Contamination of liquid streams with various organic and inorganic pollutants is a serious environmental problem affecting the quality of the global environment and represents a significant threat to human health and safety. Substantial heavy metal contamination of aquatic environments arises from commercial mining and metal extraction processes, surfaces modification and protection processes, or communal and industrial waste sites resulting from a variety of active or defunct industrial fabrication and manufacturing activities. Similarly, significant organic water pollutants, like aliphatic, aromatic, or halogenated hydrocarbons and phenols, are frequently associated with oil, gas, and coal exploration, extraction and refining, chemical industry, or large-scale farming and food processing.
In addition to potential for significant environmental damage, polluted liquid streams represent sources of desirable raw materials like heavy metals and metal oxides. For example, the Berkeley Mine Pit in Butte, Mont. alone represents an estimated 30 billion gallons of acid mine drainage which contains ˜180 ppm of copper along with other metals and thus could yield up to 22,000 tons of pure copper by use of a small treatment facility.
The prevailing method of removal of heavy metal ions from liquid solutions is chemical precipitation. This process is burdened by complexity, high cost, clear preference for extremely large facilities and high-volume operations, and little selectivity towards targeting and separating specific metallic pollutants. One fundamental disadvantage is the resulting byproduct of considerable volumes of heavy sludge which is toxic due to contamination with a mixture of toxic heavy metals. The sludge mandates further processing and costly long term disposal as a highly toxic waste. Many similar disadvantages burden alternative heavy ion removal methods that may incorporate: filtration, ion exchange, foam generation and separation, reverse osmosis, or combinations of listed processes.
Modification of polluted liquid streams can be accomplished efficiently using electrochemical processes. Electrochemical methods of reduction of metal ions or oxidation of organic pollutants do not suffer from described disadvantages of complexity, strong preference for large scale operations, or toxic byproducts. Advanced electrochemical methods like Moving Bed Electrode (MBE) electrolytic technology in various forms is relatively efficient even in the cases of comparatively low contaminant concentrations of about 1000 ppm.
Liquid stream modification using a Spouted Electrode Technology (SET) variant of the moving bed cell is well-known to prior art. U.S. Pat. No. 4,272,333 to Scot et al. discloses a method of moving bed electrolysis where motion of a particulate bed electrode; imposed by the circulation of electrolyte, prevents electrode particulates aggregation and maintains at least intermittent contacts between the moving particulates bed electrode and current feeder. Scot et al. report copper, zinc, cobalt, and manganese ions reduction from electrolytes with varying pH values.
U.S. Pat. No. 5,565,107 to Campen et al. discloses a process of electrochemical purification of “streams which contain organic and/or inorganic impurities”. The disclosed process utilizes a reactor with “a water-containing reaction zone which comprises providing a packed bed of activated carbon, applying an electrochemical potential across said packed bed and simultaneously feeding a reactant selected from the group consisting of ozone and hydrogen to said packed bed.”
Efficient electrowinning of zinc in an electrowinning cell using “moving” or “moving packed bed” SET electrode is disclosed in the U.S. Pat. No. 5,635,051 to Salas-Morales et al. from predominantly acidic electrolytes. Related U.S. Pat. No. 5,958,210 to Siu et al. discloses electrowinning of zinc in an SET electrowinning cell from alkaline electrolytes. Both patents also disclose an industrial scale eight-draft-tubes parallel electrowinning cell structure shown in side elevation in corresponding FIG. 2.
Spiegel et al. in the U.S. Pat. No. 6,298,996 report purification of metal and toxic organic compounds from polluted aqueous waste streams using “an advanced electrolytic cell technology employing a dynamic spouted electrode” SET device. Spiegel et al. also disclose a four-independent-cells device in which electrolyte is pumped in a parallel manner into the cells and returned to the reservoir.
A simplified schematic side view cross-section of a SET cell of prior art is given in FIG. 1. A typical SET cell consists of one or more anodes 10 coupled to one or more high surface area cathodes 20 in the form of spouted particulates bed, separated by a distance. Catholyte flow 30, driven by an external catholyte pumping station 40, is directed through the high surface cathode 20 to achieve vigorous particulates bed convection needed for high degree of electrode utilization. Unidirectional current is fed into the cell via anode current feed 50(+) and out via cathode current feeder device 90 and cathode current feed 50(−). The cell illustrated in the FIG. 1. is a simple planar configuration comprising cathode cell chamber 60 and anode cell chamber 70 separated by a separator (porous membrane) 80 which directs bulk electrolyte flow 30 while maintaining intimate electrochemical contact by ion transport (selective and/or non-selective) between the separated cathode 20 and anode 10. Depending upon the state of control valve system 85 the cell can operate in a batch mode processing the fluid contained in the reservoir 97, or in a flow-through mode modifying liquid streams delivered by external pipelines 95. A mode of operation created by any combination of the flow-trough and batch modes can be achieved if desired in accordance with application specific requirements and circumstances.
An important common feature of the prior art SET devices and processes is the fact that actuation of the electrode bed is achieved by vigorous circulation of electrolytes generally achieved by strong pumping action of various external pumping systems. This feature, clearly motivated by simplicity of the mechanical design, is inherently suboptimal because the achievement of optimal liquid stream flow rate is sacrificed to the requirement for vigorous spouted bed mixing and circulation. The systems of the prior art frequently comprise relatively powerful pumping stations, like one denoted by the reference numeral 10 (corresponding reference numeral 31 in the U.S. Pat. Nos. 5,635,051 and 5,958,210).
In addition to higher capital cost and higher energy consumption, pumping stations scaled up to sufficiently mix and circulate particulates beds in SET cells additionally burden the overall efficiency of the liquid stream modification processes by limiting the reactor residence time of treated liquid stream volumes. The devices and methods in accordance with the present invention are designed to essentially decouple the fluid flow and motions of the SET. This novel characteristic of the devices designed in accordance with the present invention allows for independent optimization of the SET motions necessary for improved overall efficiency of the liquid stream modification processes and efficient electrolysis, prevention of particulates aggregation, and dendrite formation, and treated fluid circulation optimized for sufficient reactor resident time necessary for efficient liquid stream treatment even in cases of streams comprising sub 100 ppm concentrations of treatment target materials.