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Fluid-Bed Electrolysis

The fluid bed cell is also manufactured in the form of ionization chamber, with large surface area, to be used to prevent the growth of bacteria and algae in metalworking fluids. Specifics ions are generated to attack and kill bacteria and algae. In addition, the charged particles generally have a mutual attraction for each other, causing an increase in size which moves dead algae and bacteria together and allowing the final filtration unit to remove them from the waste fluids. 

In addition, the purification of wastewaters with low concentration of metal ions performed by means of normal fixed cathode geometry cells gives too poor results. Typical “Fluid bed” electrolytic cell is the best answer to these problems and offers itself as the crucial system for the treatment of wastes containing low concentrations of metals. Several industrial processes discharge aqueous wastes containing heavy metal ions.

Consider for example galvanic industries, metal refineries and many organic chemistry processes that use heavy metals as catalysts. These wastes often contain metal ions at very low concentration, from 2 g/L to some p.p.m.
Removing these ions by using traditional methods such as flocculation, ion exchange, incineration or cementation is a very expensive issue.

FLUID-BED ELECTROLYSIS
Fluid-bed electrolysis is a simple removing method, which allows handling wastes with low metal concentration in a simple, economic and efficient way. As opposed to the conventional electrolytic deposition, fluid-bed electrolysis works with a moving cathode – the so called fluid bed – which is made up of a great number of round and oval metal particles, which are kept floating by the flux of the waste to be purified.

The electric contact between the suspended and continuously moving particles is ensured by a cathode with different geometries acting as current distributor. The particles suspended in the cathode compartment and electrically charged absorb the metal/s contained in the waste to be treated, by electrolytic deposition. As a result, their weight increases, leading to their separation by precipitation. A diaphragm separates the cathode compartment from the anode one, which contains a fixed conventional anode immersed in a suitable acid diluted solution.

As a result of the potential difference applied to the electrodes, the current passage causes the metal to deposit on the mobile cathode, thus increasing the metal particles and discharging a relevant anion in the anolyte solution.

The opportunity to create very large cathode surfaces (summing the surface area of each particle) by using fluid bed offers many advantages such as the opportunity to work at low cathode currents, thus obtaining higher outputs.
Moreover, fluid-bed cells allow operation with much smaller cells and higher current intensities.

By working with standard cells – i.e. having a well defined and stable cathode geometry – or with fixed or shape-limited surfaces, it is necessary to operate with solutions having metal concentrations of about 5-10 g/l or higher.  Should the values be lower, hydrogen discharges would inevitably occur. It would indeed be necessary to operate at a very low current density and with consequently large surface cathodes and bulky cells, thus incurring in great expenses.

CELL OPERATING SPECIFICATIONS
The characteristics of a conventional electrolytic cell with flat, fixed electrodes and of a cell with mobile electrodes – as the fluid bed ones – are listed below.

   Conventional cell  Fluid-bed cell
 Cathod area
 Cell M2/m3
16 3500 
 Current density
 Cell KA/m3
2.5 - 10 15 - 50
 Power transmission line KA/m2 0.15 - 1.6 0.004 - 0.015
 Metal conc. in the waste to be
 treated g/L
30 - 150 0.001 - 2

The large active cathode area allows very low specific cathode densities (A/m2), even with high current intensity. Current intensity is defined here as the amperage ratio per cell volume unit.

The specific cathode area is about 200 times larger than that of a conventional flat electrode cell, therefore current densities are 40 times lower. It is however possible to use current densities that are 5 times higher.

This enables to treat solutions with very low densities of metal ions with sufficiently high current outputs. (Please note that current output means the relationship between the theoretical value of current necessary to process deposition and the value of the current actually absorbed by the cell and outputted by the rectifier.

In conventional electrolysis processes with solutions having a concentration higher than 50 g/L, the current output value is cut to 7-8%. Thanks to the fluid-bed process, it is possible to reach output values over 80%.

SOME CONVENTIONAL APPLICATIONS OF FLUID-BED CELLS
1. Purification of water solutions containing copper, cadmium, cobalt, nickel, zinc, chrome ions such as galvanic bath rinsing waters. For example, a solution containing 100 p.p.m. of copper can be purified up to less than 1 p.p.m. of copper in a single step and with a current output of about 70%.
2. As final treatment in the purification of tannery wastes to recover chrome after acid-washing the incineration ovens.
3. Hydrometallurgy, metal (copper, cobalt, nickel etc.) recovery from solutions with low mineral acid leaching.
4. Noble metal processing to recover metals from their salts solutions.

In the treatment of industrial wastes with low metal ion concentration, the results have been very encouraging both from the technical and the economic point of view. By fully automating the plant, the operating cost are drastically cut.

According to the required treatment skills, the cells can work continuously or in batches until metals are completely removed. For this reason they are also suitable for small companies that can gather all wastes to be treated in vats or tanks.

It is then important to remember that separating the metal particles from the catholyte solution does not cause any problem. The fluid-bed electrolytic cell process allows reaching the values required by the laws in force for wastes containing heavy metal ions directly at plant inlet. The fluid-bed cell is the perfect answer to the environmental problems linked to wastes containing metals at low concentration, and offers new, highly profitable commercial opportunities.