macla nº 20. julio ‘15 revista de la sociedad española de mineralogía 59 Mixing-Induced Precipitation in Brine Mining: Reactive Transport Modeling / ALEJANDRO GARCÍA-GIL (1), MAR GARCÍA-ALCARAZ (2), ENRIC VAZQUEZ (2), CARLOS AYORA (2*) (1) Departamento de Geología. Universidad Zaragoza, Pedro Cerbuna, 12. 50009 Zaragoza (2) Grupo Hidrología Subterránea, UPC-CSIC, Instituto de Diagnóstico Ambiental y Estudios del Agua, Jordi Girona 18, 08034 Barcelona INTRODUCTION planes (probably stratification planes). It is 12 to 15 m thick and has high Continental brines have become the hydraulic conductivities. (II) A less leading raw material for lithium permeable body of 2 to 8 m thick acting production worldwide. Lithium-bearing as an aquitard which consist of halite brines are found in salt aquifers (mainly that may have gypsum levels. (III) Unit B halite) in the nucleus of salars (salt is a semi-confined aquifermade up of flats). Because of extremely high halite, 8 to 11 m thick and highly evaporation rates, the primary discharge conductive. (IV) A lowpermeability body occurs by evaporation keeping the water is at the bottom of the mined system. table below the surface. Two different brines are identified in Brines are exploited through pumping aquifers “A” and “B” with a transition wells or ditches excavated in halite brine in the aquitard separating those aquifers and evaporation of the brine in aquifers. Brine extraction is carried out solar ponds to further concentration. In via brine pumping fields, i.e. horizontal fig 2. Gypsum precipitates around in a well. this type of mining, such as that in drains (1 km length) connected with REACTIVE TRANSPORT MODELING Atacama (Chile), a fast and drastic loss vertical extraction well (Fig. 1). Hundreds in the hydraulic efficiency of the of pumping wells are distributed pumping wells threatens the feasibility according to a regular grid. Due to A reactive transport modeling of a of the exploitation. We postulate here consistency of halite rock, no casing is transversal section of the horizontal that the hydraulic efficiency loss is due used. drain has been used to investigate the to salt precipitation induced by brine processes leading to salt precipitation in mixing. The effects of mixing solutions the surroundings of a drain. To solve the has only been considered to explain multisolute reactive transport RETRASO carbonate dissolution at the seawater code (Saaltink et al., 1998) has been intrusion front in coastal aquifers used. (Sanford & Konikow, 1989), and two A explain the deep cavity formation B For each time increment, the porosity is required to allocate Mississippi Valley- updated according to the molar volume type deposits (Corbella et al., 2004). fig 1. Sketch of the brine exploitation, with two of the precipitated phases. Porosity aquifers, A and B, separated by an aquitard. The area within the dotted line represents the modeled variation modifies permeability in the The aim of this work is to study the domain. simulations according to feasibility of the hypothesis of brine Kozeny‟sequation: mixing and mineral precipitation in the brine exploitation of the Atacama Salar, During the brine extraction, a drop in the by means of reactive transport brine level in the well is observed. The level drop is caused by an exponential modeling. decrease inhydraulicconductivity and causes a rapid decrease in the brine where is the initial porosity and STUDY SITE pumping efficiency. This level drop is the intrinsic permeability for matrix . satisfactorily modeled assuming an The Atacama Salar is a closed saline „skin effect‟, i.e., the existence of a low The model domain is 40 x 500 m basin which forms part of the current permeability skin around the pumping section normal to the drain. Because inner fore arc of northern Chile. With a well. Indeed, massive precipitation of the symmetry of the problem under 1400 km2 outcrop area, the salt nucleus salts is observed in and around the wells consideration only half of the constitutes a multilayer halite aquifer. It and drains (Fig. 2). transversal section to the drain is can be distinguish (Fig. 1): (I) Unit A is an modeled (see modeled section in Fig. unconfined aquifer mainly composed of 1). The model simulates 90 days of pure halite with several discontinuity pumping with a pumping rate of 0.01 palabras clave: Salmuera, Mezcla, Precipitación, Yeso, Halita, key words: Brine, Mixing, Precipitation, Gypsum, Halite, Atacama, Atacama, Permeabilidad, Colmatación, Transporte Reactivo Permeability, Clogging, Reactive Transport resumen SEM 2015 * corresponding author: [email protected] macla nº 20. julio ‘15 revista de la sociedad española de mineralogía 60 L /s in the center of the drain. No flow is considered through the top and bottom boundaries, and constant head at the A boundary opposite to pumping. Pitzer ion -ion interaction model has been used in the geochemical calculations. R ESULTS AND DISCUSSION According to field observations, the reactive transport calculations predict a -50 thin mixing zone and gypsum B -40 precipitation in the aquitard between -30 units A and B (Fig. 3). However, most of -20 gypsum precipitation takes place -10 0.0 around the drain (Fig. 3). Precipitation is no t symmetrical. There is a zone with a low rate of gypsum precipitation which D can be called “no reactive channel” (Fig. C 3 ). Halite precipitation is two orders of magnitude lower than gypsum (not represented). Porosity decreases 0.05 towards the drain from 5%, as the initial 0.04 porosity, to 0.18 % in the drain wall. 0.03 Within the “no reactive channel” the 0.02 0.01 porosity remains above 2%. 0.005 0.002 Pumping generates hydraulic gradients forcing the upper brine to flow through figfig 3.3. A).A) Zones Zones with with brine brine A A (gree (greenn),), B B (white) (white) and and mixture mixture of of both both (orange) (orange) m markedarked by by the the concentration concentration of of a the aquitard unit towards the lower conservativea conservative element. element. Isopotential Isopotential lines lines are are plotted plotted in in the the background. background. B)B) Mass of gypsumgypsum precipitatedprecipitated 3 aquifer (Unit B). This flow fieldproduces (mol/m3).(mol/m ). FlowFlow lineslines plottedplotted inin the background. C) Detail ofof BB aroundaround thethe drain.drain. D)D) PorosityPorosity .All.All the the graphics graphics a mixing of the brines A and B. The corresponcorrespondd to to results results after after 90 90 days days calculation calculation.. mixing of two brines in equilibrium with amount of halite precipitates. The mixture has exhausted most of 12 12 12 12 10 10 10 10 x-nh4 x-na x2-ca x2-mg 8 8 8 8 x2-ca x2-mg 6 6 6 6 4 4 4 4 2 2 2 a2 salt produces supersaturation and 0 0 0 0 0.0 2.0 4.0 6.0 8.0 0.0 2.0 4.0 6.0 8.0 0.0 2.0 4.0 6.0 8.0 0.2 0.4 0.6 0.8 1.0 0.0 its precipitation potential and forms a precipitation of the salt. Indeed, Fig. 4 „non-reactive‟ channel. shows the representation of the gypsum 0.5 equilibrium equation (x*y=K). Although As hydraulic conductivity changes with activities should be represented instead 0.4 kgw porosity, after approximately 15 days molalities, the total salinity is almost 0.3 top and bottom environments of the constant, and therefore the activity mol/ ] drain has undergone mineral clogging, coefficients and the activity of water can 0.2 [SO4 and very small flow rates cross clogged be assumed constant and included in K. 0.1 areas. In this setting, ascending and Any brine equilibrated with gypsum, descending flow paths are forced to flow such as A and B, will plot within the 0.0 towards the “no reactive channel”. The equilibrium line. Any brine with higher 0.00 0.05 0.10 0.15 0.20 [Ca] mol/kgw thickness of this channel progressively Ca and/or SO4 will be supersaturated. In wanes, and the efficiency of the well contrast, any mixture of two brines fig 4. Ideal mixing of two brines (A and B) that are in equilibrium with gypsum. If mixing is non-reactive, exponentially drops. equilibrated with gypsumwill plot within theresulting mixture will be supersaturated. the straight segment AB, and therefore Returning to equilibrium requires precipitation. REFERENCES in the supersaturation field. Because of gypsum stoichiometry, precipitation will Close to the drain, descending brine. A Corbella, M., Ayora, C.,Cardellach, E. (2004): move the mixture to the equilibrium (path 1 of Fig. 3) invades the aquifer with brine B, high mixing occurs and Hydrothermal mixing, carbonate around a 1:1 slope line. The distance dissolution and sulfide precipitation in from the conservative line to the massive gypsum (and minor halite) precipitates in the area abovethe drain Mississippi Valley-type deposits: equilibrium line determines the mass of Mineralium Deposita, 39, 344-357. gypsum precipitated. It is easy to see during the initial 15 days (Fig. 3). Saaltink M.W., Ayora C., Carrera J. (1998): A Meanwhile, brine B flows from far that: 1) the more different Ca and SO4 mathematical formulation for reactive concentrations are in the mixing brines, regions through more permeable aquifer ransport that eliminates mineral the moreprecipitation occurs; and 2) B towards the drain. It mixes with concentrations. Water Resources Research, 3, 1649–1656. 50% mixing results in maximum descending brine A, which has invaded the drain area, and precipitated gypsum Sanford, W.E., & Konikow, L.F. (1989): amount of mass precipitated. The same Simulation of calcite dissolution and discussion can be made for halite. below the drain (path2 of Fig. 3). In between these two paths, brines A and B porosity changes in saltwater mixing zones However, Cl and Na concentration in A in coastal aquifers: Water Resources have been mixed up during their flow to and B brines are relatively similar, their Research, 25, 655-667.