Treatment of monazite processed effluent to recover rare earth metals (REMs)

Highlights

• Human health risk due to REMs in mine water and effluent.

• Processing of monazite effluent to reduce human health risks.

• Amberlite IR120 Na used to recuperate light REMs.

• Significantly enriched the REMs in solution for recovery.

• Pure REMs oxides can be prepared from enriched solution of REMs by precipitation.

Abstract

Improper disposal of effluent generated in rare earth mining areas and ore processing industries results in loss of REMs and miserably affects the ecosystem. Thus, their appropriate treatment is required, which can be achieved via environmentally feasible processes. In this connection, systematic scientific adsorption studies were carried out to separate REMs using cationic resin, Amberlite IR120 Na from the effluent generated during monazite processing for REMs recovery. To optimize feasible conditions for REMs recovery, bench scale studies were carried out varying different process parameters viz. pH, contact time, resin dose, etc. It was observed that adsorption of 92.63% La, 92.79% Ce, 91.45% Nd, 90.95% Pr and 95.09% Sm was achieved at aqueous/ resin (A/R) ratio 25 mL/g, pH 1.3 and contact time 10 min. Loading capacity of resin was found to hold 48.57 mg REMs/g resin. The adsorption data followed the second order reaction ((t/q) =  (1/h) + (1/q_e)(t)) and Langmuir adsorption isotherm (1/q = [(1/k₁ q_m)(1/C_e)] + (1/q_m)). The loaded REMs was effectively eluted using 15% H₂SO₄ in 10 min. The REMs enriched solution was treated to get pure REM oxides as precipitate. This technical application will be useful for REMs recovery as well as to mitigate environmental pollution.

Introduction

Huge mine-water, industrial effluent, rinse water and various lean-grade aqueous solution containing rare earth metals (REMs) create environmental problem as well as loss of valuable metals. Effluent particularly from mining and rare earth processing sectors are enriched in REMs contents [1]. But these waste effluent often goes ignored compared to large amount of metals recovered. Thus, several ecological issues related to mining, processing and disposal of REMs as effluent exists, which unavoidably affects the land and water. Massive socio-economic disturbance occurs to the population of these areas due to overdose of REMs in blood resulting in augmented diseases [2]. Some REMs also considered radionuclides (chemical elements emitting either α, β, or γ-rays, or neutrons) and thus, efficient elimination as well as solidification of these radionuclides from wastewater is reported to be essential due to their hazardous effect [3]. Attention towards these environmental issues due to exposure of REMs has been gathered by researchers globally but lack of appropriate step to strictly control their dispersion is making the situation miserable.

REMs processing techniques consumes huge amount of water and chemicals, which later gets accumulated on land surface as effluent. In China, production of 1 t oxides of rare earths evolves 200 m³ of acidic water along with the other constituents [4]. These REMs create vital and detrimental effect to the environment, biological system and human health. REMs can replace calcium present in enzymes and cell membranes; and even results in hepatic injury, damage to blood lymphocytes chromosomes and also weaken the capacity of hemoglobin to combine oxygen [5]. REMs mining areas in China were reported to have more content of cerium in their breast milk, blood plasma and serum. Regular intake of low dose REMs accrue in the bone structure resulting in genotoxicity in bone marrow cells [6]. Apart from this, human health risk issues were also assessed through vegetable consuming REM ions in mining areas. The total REMs content in vegetables were reported to be 94.08 μg kg⁻¹ and 38.67 μg kg⁻¹ in mining and control areas, respectively [7]. Therefore, in order to carefully address these environmental as well as health degradation issues, steps have been taken and strict guidelines for the management of rare earths industries has been revised. Moreover, methods are also being developed that can consistently estimate bioavailability of REMs to plants so as to evaluate their impact on human health and ecosystem [1]. Lot of advance separation techniques such as precipitation, cementation, crystallization and solvent extraction are used for the treatment of solution/effluents when concentration of metals are little high. In comparison to other hydrometallurgical separation process, the metal adsorption process using resin is feasible up to the industrial scale for the low metal containing solution/effluent. But lack of study still prevails for feasibly treating effluent generated during the processing of REMs.

Some work carried out for REMs recovery via adsorption process are discussed here. Solid-phase extraction of Tb, Dy, Ho, Y, Er, Yb and Lu from phosphoric acid using Tulsion CH-96 and T-PAR resin has been investigated where the transfer of metal to the resin phase follows ion exchange type mechanism [8]. A selective solid-phase extraction using chemically modified Amberlite XAD-4 with monoaza dibenzo 18-crown-6 ether was studied for the pre-concentration and separation of La(III), Nd(III) and Sm(III) in synthetic solution. The absorbed REMs were eluted using HCl [9]. Investigations were carried out to notice the adsorption and desorption behavior of D151 on Ce(III) [10]; D113-III on Nd(III) and Er(III) [11], [12]. Extraction of Gd from phosphoric acid medium using amino phosphonic acid resin, Tulsion CH-93 was reported where the loading capacity of Tulsion CH-93 for Gd was found to be 10.6 mg/g [13]. N-methylimidazolium functionalized anion exchange resin was used for adsorption of Ce(IV) from nitric acid medium by reducing it to Ce(III) [14]. The adsorption process for La removal from aqueous solution was also evaluated using chemically modified cellulose [15]. Ultrasonically synthesized Cyanex 572 oil droplets and Cyanex 572-impregnated resin were also used for selective extraction of heavy rare earth element from dilute solutions [16]. Selective adsorption of REE from buffered media solutions on Purolite S910 and Amberlite IRC86 weak acid resins was also studied [17]. Much more adsorbents were reported such as Lewatit FO36 nano resin, ulmus leaves and their ash, barley hull and their ash, rice husk and its ash, etc. to extract heavy metals using biosorption technique which could be utilized to extract REMs too [18], [19], [20], [21]. But as reported by researchers, these adsorbents consumes time and also require large operational areas [22]. Thus, utilization of biosorption technology on large scale is still a distant reality [23]. However, carbon materials like porous carbon, activated carbon, carbon nanotubes and activated carbon loaded with zero valent iron and silver bimetallic nanoparticles were also extensively used for waste water treatment [24], [25], [26]. But most of the research work described, lack systematic basic research, application orientation and actual system. In view of above drawbacks, the possibility of the use of feasible resin is required to be explored. Thus, considering environmental concern, related hazardous effect and loss of material, the systematic and application oriented basic studies is required urgently to comply with the thrust area of research based on zero waste discharge concept. Generally, researchers/process engineers report the work without any information regarding the treatment of generated solid/liquid waste and its disposal.

Present work reports the recovery of REMs as value added product using ion exchange technique. The novelty of this work lies in the development of a new hybrid process flow-sheet to recover REMs from the effluent generated during monazite processing. The obtained data is examined with all established scientific equations and feasibility of resin was also examined.