Silica solubility and dissolution kinetics at high saline geothermal conditions

This dataset contains solubility data for silica as a function of time, temperature and salinity. The dataset supports Chapter 2 in the deliverable “Report on mineral solubility and precipitation at high salinities, DOI: https://doi.org/10.48440/gfz.4.8.2023.001 from the H2020 project REFLECT. The silica material used as solid substrate for the dissolution studies was pro analysis sea sand (purified by acid washing and calcinated for analysis) from Merck. The sand grain size (125-250 µm) included in the experiments was obtained by sieving the material. The sieved powder was washed with tap water to remove fine grains from the samples, and dried prior to experiments. The BET surface area of the sand was measured to 0.69 m²/g and the weighted mean particle size distribution (PSD) was 118 µm. The crystallographic structure was determined by X-ray diffraction analysis (XRD) and this analysis showed that the sample contained mainly low-quartz (minimum 95% w/w) with a few unidentified impurities. SEM/EDS maps of the silica powder showed essentially pure silica with minor Al impurity. Some grains or regions are enriched in Al and K, suggesting some aluminium silicate. Some minor spots rich in Ti, Fe and Cr were also detected. The experiments conducted to study silica solubility at equilibrium conditions were performed at five different temperatures (100, 125, 150, 175 and 200°C) and four salinities (NaCl concentrations 50.9, 103.6, 215.7 and 338.1 g/kg H₂O). The columns containing SiO₂ and NaCl solutions where isolated for a reaction time of six days before fluid sampling (Table1 “Silica solubility at high saline geothermal conditions”). The experiments conducted to study silica solubility kinetics were performed for different time periods (from 1 hour up to 144 hours) to study solubility as a function of time. These tests were conducted at 200°C with NaCl concentration 50.92 g/kg and 338.09 g/kg H₂O (Table2 “Silica solubility kinetics at high saline geothermal conditions”). The experimental setup consists of packed static columns. Maximum four columns (length 40 cm, i.d. 10.22 mm, stainless steel SS316) packed with the material to study can be placed in parallel within the setup. Porous metal frits (HC276) are placed at the outlet and inlet of the columns to prevent entrainment of the material. Approximately 50 g of dried SiO₂ powder is required to fill a column completely and the pore volume was measured gravimetrically to be approximately 15 ml. Two Gilson 307 high performance liquid chromatography (HPLC) pumps are included in the setup. One for filling and displacing column pore fluid and one for diluting the fluid prior to sampling, preventing precipitation of dissolved silica due to depressurization and cooling. Pressure was maintained by a dome loaded backpressure regulator (BPR) from CoreLab at the column outlet and liquid samples were collected using a fraction collector (Gilson FC203B). The setup of columns and inlet/outlet valves was placed in a heating cabinet (Memmert). The columns were thermally insulated to prevent instabilities in temperature and hence pressure when opening the heating cabinet during sampling. The columns are flooded with degassed NaCl fluid at a low flow rate and pressurized initially to 25 bars while temperature is increased slowly to the desired level. The time of start is noted, the brine pump is shut off, and the individual columns isolated by closing inlet and outlet valves. After a period (hours, days, or weeks) samples are withdrawn from the columns and diluted at the mixing point by re-opening the valves and operating both HPLC pumps. A dilution factor of 8.5 is selected to prevent precipitation. For each sampling five samples of 2 ml is collected (totally 10 ml of fluid). The two first samples are considered to contain mainly dead volumes from tubing, fittings and valves and are therefore discharged. The three last samples represent the pore fluid from the column. These samples are analysed for Si and NaCl concentration. The NaCl concentration was analysed to keep control of the dilution step of the sampling process. SiO₂ and NaCl concentrations were analysed using inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma optical emission spectrometry (ICP-OES). The elements Si and Cl (ICP-MS) or Si and Na (ICP-EOS) were detected. The Si concentration from the analysis was reported as mg/L solution. From this concentration the concentration of SiO₂ in the samples were calculated and reported as mol/kg H₂O. The conversion from liter solution to kg H₂O was done using the OLI software for density calculations.本数据集包含二氧化硅的溶解度数据，该数据随时间、温度及盐度变化而变化。本数据集支撑H2020计划REFLECT项下交付成果《高盐度矿物溶解度与沉淀报告》（DOI: https://doi.org/10.48440/gfz.4.8.2023.001）的第2章内容。本溶解实验所用的固体基质二氧化硅材料为默克（Merck）生产的分析纯海砂，经酸洗纯化并煅烧后用于分析测试。实验所用砂粒粒径范围为125~250 μm，通过筛分法获得该粒度分布。筛分后的粉体经自来水洗涤以去除细颗粒，实验前完成干燥处理。该砂样的BET比表面积（Brunauer-Emmett-Teller）经测定为0.69 m²/g，加权平均粒径分布（PSD）为118 μm。采用X射线衍射分析（XRD）测定了该砂样的晶体结构，结果显示其主要成分为低石英（最低占比95%质量分数），含有少量未识别杂质。二氧化硅粉体的扫描电镜/能谱（SEM/EDS）表征结果显示，其基本为纯二氧化硅，仅含少量铝杂质；部分颗粒或区域富铝、钾，推测存在少量铝硅酸盐；同时还检测到少量富含钛、铁、铬的微区。针对平衡态下二氧化硅溶解度的实验，设置了5个温度梯度（100、125、150、175及200℃）与4个盐度梯度（氯化钠浓度分别为50.9、103.6、215.7及338.1 g/kg H₂O）。在流体取样前，装有二氧化硅与氯化钠溶液的反应柱需密封静置6天（详见表1《高盐度地热条件下二氧化硅溶解度》）。针对二氧化硅溶解动力学的实验，则设置了不同反应时长（1小时至144小时）以探究溶解度随时间的变化规律。该系列实验在200℃下开展，氯化钠浓度分别为50.92 g/kg及338.09 g/kg H₂O（详见表2《高盐度地热条件下二氧化硅溶解动力学》）。实验装置采用装填式静态反应柱，最多可并行放置4根装填有实验材料的反应柱（柱长40 cm，内径10.22 mm，材质为SS316不锈钢）。反应柱的进出口处均安装有HC276多孔金属滤片，以防止实验材料被流体夹带。单根反应柱完全装填需约50 g干燥二氧化硅粉体，经重量法测定其孔体积约为15 mL。装置配备2台吉尔森307型高效液相色谱（HPLC）泵：一台用于装填与驱替反应柱内的孔隙流体，另一台用于取样前的流体稀释，以避免因降压与降温导致溶解态二氧化硅析出。反应柱出口处安装有CoreLab品牌的膜片式背压调节器（BPR）以维持系统压力，液体样品通过吉尔森FC203B型馏分收集器完成采集。反应柱及进出口阀门组放置于Memmert品牌的恒温箱内，且对反应柱进行了保温处理，以避免取样时打开恒温箱导致温度波动进而引发压力不稳。首先以低流速向反应柱内注入脱气氯化钠溶液，同时将系统初始压力升至25 bar，并缓慢升温至目标温度。记录实验起始时间后关闭盐水泵，通过关闭进出口阀门密封单根反应柱。经过预设时长（小时、天或周）后，从反应柱中抽取样品，通过开启阀门并启动两台HPLC泵在混合点完成稀释，稀释因子设定为8.5以防止二氧化硅析出。每次取样采集5份2 mL的样品（总流体体积10 mL）：前2份样品主要包含管路、接头与阀门内的死体积流体，故予以舍弃；后3份样品为反应柱内的孔隙流体，用于分析硅与氯化钠的浓度。分析氯化钠浓度用于监控取样过程中的稀释步骤。二氧化硅与氯化钠的浓度采用电感耦合等离子体质谱（ICP-MS）或电感耦合等离子体光发射光谱（ICP-OES）进行测定，采用ICP-MS时检测硅与氯元素，采用ICP-OES时检测硅与钠元素。分析得到的硅浓度以mg/L（溶液）为单位上报，基于该浓度可计算得到样品中二氧化硅的浓度，最终以mol/kg H₂O为单位上报。溶液体积与水的质量之间的转换通过OLI软件进行密度计算完成。