Modelling of Metal Recovery using Sulfate Reducing Bacteria, Simulation in MATLAB and Aspen Plus

Abstract

This report presents a mathematical kinetic model describing the recovery of metal ions as metal sulfides, using sulfate reducing bacteria (SRB). Metal recovery from industrial waste is a potential beneficial and unused source of metals. The developed kinetic model is implemented and simulated in the computer programs MATLAB R2014b and Aspen Plus V8.8. The MATLAB simulation is performed on the bioreactor, where sulfate reducing bacteria produce hydrogen sulfide. The hydrogen sulfide can be used to precipitate metal ions as metal sulfides in which the metals can be removed and recovered from the industrial waste. The full metal recovery model with precipitation tanks and the bioreactor is simulated in the process simulation program Aspen Plus. A literature study was conducted before the simulation to develop the kinetic model.
The main results from this project are a kinetic growth model for sulfate reducing bacteria implemented in the two computer programs as well as a precipitation model implemented in Aspen Plus. The kinetic model includes inhibition terms for pH and hydrogen sulfide. The models are working and calibrated according to literature data. However, in the future it will be necessary to calibrate the models according to real experimental values for the given process. It can be concluded that simulating processes in computer programs makes it possible to predict how feasible the process is in reality. It can also suggest model designs and critical steps that can be avoided in the future and other potential improvements of the process. This can save a lot of time and resources spent on the process before building it.
The environmental aspects need to be considered since the process produces a lot of toxic hydrogen sulfide. It is therefore important to have an accurate and well established safety routine for this process.

Popular Abstract

Some industrial waste contains metals and other hazardous materials, which are left in the waste when deposing it. Metal recovery from industrial waste is therefore an improvement that could be done in today’s society to lower the environmental impact. The metals in the waste is also a valuable resource and it could be beneficial to recover the metals as well as removing them. Precipitation of metal ions as metal sulfides using a biologically produced hydrogen sulfide as precipitation agent is one possible metal recovery process. To implementing a new process, describing it by a mathematical model and simulating this in computer programs is a way of evaluating the feasibility of the process before it is built and it can also be used to decrease the number of experiments needed for development.

The mathematical model that was developed in this project describes a metal recovery process starting with waste that had been treated with sulfuric acid to get metal ions accessible. The metal ions then enter tanks where the precipitation occurs with hydrogen sulfide, creating insoluble metal sulfides that can be recovered as products. The hydrogen sulfide is produced in a bioreactor where sulfate reducing bacteria perform redox reactions to convert sulfate to hydrogen sulfide. The bacteria use ethanol as a substrate converting it to acetate which subsequently can be used as a substrate.
The model is built on mass balances for all substances present in the different tanks where the change in concentration over time is defined with differential equations. The mass balances are dependent on what enters, exits, gets produced and consumed in the reactor. The rate of production for the biological process uses Monod kinetic to describe the substrate uptake rate. The uptake rate equations include multiple substrate uptake, pH and product inhibition from H2S. There are also rates describing mass transfer between liquid and gas and acid-base dissociation for H2S and CO2.
The model for the precipitation tank is built in the same way but here the rates are solely assumed to be dependent on the mass transfer rate. The metals included in the model are copper and zinc. These metal precipitate at different pH, making selective precipitation possible. In this case pH 1.0 and 4.5 are used respectively for the metal ions.

The model was implemented in MATLAB and Aspen Plus, these two simulating programs are very different. In MATLAB everything is coded manually while Aspen Plus has built in mathematical relationships based on the chemical properties of the compounds. The model was successfully implemented into the two simulation programs and the outputs where compared with data from experiments obtained from literature. To improve the model further experimental values are required. Suggestions for improvement of the simulation tools are to add cost and energy calculations as well as waste management.

• author: Kvarnström, Hanna and Lönntoft, Erika
• supervisor: Ed van Niel
• organization: Applied Microbiology; Biotechnology
• course: KMB820 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Sulfate reducing bacteria, Metal sulfides, Metal Recovery, Metal precipitation, Modelling, Simulation, Aspen Plus, Matlab
• language: English
• id: 8912774
• date added to LUP: 2017-06-12 12:01:31
• date last changed: 2017-06-12 12:01:31