantly increasing. During the first year of production, the pH value gives excellent indicators, but after that it increases to 2 and higher
The pH curves are significantly different in all acidification scenarios and correspond to the acidification values. There is a significant difference in the concentration of uranium between the scenarios for only 300 days and this means that during these days the influence of acid on the horizon will be active. In addition to this, we can once again make sure that in the second phase of acidification, the acid effect will be more active than in other phases, in addition, this part of acidification considers important for both production and its dissolution rate ( Figure 8).
Figure 8 shows the rate of dissolution and the degree of extraction (I took as Net Uranium production). The data of the degree of extraction is translated into percentages, all algorithms are calculated as follows:
Figure 8. Dissolution and recovery. A) Scenario1, B) Scenario2, C) Scenario3
Table 1.
Block MSK36. U dissolution and Net uranium production of each scenariios,
Scenari o 1
I phase II phase III phase
U Dissolution Rate 15% 61% 69%
Net Uranium Production 3% 57% 67%
Scenario 2
I phase II phase III phase
U Dissolution Rate 16% 65% 74%
Net Uranium Production 3% 60% 72%
Scenario 3
I phase II phase III phase
U Dissolution Rate 15% 58% 65%
Net Uranium Production 3% 50% 61%
ACID IN PLACE
In the first days of production, the acid consumption in the model is mainly controlled by the "filling" of the tank (replacement of the pore water at pH=8 by the injected solution) and by the dissolution of calcite and goethite. These two minerals are considered to be in thermodynamic equilibrium and will therefore be dissolved instantly until stocks run out, or quickly because they are in low quantities in our model.
The longer-term consumers of acid are the clays, present in greater quantities, and distributed homogeneously throughout the reservoir. The dissolution of the
clays is linked to a much slower kinetics and will therefore take place over the entire production period of the block.
To achieve a good reproduction of the pH curve, it was necessary to adjust the kinetic law representing the acid consumption of the deposit, through the dissolution of the clays. This is acceptable in the sense that, to simplify the geochemical model and limit the species considered, a single main "long-term" acid consumer has been modeled to represent all the "long-term" acid consumers, in the form of a smectite. However, what is modeled under the name of smectite probably represents a set of several acid-consuming species that the "adjusted" kinetic law for smectite will encompass.
100% 80% 60% 40% 20% 0%
20g/l 7g/l 5g/l
** ✓ ---
/
/
1/ Volume(Ugrade) -Volume(pH<2) ---AIP
0 0.5 1 1,5 2
A
2-5 iys
AIP 20g/l
100%
100% 80% 60% 40% 20% 0%
L/S
20g/l 7-3g/l 3g/l
* — » _ - - . ^ ^ * ■ ■ ■
-Volume(Ugrade) — Volume(pH<2) ---AIP
0 0,5 1 1,5 2
2,5
L/S
Figure 9. Acid in place. A) Scenario1, B) Scenario2, C) Scenario3
Acid in place this dimensionless value shows the effectiveness of acidification in the formation, or rather where there are ore minerals. And this allows us to analyze the effect of acidification.
Volume pH < 2
Acid in place =_
Volume Uraninite grade
As we can see from Figure 9 , the results of the first and second scenarios (Figure 9 A,B) are almost the same. It seems that has been reached a limit in term of acidification capacity. The scenario 3 (Figure 9,C) has a difference compared to other scenarios. And the reason for this can be said to be the high pH value at phase 3.
300
250
200
m Z3
150
>
S 100
<J <
50
Acid consumption
"Scenariol
"Scenario2
Scenario3
100 200 300 400 500
Time [day]
Figure 10. Average acid consumption
600
700
800
0
Figure 10 shows the average acid consumption. This is the ratio of accumulated acid [t] to accumulated uranium [t]. At the beginning of production, the accumulated uranium will be zero, so I started calculating the average acid consumption after 70 days (an approximate calculation when the ratio stabilizes)
• For scenario 1 : ratio accumulated acid [t] to accumulated uranium production [t] = 95
• For scenario 1 : ratio accumulated acid [t] to accumulated uranium production [t] =142
• For scenario 3 : ratio accumulated acid [t] to accumulated uranium production [t] = 72
From this, we can conclude that scenarios 3 are more effective due to low acid consumption.
CONCLUSIONS
In the third acidification scenario, the simulated pH values slightly exceed the production data after 450500 days of extraction, depending on the blocks under consideration. This is the limit above which the solubility of Fe(III) in solution is very limited, which leads to lower concentrations of uranium in the resulting solutions. In this pH range, which is sensitive for both model and production, the proposed calibration is the best "global" adjustment of acid consumption + uranium production obtained today for this area.
There is no more benefit from 7 g/l (scenarios 1) to 14 g/l (scenarios 2) after 6 months.
This means there is no point in giving high concentrations of acid.
Considering all the results, my opinion on optimal acidification is as follows:
- In the first phase, acidification should be kept at the level of 20 g/l to L/S = 0.2-0.25;
- The second phase of acidification should be divided into several parts, and the acid concentration should be gradually reduced to 7 L / S = 1.5-2;
At the last stage of enrollment, it is necessary to reduce the acid concentration once again, but here it is constantly necessary to take into account the pH value.
References:
1. Regnault, O., Lagneau, V., Fiet, N., 2014. 3D Reactive Transport simulations 495 of Uranium In Situ Leaching : Forecast and Process Optimization.
2. Lagneau, V., Regnault, O., Descostes, M., 2019. Industrial Deployment 465 of Reactive Transport Simulation: An Application to Uranium In situ Recovery. Reviews in Mineralogy and Geochemistry 85, 499-528
3. Lagneau, V., Regnault, O., Okhulkova, T., Le Beux, A., 2018. Predictive simulation and optimization of uranium in situ recovery using 3D reactive 470 transport simulation at the block scale, in: ALTA, Perth, Australia
