Removal Of Metals From Wastewater (ExxonMobil)

Removal Of Metals From Wastewater (ExxonMobil)
United States Patent Application 20160159669
BARBOT; Elise Noelle ;   et al.   June 9, 2016
Assignee: ExxonMobil Research and Engineering Company
Abstract
A method for effecting a comprehensive removal of heavy metals from wastewater in a two stage process in which the wastewater is contacted in a first stage with a source of ferric ions under mildly acidic conditions (pH 5 to pH 8), preferably followed by the removal of the precipitated solids using a solid-liquid separation; a second stage follows in which the wastewater from the first step is contacted with a source of ferric ions under alkaline conditions (pH 8+) followed by the removal of the precipitated solids using a second solid-liquid separation. Used in conjunction with an initial oxidation step, the present method makes possible the removal of a whole suite of heavy metals present in both the anionic and cationic form in refinery wastewater. The treatment also removes metal compounds in the particulate phase. Metals concentrations can be significantly decreased from the mid to high ppb (parts per billion) range down to the low ppb range to meet the quality criteria for discharge.
BACKGROUND OF THE INVENTION
[0003] There is increasing concern over the hazards posed by the rising levels of heavy metals within the world's water supplies. Most heavy metals are toxic to some degree to life form and as a consequence of increasing concern over the concentration of heavy metals in waters discharged into the environment, industry is being required to reduce the levels of heavy metals from aqueous wastes with heavy metals typically considered to include metals and metaloids (e.g., arsenic) which have an atomic number greater than that of calcium, particularly aluminum, arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, vanadium and zinc. In the oil industry, heavy metals occur naturally in crude oil and are transferred to wastewater during refining operations. Regulations on refinery wastewater discharges vary with location but it is now common to see limits in the lower ppb range, for example, in the Water Framework Directive (WFD), the Refining BREF and the CWW BREF limits in Europe. The decrease of acceptable concentration limits makes it more and more difficult to process certain types of crude.
[0004] Common treatment of refinery and chemical plants wastewaters include oil and solids removal by sedimentation and flotation processes as well as organics oxidation by biological treatment. Such processes may remove the particulate portion of the metal content but are ineffective in decreasing the dissolved metal concentration. A tertiary treatment is thus necessary to comply with the decreasing regulatory limits. A particular difficulty encountered in wastewater treatment is to remove metals which have different solubilities in water under different conditions, e.g. in the presence of different reagents and under different pH conditions. Removal of all the eleven metals mentioned above has proved difficult at the levels in Table 1 below.
TABLE-US-00001 TABLE 1 Target Concentration Metal (ppb) Al 800 As 30 Cd 2 Cr 10 Cu 10 Pb 5 Hg 0.1 Ni 5 Se 30 V 50 Zn 60
[0005] The best available technology for metal removal is reverse osmosis, as it is capable of removing dissolved metals and ions down to very low levels but it remains an expensive technology. It generates a concentrated brine that requires very costly and additional energy-demanding processes (typically evaporation and crystallization) to generate a solid salt waste (zero liquid discharge).
[0006] Another metals removal technique is precipitation (e.g. with hydroxides, sulfides) and this is one of the most widespread technologies used in industries for metal removal. However, it is only suitable for wastewater containing high concentrations of metals and is ineffective for low feed concentrations. Ion exchange, another alternative, may be able to remove all metals to low ppb levels but is expensive at large scale for wastewaters containing low concentrations of heavy metals, as it also removes other ions present in the wastewater. Similar to reverse osmosis, it would require additional very costly and energy-demanding processes to generate a solid waste (zero liquid discharge).
[0007] U.S. Pat. No. 5,013,453 (Walker) discloses the commercial UniPure.TM. process for removing heavy metals from aqueous waste streams without the necessity of adjusting the pH of such streams to pH values above 8.0 as it had been found that the conventional alkali precipitation method which required large volumes of alkali to precipitate the heavy metals as hydroxides was, with the increasing stringency of regulatory standards, becoming excessively expensive, particularly given its inability to use the cheaper ammonium hydroxide instead of caustic soda. The method described in this patent comprises co-precipitating the heavy metal ions with a carrier precipitate which is formed in situ within the aqueous solution while maintaining the aqueous solution at near neutral pH. Ferrous (Fe.sup.2+) chloride is added at the start and injected air is used to oxidize it to ferric iron to form the precipitate; the process, however, will not provide a significant cost advantage because it requires air to be mixed into the reactor, increasing capital costs.
[0008] Single-stage iron co-precipitation at mildly acidic pH has been implemented to remove selenium from refinery wastewater but it is ineffective to remove all of the eleven heavy metals noted above since the metals have differing solubility characteristics at different pH values.
SUMMARY OF THE INVENTION
[0009] We have now devised a method for effecting a comprehensive removal of heavy metals from wastewaters such as petroleum refinery wastewater using the following steps: a first step of contacting the wastewater with a source of ferric ion under mildly acidic conditions (pH 5 to pH 8) followed by the removal of the precipitated solids using a solid-liquid separation, and a second step of contacting the wastewater from the first step with a source of ferric ion under alkaline conditions (pH 8+) followed by the removal of the precipitated solids using a second solid-liquid separation. The term "metals" is used in this specification to include metalloids such as arsenic and selenium.
[0010] The present method makes possible the removal of a whole suite of heavy metals present in both the anionic and cationic form in the wastewater. The treatment also removes metal compounds in the particulate phase. Metals concentrations can be significantly decreased from the mid to high ppb (parts per billion) range down to the low ppb range to meet the quality criteria for discharge to the environment. The process is particularly useful for removing minor amounts of metals
[0011] A number of different process variations may be used. In a preferred configuration, the wastewater is contacted in the first stage with a source of ferric ions under acidic conditions to form a co-precipitate of metals with ferric hydroxide which is then removed by solid-liquid separation to form a liquid effluent which is then passed to a second stage in which it is contacted with a source of ferric ions under alkaline conditions to form a second co-precipitate of metals with ferric hydroxide which is then removed using a second solid-liquid separation to form a purified liquid effluent suitable for discharge to the environment. In this configuration, the optimal removal of the metals is achieved by subjecting the precipitated solids removed in both stages to thickening in separate thickening steps although they may be combined and subjected to thickening in a combined thickening step. In either case, liquid effluent from the thickening step(s) is recirculated to the first stage.
[0012] In another process configuration, the wastewater is contacted with ferric ions in the first stage under acidic conditions to form the first co-precipitate which is then transferred together with the liquid effluent to the second stage maintained under alkaline conditions to form a combined co-precipitate which is removed using a second solid-liquid separation. Additional ferric ions may be added to the second stage or alternatively, the entire aliquot of ferric ions may be added at the first stage.
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