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广西壮族自治区、云南省堆积型铝土矿储量巨大、资源丰富,铝土矿山建设与氧化铝生产规模巨大。据统计,仅广西壮族自治区百色市年产氧化铝就达到1.0×107 t,且每生产1 t氧化铝约产生1.0~1.5 t赤泥[1-3]和3.0~4.5 t矿泥,赤泥和矿泥累计堆存量分别在1.0×109 t以上和3.0×109 t以上。赤泥具有碱性强、盐分高、环境风险高和资源化利用难等特点[4-8];矿泥具有含水率高、粒径小、颗粒表面带负电、高比阻和脱水难等特点[9]。由于缺乏经济可行的资源化综合利用的技术,目前赤泥和矿泥主要以堆存为主。这不但占用大量土地资源,还可能存在堆存库溃坝、土壤及水污染等环境隐患,严重制约着氧化铝行业的可持续发展[10]。
赤泥盐碱性调控是能否解决赤泥规模化的关键所在。目前,国内外对赤泥盐分调控研究较少,主要研究赤泥碱性调控。赤泥碱性调控方法主要有水洗法、无机酸中和法、石膏法、海水法和卤水法等方法[5]。其中,水洗法是水洗过滤的处置方法,难以实现赤泥中化学结合碱的有效调控,且耗水量大,技术经济性差[1,5]。无机酸中和法是利用无机酸与赤泥中自由碱和化学结合碱发生中和反应来降低赤泥碱性[11-12],但降低赤泥碱性的同时可能存在与赤泥中氢氧化铝、各种铝酸盐发生反应,加重铝的二次溶出及其环境危害。石膏法、海水法和卤水法是利用Ca2+、Mg2+与赤泥中的碱性阴离子发生反应降低赤泥pH[5,13-17],且可提供Ca2+、Mg2+,有助于植物生长,利于赤泥原位复垦和生态恢复。
铝土矿矿山采空区周边坡地和耕地可交换Mg2+质量分数低[18],而Mg2+是植物形成叶绿素不可缺少的元素,在光合和呼吸作用中起到不可或缺作用[19]。外源施加镁肥能够增强植物光合作用[20]。由崔姗姗等[21]利用CaCl2废液中CaCl2、MgCl2和酸对赤泥进行脱碱处置的研究可知,MgCl2能够降低赤泥碱性。为推进赤泥、矿泥规模化处置和矿区生态恢复工作,本研究采用MgCl2和脱水矿泥对赤泥进行盐碱性调控,以期为赤泥盐碱性调控,并与矿泥一起进行原位矿山采空区回填和地貌景观修复、复垦地块土壤生态恢复提供依据。
MgCl2和脱水矿泥对赤泥盐碱性的调控
Conditioning of salinity and alkalinity in red mud by MgCl2 and dehydrated mineral slime
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摘要: 针对赤泥盐碱性过高造成资源化利用难的问题,采用MgCl2和氧化铝工业固体废物脱水矿泥对赤泥进行盐碱性调控,并依据赤泥的pH、电导率、碱性阴离子(OH−、CO32−、HCO3−、AlO2−)和可溶性盐分阳离子(K+、Ca2+、Na+、Mg2+)质量分数以及黑麦草种子发芽率结果表征盐碱性调控效果。结果表明,当赤泥投加7‰ MgCl2,并与脱水矿泥按1∶4质量比混合处置时,其pH从11.36降至8.43,电导率从1.557 mS·cm−1降至1.166 mS·cm−1,黑麦草种子发芽率达到99.67%。赤泥pH、电导率以及OH−、CO32−、AlO2−、K+、Ca2+、Na+、Mg2+质量分数随着MgCl2投加量和脱水矿泥混合比例的增加而降低,黑麦草种子发芽率因此而随之增加,显示了良好的盐碱性调控效果。本研究结果可为氧化铝工业固废赤泥盐碱性调控提供参考。Abstract: In order to solve the problem of difficulty in resource utilization caused by excessive salinity and alkalinity of red mud, MgCl2 and dehydrated mineral slime of alumina industrial solid waste were used to control salinity and alkalinity of red mud. The effect of salinity and alkalinity regulation of red mud was characterized by pH, conductivity, mass fraction of alkaline anions (OH−, CO32−, HCO3−, AlO2−), mass fraction of soluble salt cations (K+, Ca2+, Na+, Mg2+) and germination rate of ryegrass seeds. The results showed that when red mud added with 7‰ MgCl2 and mixed with dehydrated mineral slime at a mass ratio of 1∶4, pH decreased from 11.36 to 8.43, conductivity decreased from 1.557 mS·cm−1 to 1.166 mS·cm−1, and germination rate of ryegrass seeds reached 99.67%. The pH, conductivity and mass fractions of OH−, CO32−, AlO2−, K+, Ca2+, Na+ and Mg2+ of red mud decreased with increase of MgCl2 addition and mixture ratio of dehydrated mineral slime, thus germination rate of ryegrass seeds increased, which showed a good effect on salinity and alkalinity regulation. The results of this study can provide the basis for control salinity and alkalinity of solid waste red mud in alumina industrial.
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表 1 赤泥、脱水矿泥的化学成分
Table 1. Chemical compositions of red mud and dehydrated mineral slime
% 供试样品 Al2O3 SiO2 Fe2O3 TiO2 CaO Na2O MgO 其他 赤泥 12.4 29.6 37.9 − 6.5 6.7 − 6.9 脱水矿泥 39.9 23.6 9.3 6.8 0.5 0.8 2.3 16.8 注:−表示未检出。 表 2 菜田土壤盐分分级参考标准(土水比1 g∶5 mL测定电导率)[27]
Table 2. Reference standard for soil salinity classification in vegetable fields (soil-water ratio 1g∶5mL to determine conductivity)
电导率/(mS·cm-1) 盐分等级 对作物的影响 <0.25 极低盐度 一般作物生长正常 0.25~0.60 低盐度 对敏感作物有障碍 0.60~0.80 中盐度 多数作物生长受阻 0.80~1.00 高盐度 仅耐盐作物能生长 ≥1.00 超高盐度 仅极耐盐作物能生长 注:蔬菜正常生长的电导率临界值为0.60 mS·cm−1。 表 3 赤泥、控碱赤泥、脱水矿泥和类土矿相组成
Table 3. Mineral phase contents of red mud, alkali-controlled red mud, dehydrated mineral slime and soil-like
% 矿相 赤泥 控碱赤泥 脱水矿泥 类土 方钠石 9.5 0 0 0 钙霞石 24.1 23.1 4.4 6.5 方解石 7.1 0 0 0 高岭石 0 3.6 24.9 22.5 一水硬铝石 0 0 14.3 11.0 三水铝石 3.6 9.0 25.5 24.5 水镁石 0 0.3 2.7 4.1 菱镁矿 0 33.9 0.9 8.8 石英 17.8 11.8 10.5 9.5 -
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