3.2.2 E-Waste Loading and Particle Size
An e-waste loading up to the optimum has a positive effect on the bioleaching
efficiency, whereas higher loading will limit the contact between the phases. Further,
the available cell concentration will not be sufficient for the bioleaching of the entire
loading [25]. An increased loading of e-waste for bioleaching would also result in
the increase in pH, toxic metals, and hazardous compounds (above MIC), causing a
decrease in the bioleaching rate [27, 57]. To overcome this inhibitory effect on
specific growth rate of the organism and the production of organic acids, the
bioleaching process can be accomplished by a two-step or spent medium bioleaching
process [34]. E-waste loading also affects specific metal dissolution like the
increased loading increases copper leaching by non-cyanide leaching agents,
whereas gold removal is minimal, attributed to the inhibition of cyanide production
[45]. The e-waste loading and particle size to be chosen also depend on the type of
bioleaching system operated.
Particle size of the solid is another crucial factor in the hydrodynamics of
bioleaching, phase contact for efficient mass transfer, contact surface area, and the
collision between the solids and the cells. Coarse particles have lesser surface area
for contact, and the fine particles cause collision of particles and attrition on the
microbial cells which are detrimental, thereby decreasing the bioleaching efficiency.
The metal leaching rate increases with the decrease in the size of e-waste [8, 58,
59]. A study by Li et al. [60] reveals that the set range of particle size was least
significant for bioleaching and was not considered for the pretreatment process. The
order of significance of the biotic and abiotic factors in their study reports to be
pH > e-waste loading > inoculums size > temperature > particle size. However, the
significance of parameters may vary with different bioreactor systems.
3.3 Bioreactor Design and Its Operational Parameters
Development of a large-scale bioleaching process necessitates the use of contained
systems such as bioreactors. The use of bioreactors like stirred tank reactors, column
reactors, etc. improves the efficiency of the bioleaching process. Adequately
designed bioreactors allow good mixing conditions and enhance the mass transfer
rate which is essential for any multiphase contacting processes such as bioleaching.
Bioreactors can be easily operated under controlled environment, and optimized
conditions can be maintained to maximize the recovery of metals. The material to be
used to construct the reactor should be carefully chosen, as the walls or the internal
components of the reactor might be subjected to biocorrosion by the microorganism
used for bioleaching. Bioreactor design, physical components, material used, its
capacity, and operational parameters affect the efficiency of the bioleaching.
Bioleaching rate vary in different reactors. Some of the commonly used bioreactors
include column reactors like mechanically agitated stirred tank reactors, rotating
drum bioreactor, packed bed reactors, pulsed plate bioreactor, bubble column
34
M. Minimol et al.
An e-waste loading up to the optimum has a positive effect on the bioleaching
efficiency, whereas higher loading will limit the contact between the phases. Further,
the available cell concentration will not be sufficient for the bioleaching of the entire
loading [25]. An increased loading of e-waste for bioleaching would also result in
the increase in pH, toxic metals, and hazardous compounds (above MIC), causing a
decrease in the bioleaching rate [27, 57]. To overcome this inhibitory effect on
specific growth rate of the organism and the production of organic acids, the
bioleaching process can be accomplished by a two-step or spent medium bioleaching
process [34]. E-waste loading also affects specific metal dissolution like the
increased loading increases copper leaching by non-cyanide leaching agents,
whereas gold removal is minimal, attributed to the inhibition of cyanide production
[45]. The e-waste loading and particle size to be chosen also depend on the type of
bioleaching system operated.
Particle size of the solid is another crucial factor in the hydrodynamics of
bioleaching, phase contact for efficient mass transfer, contact surface area, and the
collision between the solids and the cells. Coarse particles have lesser surface area
for contact, and the fine particles cause collision of particles and attrition on the
microbial cells which are detrimental, thereby decreasing the bioleaching efficiency.
The metal leaching rate increases with the decrease in the size of e-waste [8, 58,
59]. A study by Li et al. [60] reveals that the set range of particle size was least
significant for bioleaching and was not considered for the pretreatment process. The
order of significance of the biotic and abiotic factors in their study reports to be
pH > e-waste loading > inoculums size > temperature > particle size. However, the
significance of parameters may vary with different bioreactor systems.
3.3 Bioreactor Design and Its Operational Parameters
Development of a large-scale bioleaching process necessitates the use of contained
systems such as bioreactors. The use of bioreactors like stirred tank reactors, column
reactors, etc. improves the efficiency of the bioleaching process. Adequately
designed bioreactors allow good mixing conditions and enhance the mass transfer
rate which is essential for any multiphase contacting processes such as bioleaching.
Bioreactors can be easily operated under controlled environment, and optimized
conditions can be maintained to maximize the recovery of metals. The material to be
used to construct the reactor should be carefully chosen, as the walls or the internal
components of the reactor might be subjected to biocorrosion by the microorganism
used for bioleaching. Bioreactor design, physical components, material used, its
capacity, and operational parameters affect the efficiency of the bioleaching.
Bioleaching rate vary in different reactors. Some of the commonly used bioreactors
include column reactors like mechanically agitated stirred tank reactors, rotating
drum bioreactor, packed bed reactors, pulsed plate bioreactor, bubble column
34
M. Minimol et al.