Recovery of Ni, Co and rare earths from spent Ni–metal hydride batteries and preparation of spherical Ni(OH)₂

Abstract

A hydrometallurgical procedure has been developed for the separation and recovery of nickel, cobalt and rare earths (RE) from spent Ni–MH batteries. In the process of separation and recovery, the positive and negative electrode materials were merged and leached with 3 M H₂SO₄ at 95 °C. In this stage, about 94.8% of rare earths were separated from other metals due to the low solubility of RESO₄ at relatively high temperature. Then iron, zinc and manganese (together with the remaining 5.2% rare earths) were almost completely separated from nickel and cobalt by solvent extraction with 20% P204. Rare earths in the organic phase were recovered by stripping with 2 M HCl after scrubbing Zn and Mn. Most rare earth sulphates remaining in the leach residue were transformed to RE(OH)₃ by treatment with NaOH and then redissolved from the filter cake as RECl₃ by using the rare earth strip liquor and additional HCl. Cobalt was easily separated from nickel by solvent extraction with 20% Cyanex 272 and pure cobalt and nickel sulphates were recovered from evaporation of the strip liquor and raffinate respectively. Finally, spherical nickel hydroxide powder was synthesized from the pure nickel sulphate, which is an important starting material for Ni–MH batteries. The overall recovery efficiency for rare earths, cobalt and nickel exceeded 98%.

Introduction

Ni–MH batteries have been applied widely in many fields since they were commercialized in 1990. Their electrodes consist of foamed-nickel or copper plating strip as substrate and nickel hydroxide and AB₅-type hydrogen-storage alloy as active electrode material. Therefore Ni–MH batteries contain valuable metals in which the average content of Ni, Co and light rare earths is about 30% 4% and 10% respectively (Zhang et al., 1999). The output of Ni–MH batteries is steadily rising due to their low cost, high electrochemical capacity, quick charging capability, long cycle life, good security, good environmental compatibility, wide range of usable temperature, etc. It is estimated that the Ni–MH batteries will occupy ~ 30% of the international battery market in 2010 (Tzanetakis and Scott, 2004a, Tzanetakis and Scott, 2004b) and have an average service life of about two years. In 2005, it was determined that the total number of spent Ni–MH batteries reached one billion, in which there exists about 7500 ton Ni, 1000 ton Co and 2500 ton light RE. Because the arbitrary dumping of spent Ni–MH batteries could lead to serious heavy metal pollution, the recovery of valuable metals from spent Ni–MH batteries has received much attention (Espinosa et al., 2004, Bertuol et al., 2006, Muller and Friedrich, 2006, Rabah et al., 2008).

Many hydrometallurgical processes have been developed to recover metals from spent Ni–MH batteries in recent years. Tenorio and Espinosa (2002) used ore-dressing methods to recover the nickel-based alloy from Ni–MH batteries whilst Wang et al. (2002) regenerated the hydrogen-storage alloy from Ni–MH batteries, whose structure and electrochemical properties could be restored. Zhang et al., 1998, Zhang et al., 1999 reported a hydrometallurgical process for the separation and recovery of metal values such as nickel, cobalt and rare earths from spent Ni–MH batteries. In their research work, the electrode materials were dissolved in 2 M H₂SO₄ at 95 °C with a leach efficiency of 97% Ni, 100% Co and 96% RE. The rare earth values were recovered from the leach liquor by solvent extraction with 25% D2EHPA, followed by precipitation with oxalic acid and calcination. The cobalt and nickel in the raffinate were then separated by 20% Cyanex 272 and recovered as oxalates by the addition of oxalic acid. More recently, Tzanetakis and Scott, 2004a, Tzanetakis and Scott, 2004b also used D2EHPA to extract rare earths from the leach solution, then nickel and cobalt powders were separated and recovered through electrochemical deposition.

Because the solubilities of rare earth sulphates are low, especially at higher temperature, a large ratio of liquid to solid must be used to ensure a high leach recovery of rare earths, which makes the final nickel concentration low. This leads to large amounts of wastewater and high energy consumption.

This work develops a facile and low-cost flowsheet to recover and separate rare earth, nickel and cobalt from the electrode materials of spent Ni–MH batteries. Higher concentrations of H₂SO₄ and lower ratios of liquid to solid were adopted in the leaching step, which precipitated most of the rare earths as sulphates in the leach residues. Consequently, the leach liquor contained relatively low concentrations of rare earths and high concentrations of nickel, which decreased the consumption of extractant and the emission of wastewater. Subsequently, a high-density spherical Ni(OH)₂ with wide size distribution was prepared from the concentrated nickel solution by the “controlled crystallization method”. This material can be used to produce the positive active material of Ni–MH batteries and thereby realize the recycling of nickel from Ni–MH batteries.