Electrometallurgy 2012 Edited by: Michael Free, Michael Moats, Georges Houlachi, Edouard Asselin, Antoine Allanore, Jim Yurko, andShijie Wang

TMS (The Minerals, Metals & Materials Society), 2012

DESIGN AND COMMISSIONING OF A LABORATORY SCALE ELECTROCOAGULATION REACTOR

Eduard Guerra1, Padmavathy Mahadevan2-, Samir Chefai3

'Laurentian University, School of Engineering; 935 Ramsey Lake Road, Sudbury, Ontario, P3E 2C6, Canada

2Barrick Gold Corporation, Barrick Technology Centre, 323 Alexander Street, Vancouver, British Columbia, V6A 1C4, Canada

3Barrick Gold Corporation, Operations Support, 3700-161 Bay Street, Toronto, Ontario, M5J 2S1, Canada

Keywords: Electrocoagulation, Laboratory Reactor, Arsenic Removal, Current Distribution

Abstract

Electrocoagulation is a technique for treating wastewater streams from metallurgical plants whereby electrodes are corroded to generate gases (most notably oxygen at the anode and hydrogen at the cathode) and solid particulates which acts as substrates for coprecipitation of impurities. The performance of electrocoagulation reactors, in terms of their efficiency for removing a particular impurity, is highly dependent on several factors, most notably: the nature of the electrodes, the spacing between electrodes, the current density, the height of the electrodes, the solution flow rate, and application of periodic current reversal. Because these variables have complex interactions, in order to ensure reliable scale-up of laboratory results, experiments should be conducted under conditions that closely match those that would be employed at an industrial scale. To that end, this article describes the design and commissioning of a versatile laboratory scale electrocoagulation reactor that allows for the control of all the aforementioned variables along with the ability to measure the instantaneous spatial distribution of current within the reactor.

Introduction

Arsenic is often removed from metallurgical waste streams in stirred tank reactors by coprecipitating it with ferric ions as an arsenical ferrihydrite [1]. Electrocoagulation is an electrolytic technology that can accomplish the same task, but has not yet been widely adopted to treat metallurgical waste streams [2]. The electrocoagulation process involves the use of an external DC power supply to drive the dissolution of a relatively non-toxic metal into the wastewater stream, usually either iron or aluminum, which then largely precipitates as oxyhydroxide phases that adsorb undesired impurities [1]. At the anode, the metal is dissolved and oxygen is often evolved, lowering the local pH and enhancing metal ion solubility.

M ^ M n + + ne" (1) 2 H 2 0 - > 4 H + + 0 2 + 4e" (2)

The dominant reaction at the cathode is hydrogen evolution, which consumes acid and creates locally high pH values.

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2H+ + 2e" -> H2 + 4e' (3)

Metal ions are convectively transported, largely by the action of the rising gas bubbles, from regions of low pH, near the anode, to those of higher pH, near the cathode, where they rapidly precipitate and coagulate into large oxyhydroxide particles that adsorb impurity ions.

Mn+ + (n+x)H20 - MO(OH)(n.,)(H20)(1+x) + nH+ (4)

For example, the adsorption of ASO43" by ferrihydrite may be written as follows [1]:

FeO(OH) (H20)(i+x) + As043" -► FeO(OH) (H20)<i+X).As04

3" (5)

Industrial electrocoagulation is often conducted in parallel plate reactors that operate at a fixed spacing and current density (electrical current per unit electrode area), having similar basic designs to one patented a century ago [3]. However, the performance of a specific electrocoagulation reactor in terms of its efficiency for removing a particular impurity and its energy consumption is highly dependent on several factors, most notably: the nature of the electrodes, the spacing between electrodes, the current density, the height of the electrodes, the solution flow rate (superficial solution velocity), and cycles of periodic current reversal [2]. Because these variables have complex interactions, the prediction of their effects is not generally amenable to computer modeling. For example, the electrical current passing through the cell generates bubbles which, as mentioned, enhance convective mixing within the cell. However, these same bubbles act as insulating spheres and the population of bubbles increases with height since they are generated along the entire surface of the electrodes and rise from bottom to top. This has the effect of creating a non-uniform spatial distribution of electrolyte conductivity, which in turn creates a tendency for a non-uniform spatial distribution of current within the cell because the current follows the path of least resistance. An uneven current distribution results in uneven local corrosion of the electrodes and uneven gas bubble generation rates, altering the pH gradients within the cell. These affect the precipitation of the metal ions, which affects the efficiency of impurity removal, and so on. As such, a laboratory scale electrocoagulation reactor was designed and built to control the aforementioned parameters along with the ability to measure the instantaneous spatial distribution of current within the reactor, in order to facilitate the design of full-scale industrial electrocoagulation reactors and processes.

Materials and Methodology

In designing the electrocoagulation reactor it was important to create a versatile reactor that could be rapidly and reliably assembled and disassembled in various configurations without being overly complex. The reactor created is essentially a channel reactor with electrodes bolted to exterior plates that are themselves bolted to flanges on either side of the reactor (see Figure 1). The reactor was constructed of PVC, which is the most common material of construction for electrocoagulation reactors [2] because of its relatively low price, common availability, excellent chemical resistance, ease of joining, good stiffness, and ease of machining. In order to ensure watertight and corrosion resistant sealing of the reactor, gaskets were created from a sheet of 1/16" expanded PTFE. The electrodes themselves were made by cutting 3"x3" squares from Ά"

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plate of mild steel and then threading steel bolt ends into their backs. Electrodes of other metals, such as aluminum, can be easily created in the same manner. Each electrode segment can also be replaced by a PVC blank in order to adjust the overall height of the electrode and spacer plates and gaskets can be inserted behind the electrodes in order to adjust the interelectrode gap from 1/8" to 1" in 1/16" increments.

The cell is powered by a Sorensen digitally controlled power supply, Model DCS 20-50E, which can deliver 50 ± 0.2 A at 20 ± 0.02 V. This power supply has also been equipped with Ethernet control and polarity reversal relays, M135 option, for the ability to control it remotely and apply periodic current reversal, respectively. The cell contains a single pair of electrodes so as not to require excessive amounts of wastewater to be treated during the experimental runs. A Pulsatron Series D, Model LFC4, diaphragm pump with PTFE and PVC internals is used to feed the cell. This pump can deliver flow rates between 48 gal/day to 0.48 gal/day, which should allow for relatively precise control of superficial solution velocity, which is necessary to reliably emulate the performance of stacks of parallel plate electrodes. The symmetrical design also allows for the reactor to be fed from the top or the bottom.

Electrode Plate (Iron) with threaded bolt ends

Hex Nut and Washer Expanded PTFE Gaskets

PVC Reactor (with clear sides and pipe threaded inlet and outlet)

Hex Bolt and Washer \

Hex Mut and Washer

PVC Exterior Plate

Figure 1. Schematic diagram of the laboratory electrocoagulation reactor.

A custom breakout box was constructed (schematically illustrated in Figure 2) for data aquisition. Each anode segment in the cell is connected to it through an integrated Hall Effect based current sensor, ACS712 by Allegro MicroSystems Inc., which produces an output voltage between 0 and 5 V that is linearly proportional to the electrical current passing through it. The Hall sensors were purchased in ready-to-solder breakout boards from Sparkfun.com, as were the

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100 nF capacitors (added for noise suppression) that are illustrated in Figure 2. The ASC712 sensor is rated to measure up to 5.0 A, corresponding to a maximum current density of approximately 860 Am"2 on each electrode segment. The voltage data are acquired using a USB-6211 DAQ card from National Instruments Inc. and collected to a laptop computer using LabView Signal Express 2010 software. Two of the 16 analog inputs channels on the DAQ card are employed in a differential channel configuration to measure the cell voltage, while five others channels, configured as ground referenced single ended, are dedicated to measure the output voltages from the Hall effect sensors. Since the USB-6211 card can supply 100 mA of current through its +5 V channel and each of the sensor chips only draws up to 20 mA, the +5 V channel from the card is employed to supply power to the five Hall effect chips and no separate +5 V power supply is necessary.

Figure 2. Schematic of the breakout box for the electrocoagulation reactor.

Results and Discussion

A sample set of data collected from the current sensors connected to an array of 4 mild steel anodes and 1 PVC blank is plotted in Figure 3. In this experiment, 'Anode Γ is the PVC blank located in the uppermost position of the electrode array, so the voltage output from the corresponding sensor outputs a relatively constant voltage throughout the experiment thus indicating zero current. Anodes 2 through 5 are arranged in descending order from top to bottom in the electrode array. The output values from these current sensors indicate changes in the current passing to the cell: 5.0 A at ca. 20 seconds, 10.0 A at ca. 70 seconds, and 15.0 A at ca.

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130 seconds. The actual current flowing through each of the anode segments may be calculated by subtracting the instantaneous output voltage from the quiescent output voltage (output voltage when no current was flowing) and then dividing this value by the experimentally determined linear proportionality constant of the sensors, 0.182 VA'1. In this case, the value of 0.182 VA"1 is close to the typical value of 0.185 VA"1 quoted in the specification sheet for the ACS712 sensor and is well within the range of typical acceptable values.

One undesired feature of the data in Figure 3 are the spikes in the voltage signals. These are most likely related to electrical noise from the laptop affecting the grounding plane, since the ground of the card is ultimately derived from the laptop and it is not itself connected to an external electrical ground. In addition, the DAQ card being used is at the limit of its capability to deliver current to the sensor array, making its performance more susceptible to electrical noise from the computer. These problems might be obviated by employing a desktop computer and connecting the output voltage from one of its spare USB ports to the +5 V channel of the DAQ card. This should not only improve the stability of the grounding plane, but also increase maximum current output to 0.6 A or 1.0 A, depending on whether the port is USB 2.0 or USB 3.0, respectively. This added power could then be used to control the pump or the power supply using the DAQ card and/or to make other measurements such as pump speed and pH using some of the spare analog input channels.

Figure 3. Sample data collected from an electrocoagulation experiment.

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Conclusions

A versatile laboratory scale electrocoagulation reactor has been created to control key operating parameters as well as to measure the instantaneous current distribution within the cell. The reactor is intended to perform experiments under a wide range of operating conditions to acquire data that will be useful for the design of industrial scale electrocoagulation processes and reactors. Hopefully, the use of this reactor will lead to improvements in the application of electrocoagulation in the treatment of wastewater streams from metallurgical plants.

References

1. P.A. Riveros, J.E. Dutrizac, and P. Spencer, "Arsenic Disposal Practices in the Metallurgical Industry," Canadian Metallurgical Quarterly, 40 (4) (2001), 395-420.

2. M.Y.A. Moliah et al., "Fundamentals, Present and Future Perspectives of Electrocoagulation," Journal of Hazardous Materials, 114(1-3), (2004), 199-210.

3. L.A. Fitzer, "Electric Water Purifier," US Patent 1,080,005 (1913).

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