Leaching behaviour of a galvanic sludge in sulphuric acid and ammoniacal media

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Abstract

Leaching studies of a sludge produced by the physico-chemical treatment of wastewaters generated by a Ni/Cr plating plant were carried out in both sulphuric acid and ammoniacal media aiming to decide which of them would be the best treatment for this kind of waste material. The dissolution behaviour of some metals (Cu, Ni, Cr and Zn) was studied in order to assure the best metal recovery conditions in subsequent processes by the use of some separation methods such as solvent extraction and precipitation techniques. Therefore, the study here presented deals with the first chemical stage of an integrated treatment process. For the sulphuric acid leaching, maximal conversions obtained were 88.6% Cu, 98.0% Ni and 99.2% Zn for the following experimental conditions: a 100 g L−1 acid concentration, a 5:1 liquid-to-solid ratio (L/S), a particle size less than 1 mm, a digestion time of 1 h, a stirring speed of 700 rpm (all at room temperature and under atmospheric pressure). As expected, no selectivity was achieved for the sulphuric acid leaching, despite this option yielding much higher metal ion dissolution when compared with that reached by ammoniacal leaching. The use of this latter medium allowed the extraction of Cu and Ni without Cr species, but rates of conversion were only about 70% for Cu and 50% for Ni, much lower than those obtained for sulphuric acid leaching.

Introduction

Metal plating has been identified as an environmental risky industrial sector concerning the potential hazardous nature of its own waste streams, since they often contain reasonable amounts of metals (e.g., nickel, copper, zinc, chromium, etc). The concentration of those metals in the wastewater might reach 30% (w/w, dry weight) making their recovery an interesting issue as environmental and economical reasons are also subjacent [1]. Despite these arguments, the usual destination for sludges generated in wastewater treatment industrial units in Portugal is their direct disposal as hazardous waste in special landfills located outside the country. This solution involves enormous costs to the waste producers, since there is a complete absence of accessible alternative treatments within the country [2]. Moreover, the average dimension of the existing units (small-medium enterprises, SMEs) contributes to the lack of real alternatives attempting to an efficient recovery of the valuable species before disposal. Hence, it is extremely important to develop viable ways to recycle those sludges if either environmental or economical aspects are to be taken into account.

A good way to treat and recycle these sludges seems to be achieved by the use of hydrometallurgy. Hydrometallurgical processes have been applied in metallurgy since the Middle Age, and actually one of their utilisations has been in the reduction of the environmental impact of several waste streams, either by lowering the build up of hazardous materials on the earth's crust or by recovering the involved metals for re-use. In fact, this area has proved to be a fruitful sector for innovation. The use of hydrometallurgical techniques is becoming more attractive, since the need of high energy requirements or eventual gas collectors and cleaning systems, always necessary in the pyrometallurgical processes, are avoided [3], [4].

The most determinant step of a hydrometallurgical process is generally the first, which is the leaching or dissolution of the valuable metals presented in the raw material suffering this sort of treatment. The overall recovery of each metallic species and the difficulty of their separation from other undesirable metals are generally governed by the efficiency and selectivity of the leaching process.

After ammonia, sulphuric acid is the main solvent used in the industrial leaching of laterite, the most important nickel ore, and chalcopyrite, the major source of copper [5], [6], [7], [8]. Jha et al. [4] made an extensive review concerning several industrial hydrometallurgical processes, including leaching, to recover zinc from industrial wastes. They have established that the sulphuric acid and ammoniacal solutions are the most effective reagents for the treatment of wastes/secondaries generated in metallurgical industries such as galvanizing, casting, smelting, scrap recycling, etc.

In many hydrometallurgical processes, sulphuric acid has been selected as the cheapest and the most effective leachant [9], [10]. Ivascanu and Roman [11] studied the extraction of nickel from such kind of spent catalysts through its leaching with sulphuric acid. Almost all the nickel (99% Ni) was recovered as nickel sulphate by adjustment of a set of suitable operational conditions (sulphuric acid concentration of 80%, reaction time of 50 min and reaction temperature of 70 °C). Al-Mansi and Abdel Monem [10] investigated the possibility of extracting nickel (99%) from Egyptian spent catalysts and optimal experimental parameters were defined as follows: a 50% sulphuric acid concentration, a solid-to-liquid ratio of 1:12 and a contact time over 5 h. Abdel-Aal and Rashad [12] focused their work on the kinetics of the leaching process of spent nickel oxide catalysts with sulphuric acid and they obtained extraction levels of about 94% Ni by using a −200 + 270 mesh spent particle size, a reaction temperature of 85 °C for 150 min and a sulphuric acid concentration of 50%.

On the other hand, ammonia leaching has been applied to metal extraction for a long time [13] and the application of this technology is gradually expanding. In fact, ammonia is widely used as a leachant for its mild alkaline characteristics, ease of use, and potential ability to be recycled [14]. Tsai and Tsai [15] have proposed a flowsheet in which a fly ash residue was firstly leached with an ammoniacal solution containing ammonium sulphate to recover nickel, and then the remaining solid fraction was leached in sodium hydroxide solution to recover vanadium.

The work described in this paper is an attempt to optimise a leaching process to recover nickel, copper and zinc from an industrial galvanic sludge under room temperature/atmospheric pressure and without gas injection. These sludges generally contain a unique mixture of metallic species not normally found in natural ores and concentrates. Furthermore, recycling processes must be as simple and inexpensive as possible. With this idea in mind, a hydrometallurgical process was selected since it can be developed at lower operational costs than those required for the treatment of mineral deposits. This particular paper focuses on the leaching operation itself as a part of an integrated process involving neutralization, precipitation methods, solvent extraction (SX), crystallization, etc, aiming to find an efficient, eco-friendly and economical hydrometallurgical route to recover valuable/hazardous metals and reduce the hazardous character of the waste stream before final disposal.

Section snippets

Physico-chemical characteristics of the sludge

The waste used in this study is a sludge produced by the physico-chemical treatment of wastewaters generated by a Ni/Cr plating plant located at Braga, in the north of Portugal. This sludge is mostly composed of metal hydroxide gels, sulphate salts and occasionally chlorides, thus showing a relatively complex structure and a reasonable high chemical variability, depending on the producing unit, time, product specifications, processing/operation variations and stocking age [16]. The main metal

Physico-chemical characteristics

As can be seen in Table 1, the sludge contains high contents of valuable metals such as Cu, Ni and Zn, though it presents a much higher concentration (60 wt.%) of silica, an inert material. At the same time, several other metal species are present, like Cr, Fe and Pb; these latter metals can be considered contaminants as they affect the leaching process through their co-dissolution together with the valuable metals, thus reducing the purity of the resulting liquor leach. It can also be noted

Conclusions

The leaching efficiency of sulphuric acid towards several metallic elements such as Ni, Zn and even Cu, contained in a galvanic sludge, is much higher than that observed by using ammonia/ammonium carbonate. In fact, the metals dissolution by H2SO4 is almost complete within 1 h, while leached levels reached upon by ammoniacal solutions are only half (for Ni and Zn) or two thirds (for Cu) of those registered in acidic media, even after 24 h. Then, amongst all the leaching experiments carried out

References (23)

  • D.G.E. Kerfoot et al.

    The hydro and electrometallurgy of nickel and cobalt

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