Bio-processing of copper from combined smelter dust and flotation concentrate: A comparative study on the stirred tank and airlift reactors

Abstract

To scrutinize the influence of the design and type of the bioreactors on the bioleaching efficiency, the bioleaching were evaluated in a batch airlift and a batch stirred tank bioreactors with mixed mesophilic and mixed moderately thermophilic bacteria. According to the results, maximum copper recoveries were achieved using the cultures in the stirred tank bioreactors. It is worth noting that the main phase of the flotation concentrate was chalcopyrite (as a primary sulphide), but the smelter dust mainly contained secondary copper sulphides such as Cu₂S, CuS, and Cu₅FeS₄.Under optimum conditions, copper dissolution from the combined flotation concentrate and smelter dust (as an environmental hazard) reached 94.50% in the STR, and 88.02% in the airlift reactor with moderately thermophilic, after 23 days. Also, copper extractions calculated for the bioleaching using mesophilic bacteria were 48.73% and 37.19% in the STR (stirred tank reactor) and the airlift bioreactor, respectively. In addition, the SEM/EDS, XRD, chemical, and mineralogical analyses and studies confirmed the above results.

Introduction

The production of solid waste streams such as dust, slag, etc. is usual in metallurgy and recycling industries. As such, owing to the harmful effects of the wastes to the environment if released, environmental-friendly methods are extremely of high importance [1].

The environmental lead pollution particularly around mining and smelting operations has become a global problem, and quite a few countries seriously have investigated and presented methods to control the pollution [2]. Arsenic compounds (As (V), arsenate; As (III), arsenite; As (0), and As (−III), arsine) discharged into atmosphere during copper smelting operation is classified as a very hazardous materials in the US Environmental Protection Agency (EPA) [3].

Two reverberatory and converter furnaces in the Sarcheshmeh copper complex (south-east of Iran) produce nearly 50 tpd copper dust with about 30% copper content. According to our investigations at the complex, the hazardous dust contains about 8.63% As, and 3.14% Pb. At present, recycling the dust to the furnaces results in the reduction of their efficiencies, increases the required energy for their smelting, damages the refractory bricks, and imposes a circular load on the furnaces. Also, in this copper complex about 1500 tpd chalcopyrite concentrate is produced with about 27% copper content [4].

Therefore, the primary aim of the study was to find a safer and cleaner technology to treat the hazardous materials. In spite of the reports in this regard [4], [5], two questions are central to the debate here: firstly, which is the best option for the bioleaching operation, stirred tank or airlift reactors?

The advantages of using tank bioreactors, the higher costs of which restrict their use to the bio-oxidation of high-grade ores and concentrates [6] are: more homogenous reacting mass, better control of the main process variables such as pH and temperature, resulting in the high rate bioleaching of minerals, a better performance, and much shorter turnover times (the times required for mineral processing to be effectively completed). As a result, parameters such as volumetric productivity and degree of extraction can be significantly increased [7], [8], [9], [10]. Stirred tank reactors (STRs) are also favorable mass transfer devices providing a large gas–liquid interfacial area and high shear stress to enhance the mass transfer [11].

On the other hand, in the STRs, high pulp densities negatively affect both extraction rate and final recovery of metals. Some downsides of the STRs are as follows: oxygen and carbon dioxide availability, low bacteria–solids ratio, metabolic stress by high shear stress and abrasive conditions, inhibition of bacterial attachment, and the build-up of toxic leach products or other detrimental substances (such as some flotation reagents) [12]. However drawbacks the reactors have, they can be greatly compensated by adaptation to high pulp densities and the STR conditions.

Because of simple design and construction, low power input, low shear, good mixing characteristics and high heat and mass transfer, many chemical, petrochemical, mineral processing and biotechnological industries are using the airlift reactors [13], [14], [15]. In these reactors, owing to a pressure gradient due to gas retaining in the riser and the down-comer, the fluid flows from the bottom of the down-comer towards the riser, creating liquid velocity and mixing. In the airlift bioreactors, there is a complex connection between solids loading and mechanical stress for three-phase (solid–gas–aqueous) systems. Low solids loadings of about 2% seem to reduce particle stress whereas high solids loadings of up to 10% cause higher mechanical stress compared to two-phase mode [14].

Practically speaking, selection and design of a bioleaching reactor is driven by the physical, chemical and biological properties of the leaching system. Design is further complicated by the multi-phase nature of the pulp, consisting of an aqueous solution with cells adhering to the mineral in suspension, and also a suspended solid and air bubbles [7], [16].

Two important parameters that significantly affect a bioleaching process are: type of culture used in the process and mineralogical properties of copper sulphides [4]. Copper extraction from combined flotation concentrate and smelter dust has been studied [5]. The results of the study revealed that it is possible to efficiently extract copper from the combined using STRs at 50 °C inoculated by moderately thermophilic bacteria [5].

Most secondary copper sulphides, such as chalcocite, digenite, bornite and covellite, can be bioleached successfully at 35–45 °C, using mesophilic bacterial cultures [17]. However, the bioleaching of chalcopyrite is still a major challenge due to relatively slow kinetics and poor extractions [4], [5], [12], which have mainly been attributed to passivity [18], [19], [20], [21], [22].

Thus, the second question is: can mesophilic microorganisms enhance the rate of dissolution of the combined as moderately thermophilic bacteria do?

All in all, the present study was conducted to comparatively evaluate bioleaching process for copper recovery from mixed samples containing 50% metallurgical dusts and 50% flotation concentrate using mixed mesophilic and mixed moderately thermophilic bacteria in both stirred and airlift tank reactors.