Transfer of cobalt and nickel from sulphate solutions to spent electrolyte through solvent extraction and stripping

Abstract

We report in this paper studies carried out on the separation and recovery of cobalt and nickel from sulphate solutions through solvent extraction using acid form of PC 88A and D2EHPA diluted in kerosene and stripping of metals from loaded organic phase with synthetic spent electrolyte solutions to generate respective pregnant electrolyte solutions. The typical composition of sulphate solution used for the present study contains Co: 1.91 g/L and Ni: 14.83 g/L. Variation of aqueous phase equilibrium pH in the range from about 3.1 to 6.5 using 0.15, 0.2 and 0.25 M PC 88A indicated increase of cobalt extraction and reaches maximum (∼92–95%) around 5.5 pH with co-extraction of nickel from 0.1 to 5.7%. Comparison of cobalt extraction behavior with the three extractant concentrations showed 0.2 M PC 88A as the best with highest separation factor of about 730 and about 3% co-extraction of nickel only. Using 0.2 M extractant concentration, almost quantitative extraction of cobalt in two counter-current (CC) stages at an aqueous to organic (A:O) phase ratio of 1.5:1 and stripping of metal from loaded organic (LO) in two CC stages at A/O ratio of 1.7 with synthetic spent electrolyte (SE) solution was achieved, generating a pregnant electrolyte solution of pH around 3 suitable for electrowinning. Similar optimum conditions for the treatment of cobalt raffinate to recover nickel using 0.7 M D2EHPA (pH 6.6, unit phase ratio, two stages) and stripping of metal from LO (O/A: 1.7, two stages) with synthetic nickel SE solution were established. Finally, complete process flowsheet for the separation and recovery of cobalt and nickel is demonstrated.

Introduction

Cobalt and nickel are produced primarily from the land based resources such as cobaltiferrous pyrite concentrate/sulphide or oxide nickel ores. In addition, some secondaries, such as spent catalysts, alloy scrap, sludge, dust, etc., are being effectively utilized for cobalt/nickel production. Further, manganese nodules found in the North and South Atlantic Ocean, the Indian Ocean and the South Pacific Oceans are considered to be potential futuristic resources for nickel and cobalt. The vast size of these deposits, for example, of the order of 10¹¹ tonne in the Pacific Ocean, is the main reason for the continued research and development [1]. Out of these, the economic criteria reported for the exploitation of manganese nodules was based on three factors such as (i) the distribution should be >5 kg/m², (ii) the grade of mineral contents should be >2.25% in terms of Ni and Cu and (iii) the scale of production should be >300 million tonne per year on a dry basis [2]. The most prospective area of manganese nodules is the Clarion and Clipperton fracture zones with an area of ∼2.5 million km². Available data shows that about half the area contains nodules in concentration >5 kg/m² and averages ∼11.5 kg/m² [3], which would give ∼2.1 billion dry metric tons of recoverable nodules containing about 1.3% Ni, 1% Cu, 0.22% Co and 23% Mn [3]. The most comprehensive coverage of the patented literature related to the research and development effort in the field of solvent extraction of non-ferrous metals for the periods from 1972 to 1976 including the treatment of nodules leach liquors was reviewed [4], [5]. Deep Sea Ventures, Kennecott Copper Corp. (KCC), International Nickel Co. (INCO) and Metallurgie Hoboken Overpelt (MHO) are the main agencies that have contributed in the processing of sea nodules; their contributions were reviewed by Mohemius [6]. Deep sea Ventures patented a process on the leaching of nodules with hydrochloric acid to soluble metal chloride complexes and solvent extraction separation of copper from Ni, Co, Mn, using Kelex or LIX reagent. KCC process tested on pilot plant consists of ammoniacal reductive leaching of nodules around 50 °C, producing selective recovery of copper, nickel and cobalt as amine complexes. INCO process consists of a combined drying and reduction smelting of nodules at high temperature (∼1000–1300 °C) using fuel oil and coal to produce alloy. This alloy is oxidized and sulphided to produce a matte containing about 25% Cu, 40% Ni, 5% Co, 5% Fe and 25% S, which was further ground and leached with H₂SO₄ in an autoclave to generate concentrated leach liquors with little iron. In all the cases, the leach liquors are processed by the conventional precipitation and solvent extraction using LIX and organophosphorus reagents.

The Minerals and Materials Division of the Korea Institute of Geosciences and Mineral Resources (KIGAM), Korea has been working for the past 10 years on the processing of Pacific Ocean manganese nodules for metal recovery. The processing of Pacific Ocean nodules involve the pyrometallurgical smelting reduction–sulphidation route to produce Cu–Ni–Co rich matte and Mn slag [7] followed by dilute sulphuric acid pressure leaching of matte [8] to obtain metals in soluble form for further processing. This route produces low volumes of matte with high metals content suitable to produce leach liquor containing high concentrations of copper, nickel, cobalt. In the previous study on selective separation of copper from a typical sulphate leach liquor containing (g/L) 10.68-Cu, 15.13-Ni, 1.95-Co, 0.07-Fe using 40 vol.% LIX 84 in kerosene, resulted about 92% copper extraction efficiency, while leaving all of Ni, Co and Fe in the raffinate [9]. Further, the raffinate before taken for Co–Ni separation was treated with alkali solution to remove the impurities (0.4 g/L Cu and 0.07 g/L Fe) by adjusting to pH around 7.5. The percentage loss of Co and Ni was about 2.2–2.7%. In the present study, a synthetic solution similar to copper free raffinate was prepared for Co–Ni separation using PC 88A as the extractant.

The solvent extraction separation and recovery of cobalt and nickel from sulphate solutions using organophosphorus compounds has been extensively studied and some industrial plants have been operated. The first commercial process for the separation of cobalt and nickel was developed [10] using di(2-ethylhexyl) phosphoric acid (D2EHPA), while similar process using HDEHP was used by Mathey Rustenburg Mining [11]. The main drawbacks of the process were the poor selectivity at high Ni:Co ratios and high temperature used (>50 °C). As a result, many efforts were made to synthesize more selective compounds such as phosphonic (PC 88A/Ionquest 801) and phosphinic acids (Cyanex 272), which led to successful Co/Ni separations on commercial scale [12], [13], [14], [15], [16], [17], [18], [19]. The Co/Ni separation factor increases in the order: phosphoric < phosphonic < phosphinic acid [20].

The present paper describes the research work carried out in the frame work of a project on processing of manganese nodules for the separation of cobalt and nickel from sulphate solutions similar to leach solution obtained by the dilute sulphuric acid pressure leaching of Pacific Ocean Nodules matte using PC 88A and D2EHPA as the extractants. The loaded organics were stripped with respective synthetic spent electrolytes in order to produce pregnant solutions suitable for Co and Ni electrowinning. The optimization of various process parameters studied includes pH, extractant concentration, aqueous to organic ratio, scrubbing, counter-current extraction and stripping stages.