Investigating the effect of acid stress on selected mesophilic micro-organisms implicated in bioleaching

During start-up of heap bioleaching, low grade ores are typically treated with acid for agglomeration and to combat the acid neutralising capacity of the gangue minerals. This may stress the bioleaching inocula, particularly upon inoculation during ore agglomeration. Acid addition for agglomeration varies across operations, ore types and their neutralising capacity, with limited information published on recommended concentrations. The initial pH in the agglomeration mix is typically below pH 1.0 and may be as low as pH 0.5. This paper investigates the effect of acid stress in terms of initial acid concentration and exposure duration in submerged culture on mesophilic bacteria typically implicated in mineral sulphide bioleaching and critical for heap colonisation at start-up. Following acid stress, cultures were returned to standard operating conditions in batch stirred slurry reactors and their performance assessed in terms of mineral leach rates, ferrous oxidation and the rate of microbial growth. Increasing acid stress resulted in an increase in the lag period before onset of microbial growth and iron oxidation. Following adaptation, typical growth and ferrous iron oxidation rates were observed under low stress conditions while reduction in the rate and extent of microbial growth and ferrous iron oxidation persisted at extreme conditions. A reduction in yield (microbial cells produced per kg iron oxidised) was observed with increased acid concentration over comparative times. Microbial speciation analysis indicated a substantial decrease in the diversity of surviving bacterial species

The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture: the effect of pH

The kinetics of ferrous ion oxidation by Leptospirillum ferriphilum were studied in continuous culture with a focus on the effect of solution pH (pH 0.8–2.0), assuming that the effect of pH on cell metabolism can be independently studied of reactor context and other reactions common in bioleach heaps. A simplified competitive ferric ion inhibition model and the Pirt Equation were used to analyze the experimental data. The results showed that the maximum specific activity of L. ferriphilum has a symmetrical bell-shaped curve relationship with pH. The maximum specific ferrous-iron oxidation rate,qFe2 +maxgave a highest value of 14.54 mmol Fe2+(mmol C h)− 1 at pH 1.3, and was described by a quadratic function. The steady state carbon biomass in the reactor and the apparent affinity constant, K′Fe2 +, also increased with increase in pH; however, a slight increase in the carbon biomass was observed beyond pH 1.6. The results also showed that ferric ion precipitation is significant beyond pH 1.3 and about 13% total iron from the feed was lost at pH 2.0. The maximum biomass yield increased linearly with pH, while the culture maintenance coefficient was significantly small in all experiments and was minimum at pH 1.3. The values are indicative of actively growing chemostat cultures. This study shows that microbial ferrous ion oxidation by L. ferriphilum may be sustained at pH lower than pH 0.8 as the microbial activity is much higher than reported values for common mesophilic acidophiles. This may have implications on how bioleach heap operations can be started-up to improve metal recovery

The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture: the effect of temperature

A typical bioleach heap is characterized by wide variation of temperature across the heap bed, leading to oxidation of target minerals occurring at different rates. Previous studies on the effect of temperature on the microbial oxidation of ferrous-iron were limited to a narrow range of temperatures (30–40 °C) near optimum conditions and mostly toAcidithiobacillus ferrooxidans. The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum were studied in continuous culture. In this paper we focus on the effect of temperature (18–45 °C) on these kinetics. The study was based on the assumption that the effect of temperature can be studied independently of other, equally important factors such as pH, dissolved salts, etc. and independent of the reactor context. The experimental data were correlated using both, a simplified ferric-iron inhibitory model and the Pirt Equation. The results showed that the maximum specific ferrous-iron oxidation rate, increased with increasing temperature to a maximum at 42 °C. This trend can be described adequately by the Arrhenius Equation with an activation energy, Ea of 34.46 kJ mol−1 and frequency factor,K0 of 1.05 × 107 mmol Fe2+(mmolC)−1 h−1. An increase in temperature slightly reduces the steady state carbon biomass in the reactor, while the apparent affinity constant, K′Fe2+ increases. The investigation further suggests that at low temperature (18 °C) and beyond the maximum temperature (42 °C), the culture cannot be sustained in a continuous mode. The maximum biomass yield followed a linear decline with increasing temperature, while cell maintenance on ferrous-iron followed a quadratic trend, although the small values indicates that it is not significant, as would be expected in continuous culture. The results indicate that L. ferriphilum is likely to perform optimally, at warm temperatures (25–42 °C) in heap bioleach operations before being taken over by thermophiles at higher temperatures

Biooxidation kinetics of Leptospirillum ferriphilum under heap bioleach conditions

Although the kinetics of biological oxidation of ferrous to ferric iron–the key step in any bioleaching process–have been studied for a variety of organisms, the focus has always been on conditions typical of tank-bioleaching. In heaps, parameters such as pH and temperature vary widely across the heap and are usually far from optimal. Total iron concentrations are usually much lower than the tanks(except in gold heaps), and the presence of dissolved gangue mineral is usually much more significant

A comparative study of the hydrolysis of gamma irradiated lignocelluloses

The effect of high-dose irradiation as a pretreatment method on two common lignocellulosic materials; hardwood (Khaya senegalensis) and softwood (Triplochiton scleroxylon) were investigated by assessing the potential of cellulase enzyme derived from Aspergillus flavus Linn isolate NSPR 101 to hydrolyse the materials. The irradiation strongly affected the materials, causing the enzymatic hydrolysis to increase by more than 3 fold. Maximum digestibility occurred in softwood at 40kGy dosage of irradiation, while in hardwood it was at 90kGy dosage. The results also showed that, at the same dosage levels (p < 0.05), hardwood was hydrolysed significantly better compared to the softwood

A review of rate equations proposed for microbial ferrous-iron oxidation with a view to application to heap bioleaching

In view of the fact that the microbial oxidation of ferrous iron to the ferric form is an essential sub-process in the bioleaching of sulphide minerals, the development of a comprehensive rate equation for this sub-process is critical. Such a rate equation is necessary for the design and modelling of both tank and heap bioleach systems. Most of the rate equations presented in the literature define the specific microbial growth rate using a Monod-type form for ferrous substrate limitation, with further terms added to account for ferric product inhibition, ferrous substrate limitation and inhibition. A few of the published rate equations describe the specific substrate utilization rate in terms of a modified Michaelis–Menten equation and include the maximum yield constant and cell maintenance via the Pirt equation. Other rate equations are based on chemiosmotic theory or an analogy with an electrochemical cell. In the present paper a selection of rate equations are compared against each other by calibrating them against the same set of data and comparing the fits. It was found that none fits the data particularly well and that some of the underlying assumptions need to be questioned. In particular, it appears that ferric inhibition is perhaps not as significant a factor than previously assumed and that rate control by the availability of ferrous is more significant. Some rate equations include terms to account for the effects of temperature, pH, biomass concentration, ionic strength as well as inhibition due to arsenic. In general these effects have been studied in isolation and in ranges not too far off the optimum. Few rate equations combine more than 2 effects and there is no clarity on how a comprehensive model to account for all effects should be constructed. Rate equations have been applied to tank bioleach systems, which usually operate under controlled conditions near the optimum. Heap bioleach systems, on the other hand, often operate far from optimum conditions with respect to temperature, pH, solution conditions, etc., at the same time. The kinetics of such sub-optimal systems are still poorly understood. Future studies should be directed towards the development of a comprehensive rate equation useful for describing the kinetics of heap bioleaching over a wide range of conditions

Optimization of wood ash-catalysed biodiesel production from used vegetable oil using response surface methodology

9th World Congress of Chemical Engineerin