Optimizing the re-use of uranium contaminated water from a flooded mine

ANNUAL OF THE UNIVERSITY OF MINING AND GEOLOGY “ST. IVAN RILSKI”, Vol. 57, Part ІI, Mining and Mineral processing, 2014

The Communicator module allows to store pre-calculated GWB situation in PostgreSQL database and using similarity algorithms to select these results during a call from the transport model. It also allows further analysis of times and time periods consumed in individual phases of calculation or operations in a database system. The aim is to reduce the total time of numerical simulations.

Basic points: 1. Introduction – history of deposits exploitation, flooding time, water resources, chemistry; 2. FEFLOW model of the deep mine presentation; 3. Main geochemical components; 4. FEFLOW and geochemical code coupling; 5. Conclusion – optimizing of pumping wells net.
ОПТИМИЗИРАНЕ НА ПОВТОРНАТА УПОТРЕБА НА ВОДА ЗАМЪРСЕНА С УРАН ОТ НАВОДНЕНИ МИНИ

Павел Щроф

ДХИ а.с., Прага, Репблика Чехия, p.strof@dhi.cz
РЕЗЮМЕ: Урановото находище в Олши–Драхонин е разработвано в периода между 1959 и 1989г. В момента, в който се преустановява експлоатацията, минните дейности са на дълбочина по-малко от 467 м под нивото на терена, като находището е било отворено чрез „blind shaft“ с дълбочина около 859 метра под нивото на терена. От 1997 г., нивото на руднични води се поддържа чрез изпомпване на нива от 1,5-7,0 m под преливното ниво (т.е. под пода на дренажния проход), което е на дълбочина по-малко от 35 м под нивото на терена. След наводняване/заливане, бе наблюдавано движение на водата в плитката зона, контролирано чрез инфилтрацията на валежите и анализирано геохимичното равновесие в по-дълбоките части.

Модел на подземния поток и на транспорта на разтворените вещества бе реализиран във FEFLOW среда (http://www.feflow.com). Геохимичният модел на равновестното състояние на разтворените вещества в скална среда е създаден чрез Geochemist's Workbench (GWB, http://www.gwb.com). За връзка между двата числени симулации алгоритми Модулът Communicator бе планиран, използвайки XML файлове, всички напълно конфигурируеми с цел управлението на съвместната работа на двете системи.

Модулът Communicator позволява да съхраните предварително изчисленото състояние чрез GWB в PostgreSQL база данни и използвайки подобни алгоритми да изберете тези резултати при „запитване“ от страна на модела за транспорт. Той също позволява по-нататъшен анализ на продължителността на времевите периоди използвани при отделните фази на изчисление или работа в системата от база данни. Целта е да се намали общото време на числените симулации.

Основни точки: 1. Въведение - История на експлоатация на находището, периоди на наводнения/ заливания, водните ресурси, химия; 2. FEFLOW модел за дълбочинно представяне на мината; 3. Основни геохимични компоненти; 4. FEFLOW и връзката му с геохимичния код; 5. Заключение - оптимизиране на мрежа от помпени кладенци.

1. 
   Introduction
   

Uranium ore deposit Olší-Drahonín is a part of the Rožná ore district. The Rožná ore district has the last active uranium mine in Europe, nevertheless some of the shafts were already closed and flooded. The Rozna uranium ore district is located in the northeast of the Ceskomoravska highland at Tisnov city. Its natural boundaries are Svratka River in the southeast, the Loucka River in the west and the Nedvedicka River in the east. The highest elevation lies is in the North 631 m a.s.l., the lowest part is near the confluence of the Svratka and the Loucka River in the South at 254 m a.s.l. The ore district is formed by two flat crests divided by the Rozsochy syncline. The regional geology consists mainly of metamorphic rocks of the Northwest Strazek part of the Moldanubien. Within the area, there are predominantly paragneiss, orthogneiss, migmatite, granulite and amphibolite. Marble, quartzite, pegmatite and aplite are much less represented. Nappe structure of Strazek part of the Moldanubien is caused by Variscan orogenesis. Groundwater flow is connected with porous and mainly fractured media. Shallow aquifer is composed of poorly-developed quaternary sediments and the upper part of weathered bed rock. Deeper aquifer is composed of various hard rocks with only fracture porosity, the main uranium ore minerals are uranite and coffinite. Deposit Olší-Drahonín was closed in 1989 and completely flooded in the year 1995.

2. Site characterization and methods of solution

When uranium ore exploitation was finished in the mines and the pumping systems were shut down, the process of spontaneous mine flooding started. This process took several years, depending on the amount excavated, the depression cone area and the hydrogeological conditions of the deposit. During this time, conditions for proper mine water management in the “collection – controlled draining from underground spaces – purification – discharge” mode had to be created in advance. This ensured that shallow underground and surface water would not be threatened by uncontrolled leakage of contaminated water from the flooded mine in future.

The shut-down of uranium mines on the vein, zone and metasomatic deposit types consisted primarily of filling the main mine outlets at ground level, the so-called main shafts. Raises and mining areas coming up to ground level were filled as well. Under the geological and hydrogeological situation in these locations, mining methods used, and in many cases, the considerable underground mining depth, it was not necessary to backfill other mine workings or other open underground spaces in connection with their liquidation. Unconsolidated backfill was used to fill the shafts, raises and near-surface stopes; untreated material from mine dumps created during excavation was used as a backfill material.

For the detailed research, a pilot locality has been selected, namely the Olší-Drahonín uranium deposit, where exploitation was finished in 1989 and since the year 1996, excess waters have been discharged (under control) from the deposit and subsequently purified. In the deposit, hydrological steady-state exists and sufficient data was available for project use. Basic research is therefore being carried out in this locality.

Within the terms of the research we have been conducting, just those waters accumulated in deeper parts of the former mine, in a so-called quasi-stagnant regime, form the environment of interest. The extent of shallow circulation depends on the hydrogeological conditions and the method in which the deposit was developed, as well as the flow of waters induced by controlled drainage of mine waters (either by pumping or by gravity); whereas quasi-stagnant waters are impounded in the mine, almost without movement and the concentration of dissolved substances is markedly higher than in the shallow circulation waters. The expected distribution of mine waters in the flooded mine is illustrated in Figure 1.

Fig. 1. Diagram of expected distribution of mine waters in the flooded mine under closure

The project at the former, already flooded, uranium mine at Olší – Drahonín required the application of a modelling code that could simulate double porosity flow as well as preferential flow along mine workings.

2.1. Software “FEFLOW”

The FEFLOW code (Diersch, 2014) was selected as the best available software since the flexibility of finite elements mesh design to enable the geometrization of the uranium ore deposit to an acceptable level of simplification. In addition to 3D elements, it is possible to work with a combination of planar and linear elements applicable for simulation of fractures as well as vertical and horizontal mine workings. Within these elements, there is a choice of hydraulic calculations based on Darcy’s law for porous media, the Hagen-Poiseuille law for fracture flow, or the Manning-Strickler law for channel flow. The problem in conceptualisation and modelling of the mining environment consists of correctly describing and quantifying the hydraulic properties of preferential pathways. Depending on the site, one can decide to use either the Darcy or Manning-Strickler equations for mine workings. A three-dimensional model of mine workings was built for the Olší – Drahonín Mine using FEFLOW environment – see Figure 2.



Fig. 2. FEFLOW model mesh with mine workings

2. Geochemist's Workbench

Complex modelling of geochemical evolution of mine water and thermodynamic modelling of particular component stability in groundwater and in water discharged from the mine was performed with the aid of Geochemist´s Workbench 10.0 software package (Bethke, 2014). Concentration of dissolved uranium in mine water is determined by redox potential and depending acidity (see Figure 3). The mine water is saturated with respect to the uranite UO₂ containing four valent uranium and other solid phases as U₃O₉ and U₃O₈. Natural uranite is usually a mixture of oxides binding four and more valent uranium as for example U₄O₉ = 2 U^IVO₂×U^V₂O₅ and U₃O₈ = U^IVO₂×U^VI₂O₆. According to results of geochemical modelling, the equilibrium between dissolved uranium and solid phases can be described by chemical equations:

USiO₄ + 3 HCO₃^– ↔ 3 H⁺ + 2 e^– + UO₂(CO₃)₃^4– + SiO₂(aq) (1)

U₄O₉(c) + 12 HCO₃^– ↔ 10 H⁺ + H₂O + 6 e^– + 4 UO₂(CO₃)₃^4– (2)

U₄O₉(c) + 8 HCO₃^– ↔ 6 H⁺ + H₂O + 6 e^– + 4 UO₂(CO₃)₂^2– (3)



Fig. 3. Redox-pH stability diagrams for uranium in mine water at 13.2°C

This equilibrium is strongly dependent on redox conditions in ground water. Compare conditions in water from borehole and mine water discharged from the mine: Blue stars – conditions in mine water from borehole at depth 170 and 210 m below surface, red circles – mine water discharged from mine at dewatering adit (see Figure 4).

Fig. 4. Activity-pH stability diagrams for uranium in mine water at 13.2°C

In principle, the pore water in ore deposit rocks contains concentrated levels of various components. Usually the components are saturated with respect to secondary minerals formed during mining due to oxidation processes. This can be due to dissolved uranium which is accumulated in pore water in the form of uranyl ion as a result of primary uranium ore oxidation (uraninite UO₂, coffinite USiO₄).

UO₂(uraninite) + 2 H⁺ + 0,5 O₂(aq) → H₂O + UO₂²⁺ (4)

USiO₄(coffinite) + 2 H⁺ + 0,5 O₂(aq) → SiO₂(aq) + H₂O + UO₂²⁺ (5)

Other components, for example iron, are released into pore water by pyrite and other sulfide minerals oxidation in the form of divalent ion

FeS₂(pyrite) + H₂O + 3,5 O₂(aq) → 2 H⁺ + 2 SO₄^2– + Fe²⁺ (6)

are further oxidized to trivalent ion

Fe²⁺ + H⁺ + 0,25 O₂(aq) → Fe³⁺ + 0,5 H₂O (7)

and immobilized and accumulated in the form of oxides and hydroxides mixture of ferric iron in host rocks

Fe³⁺ + 3 H₂O → Fe(OH)₃(am) + 3 H⁺ → FeO(OH)(am) + H₂O (8)

Gradually there is an accumulation of components in pore water and in secondary products during mining within part of ore deposit due to oxidation and this forms a resource of individual components. After flooding, ore deposit pore water components are washed out into mine water. Furthermore, accumulated oxidation products (minerals) are again transferred into reducing conditions after flooding. Most of them are mobilized reduction processes. As an example: oxohydroxides of ferric iron or fourvalent manganese

Fe(OH)₃(am) + 3 H⁺ + e^– → 3 H₂O + Fe²⁺ (9)

MnO₂(pyrolusite) + 4 H⁺ + 2 e^– → 2 H₂O + Mn²⁺ (10)

With these processes, the individual components are released into mine waters and cause their increasing concentration. Some products of oxidation in originally mined and oxidized area of ore deposit undergo reductive immobilization. For example, dissolved hexavalent uranium in the form of uranyl ion UO₂²⁺ is reduced to fourvalent uranium and precipitates in the form of uraninite or other fourvalent uranium minerals

UO₂²⁺ + 2 e^– → UO₂(uraninite) (11)

Sulphate ions are reduced to hydrogen sulphide

SO₄^2– + 8 e^– + 10 H⁺ → H₂S + 4 H₂O (12)

which forms with formerly released ferrous iron highly insoluble sulphide – secondary pyrite

H₂S(aq) + 0,5 Fe²⁺ → 0,5 FeS₂(pyrite) + e^– + 2 H⁺ (13)

With these processes, mine waters are depleted and their concentrations are gradually decreasing. Portions of these components are drained away from flooded ore deposit by discharged mine waters.

3 Communicator

For the communication between the transport model (FEFLOW) and geochemical processes simulation (GWB), a library Communicator was implemented. It is a DLL component written in C # which could interact with the database system PostgreSQL. The component takes a form of plugin for FEFLOW 6.2 and it is running in the FEFLOW’s memory space. It also requires the installation of GWB on the same computer where FEFLOW is installed. For a successful call it requires the REACT.EXE engine, which is a tool of the GWB system.

Debugging of system of equations that describes the geochemical system is completed in GWB and the Communicator component ensures the transfer of chemical components from the FEFLOW transport model to GWB and also takes care of return values after simulation of kinetic geochemical processes in the specified time step for each FEFLOW model mesh node (see Figure 5 at the end of this article).

The component Communicator is written as a universal interface between the transport and chemical numerical simulation environment. Using XML configuration files the appropriate species are mapped from one environment to another (see Figure 6 at the end of this article).

The component Communicator can be easily adapted to co-operate with other geochemical models (e.g. PHREEQC) using the XML configuration file which defines commands Init, Set (Go) and Close according to the syntax of considered geochemical program (see Figure 7 at the end of this article).

When using component’s Communicator in co-operation with the PostgreSQL database environment, we can benefit from saving results of geochemical calculation module. In following transport model time steps, the system could evaluate the criterion of similarity of node species composition and possibly instead of calling geochemical calculation, use the return value from the database saved in previous time steps. In this way, a significant reduction of time required for the total reaction-transport simulation calculation may be achieved.

Fig. 5. Component Communicator - diagram of the processes



Fig. 6. Component Communicator – transport to react mapping


Fig. 7. Component Communicator – GWB commands syntax configuration

4 Conclusions

Mining activity has a deep impact on the surface environment. Amongst the most significant, is the deterioration of ground water quality. Changes in oxidation-reduction conditions during mining and again after flooding of the ore deposit were identified as the key factor which determines quality and long term geochemical evolution of mine water. These processes play an important role especially in evaluation of mine water quality in the uranium mine sites due to complicated behaviour of uranium forms in various Eh - pH conditions. Geochemical tools like GWB are suitable for a description of these relationships. The FEFLOW environment allows for incorporation of mine workings into a model network and a simulation of water flow and transport of substances is thus processed in an appropriate geometrical arrangement.

The above method of transport and reaction modules coupling is after intended to enable a prediction of water composition in flooded uranium mine. The aim of this work is to optimize the pumping scheme of these waters for surface treatment and to verify the possibility of their re-use for the extraction of uranium.

Acknowledgement. This research was financially supported by the Technology Agency of the Czech Republic (TACR, research project TA 02021132).

References

Bethke, C. M., (2014) The Geochemist’s Workbench® Release 10.0. GWB esentials guide. University of Illinois, Urbana

Diersch H.J.G. (2014) FEFLOW Finite Element Subsurface Flow and Transport Simulation System. Reference Manual, DHI - WASY GmbH Institute for Water Resources Planning and Systems Research.

Recommended for publication by Editorial board.

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