Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
1998-10-30
2001-01-16
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C205S745000, C210S717000, C210S724000, C210S748080, C210S906000
Reexamination Certificate
active
06174444
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to the treatment of an effluent and more particularly is concerned with the treatment of a waste solution such as acid mine drainage.
South African patent No. 95/10009 describes a separation process for separating solids from an electrolytic liquid stream wherein iron ions are released into the electrolyte from an iron electrode. The metal ions react with phosphate in the electrolyte and settle out as iron phosphate. Settling is enhanced by the coagulation and flocculation effect of the iron ions.
The aforementioned process is carried out while maintaining the pH of the electrolyte within the boundaries of 4 to 10.
It is not evident, from the specification of the aforementioned patent, in which way the pH of the solution is maintained in the indicated range and there would appear to be no control over the pH of the liquid which is discharged after settling of the iron phosphate takes place. It would also seem that the technique described in the aforementioned patent specification is only effective for the removal of phosphate in the form of iron phosphate.
Acid mine drainage solutions, and similar acidic liquors, generally contain sulphate, often in combination with phosphate. The treatment of this type of effluent poses a problem of many parts in that, ideally, the pH of the effluent should be raised to a substantially neutral level; at least the harmful contaminants such as sulphate and phosphate should be reduced to acceptably low levels; and potentially harmful base metals, which may be dissolved in the effluent, should, preferably, be removed from the effluent as part of a single treatment process.
SUMMARY OF THE INVENTION
The invention provides, in the first instance, a method of treating an effluent which includes the steps of:
(a) introducing the effluent as an electrolyte into an electrolytic reactor which includes at least one sacrificial metallic anode;
(b) dissolving metallic cations from the anode into the effluent;
(b) (i) to increase the pH of the effluent, and
(b) (ii) to allow the formation of a metal-contaminant-complex, where the contaminant is selected from phosphate and sulphate; and
(c) removing the complex from the effluent.
The method may include the step of precipitating base metals in the effluent as hydroxides (i.e. directly from the reactor.
Preferably the metallic anode is a zinc anode. The invention is however not limited to the use of a zinc anode and any other metal which is capable of increasing the pH of the effluent, and forming a complex of the aforementioned kind, and which is sufficiently inexpensive, may be used in place of zinc.
When use is made of a zinc anode then the complex may be a zinc phosphate or sulphate complex, both of which are insoluble at certain pH values. In the latter case the complex may be a zinc-hydroxyl-sulphate complex.
The complex may be removed from the effluent in any appropriate way but, preferably, is precipitated from the reactor. To this end the pH may be controlled to render the complex insoluble in the effluent.
The method may include the step of treating the complex to recover at least some of the metal therefrom. Thus, in the case where the method makes use of a zinc sacrificial anode, the precipitated zinc complex may be subjected to a zinc recovery process, which may be electrolytic, and the zinc recovered from the process may be recycled and, ultimately, may be available for reuse in the method of the invention.
Base metals may be precipitated from an overflow of effluent from the reactor or, as noted, directly from the reactor.
The invention preferably includes the step of controlling the parameters of the electrical energy input to the reactor to control the pH of the effluent at a value which maximizes the formation of the metal-contaminant complex. Thus the voltage which is applied to the reactor and the electrical current which flows through the effluent (i.e. the electrolyte), and the period of time for which the current is passed, may be regulated to control the quantity of electrical energy or charge introduced into the reactor.
Preferably the pH is raised to a level of from 5 to 7, e.g. from 5.5 to 6.5, to maximize the formation of the said complex. The exact pH value depends on the chemical condition and speciation of the effluent, and the electrochemical characteristics, i.e. the ion content, of the effluent.
For a particular acid mine drainage solution the pH level was in the range of 6 to 7. This pH level effectively promoted the formation of insoluble zinc-hydroxyl-sulphate, which was then precipitated from the solution.
On the other hand insoluble zinc phosphate was formed at a pH level in the range of 2 to 4.
In one form of the invention the electrical energy input to the reactor, expressed as a ratio of coulombs/liter of effluent in the reactor, is in the range of 1000 to 2500. A preferred ratio is approximately 2000.
Further electrical parameters which may be controlled, according to requirement, include the profile or shape of the voltage which is applied to the reactor, the amplitude of the current flow, if the voltage is pulsed the frequency thereof; and the voltage polarity which may be reversed at intervals to reduce unwanted deposits on the electrodes.
If use is made of a plurality of cathodes and anodes then the spacing or interpolar distance between the electrodes may be adjusted, if necessary on an empirical bases, to achieve an effective precipitation of the metal-contaminant complex.
More particularly the pH of the system is controlled by balancing the charge delivery rate to the solution, i.e. coulombs/liter, and the effluent flow rate, i.e. liters/second, which regulates the retention time of the solution in the reactor.
According to a second aspect of the invention there is provided a method of treating an effluent which includes the steps of introducing zinc cations into the effluent in a reactor to increase the pH of the effluent, allowing at least some of the cations to react with hydroxyl ions and sulphate anions in the effluent to form a zinc-hydroxyl-sulphate complex, and precipitating the complex from the effluent to produce an overflow.
The method may include the step of precipitating base metals from the overflow,
In a variation of the invention which holds particular commercial utility there is provided a method of treating an acid mine drainage solution wherein the pH of the solution is increased by dissolving zinc ions into the solution, and the pH of the solution is controlled so that at least some of the zinc ions are allowed to react with sulphate ions in the solution to form an insoluble zinc-hydroxyl-sulphate complex which is then precipitated from the solution.
by balancing the flow rate of the effluent (containing the sulphate) and the electrical energy introduced into the effluent, expressed as coulombs/liter, the pH can be controlled at an optimum value which promotes the formation, and hence precipitation, of insoluble zinc-hydroxyl-sulphate. These aspects are important for they enable the effluent to be treated on a continuous basis, as opposed to a batch basis.
The invention further extends to apparatus for treating an effluent which includes an electrolytic reactor into which the effluent is introduced and in which are located at least one cathode and at least one metallic anode, an electrical energy supply connected to the cathode and the anode, the parameters of the electrical energy supply and the anode being such that metallic cations from the anode are dissolved into the effluent to increase the pH of the effluent and to cause the formation of a metal-contaminant complex, where the contaminant is selected from phosphate or sulphate anions in the effluent, means for separating precipitated complex from the effluent in the reactor, and means for clarifying overflow from the reactor.
The metal phosphate, and hydroxides in the effluent, are insoluble and precipitate at pH values above a lower limit which, depending on conditions, may be in the range of the 2 to 4. On the other hand met
Arent Fox Kinter Plotkin & Kahn PLLC
Hruskoci Peter A.
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