Removal of Heavy Metals From Aqueous Solutions

Some aqueous process streams or waste water streams contain heavy metal values, e.g., Pb, in solution. Such heavy metal values are often deleterious in subsequent processing of the aqueous stream. If the aqueous stream is a waste water stream destined for merging with public waters, the heavy metal may pose an ecological or environmental problem. Government regulations have been established, setting very low acceptable levels of heavy metals, e.g., Pb, in waste streams destined to merge with public waters. Pb values are present, for example, in aqueous streams rich in MgCl.sub.2 which emerge from the production of Grignard reagents.

There is a need, then, for a method of substantially removing heavy metals, such as Pb, from aqueous solutions.

It is an object of the present invention to provide for removal of heavy metal values from aqueous streams.

It is a further object to provide a method for substantially removing heavy metal values from aqueous alkali metal or alkaline earth metal salt solutions.

Another object is to provide a method for removing heavy metal values from aqueous MgCl.sub.2 solutions destined for use as a feed material for other processes which require Mg values, or else are destined for merging into public waters, such as bays or estuaries.

Yet another object is the removal of Pb values from MgCl.sub.2 brine which emerges from a process in which Grignard compounds are made.

These and other objects are attained by the invention disclosed herein.

Aqueous solutions containing heavy metal values, such as Pb, are intimately contacted with chromate or dichromate ions under conditions conducive to oxidation of any organics and the compounds containing heavy metal values, then raising the pH to above about 7.0 by adding OH.sup.- ions thereby precipitating Pb chromate, then separating the precipitate from the aqueous solution.

Generally speaking, the present invention comprises a method for substantially removing heavy metal values, such as Pb, from aqueous solution by adding soluble chromate (or dichromate) ions to the solution in an amount sufficient to react with the heavy metal compounds, then subjecting the solution to conditions conducive to cause oxidation of any organics and compounds containing the heavy metal compounds, then raising the pH to above about 7.0 by the addition of soluble caustic (OH.sup.- ion source) thereby causing precipitation of heavy metal chromate, and then separating the precipitate from the aqueous medium.

As used herein, the term "chromate" or "chromate ion" includes dichromates as well as chromates such as are obtained by dissolving a soluble metal chromate or dichromate in aqueous solution. Potassium chromate (K.sub.2 CrO.sub.4), potassium dichromate (K.sub.2 Cr.sub.2 O.sub.7), magnesium chromate (MgCrO.sub.4) and magnesium dichromate (MgCr.sub.2 O.sub.7) are examples of chromates operable in the present invention. Chromates and dichromates of other alkali metals or alkaline earth metals, such as Li, Ca, Na and Be are also operable. Chromic acid (H.sub.2 CrO.sub.4) may be used, but because of handling problems, is not generally preferred. The potassium chromates are generally preferred; they are not difficult to handle and are readily and economically available. The chromates and dichromates are known to be oxidizing agents.

The aqueous solutions within the purview of the present invention are those which contain deleterious amounts of heavy metal ions such as Pb, Fe, Cr, or Ag. Of special interest are metal salt brines, such as aqueous MgCl.sub.2. In a particular preferred embodiment there is used a MgCl.sub.2 brine containing Pb values which emerges from a process in which Grignard compounds (RMgCl where R=alkyl) are made. This MgCl.sub.2 brine may in some instances be destined for use as a process stream to subsequent chemical processing or may be destined to merge with public waters as a waste stream. In either case, the substantial removal of Pb values is desired.

The OH.sup.- ions employed in the present invention to raise the pH may be any water-soluble metal hydroxide, with NaOH or KOH being preferred. NaOH is readily and economically available in large quantities, and is the most preferred. Lime (CaO) will hydrate to form CaO.H.sub.2 O (or Ca(OH).sub.2) in water and will raise the pH but tends to precipitate as Ca(OH).sub.2 if the pH reaches about 7.4 or higher; likewise Mg(OH).sub.2 precipitates from alkaline solutions at a pH from about 7.4 or higher so "causticizing" with MgO or dolime (CaO.MgO) is not generally preferred unless the aqueous medium already contains Mg or Ca ions which are desired to be kept in solution. The operation of the present invention requires raising the pH from acidic up to about 7.0 or more, but if one employs an alkaline earth metal hydroxide (or hydrated alkaline earth metal oxide) care should be exercized to avoid going above about 7.4. Furthermore, if the brine itself contains alkaline earth metal salts, such as MgCl.sub.2 or CaCl.sub.2, then raising the pH above about 7.4, even using a completely soluble caustic, such as NaOH or KOH, can cause precipitation of Ca(OH).sub.2 or Mg(OH).sub.2 and it is likely that such precipitation would need to be avoided in those instances in which the Mg and/or Ca is desired to be kept in solution.

Thus, there are embodiments of the present invention wherein the precipitation of the heavy metal chromate is preferably done within a limited pH range between about 7.0 and about 7.4, most preferably about 7.2 .+-. 0.1. In those embodiments in which precipitation of alkaline earth metal hydroxides is not a problem and avoidance of such precipitation is not required, then such close control of pH is not necessary and the pH can be raised to above about 7.0 without taking precaution to keep it below 7.4. At higher pH's (about 7.4 or more) the heavy metal chromates are more efficiently precipitated and the need for excess amounts of chromate/heavy metal is lessened.

In obtaining oxidation of the heavy metal compounds such as Pb compounds, the pH of the solution should be acidic (below about 7.0 pH, preferably below about 6.7 pH). It is preferred to employ HCl as the acidifier, though other acidifiers will work. Organic acids are not normally preferred as acidifiers because the carbonaceous material would tend to compete with the heavy metal compounds in the oxidation step, thus requiring an excess of oxidizer (chromate ion) beyond that required by the heavy metal compound. Furthermore, organic acids are not generally as economial as HCl. Other mineral acids may be used, but where the aqueous solution being treated is a brine (metal chloride) material, it is generally best not to introduce different cations than are already present, viz chloride ions.

The oxidizer (chromate ion) is preferably added in an amount which is at least the stoichiometric amount for oxidizing all the heavy metal compound. Most preferably, the oxidizer is added in an excess of about 1.5 to about 4.0 times the stoichiometric amount. The heavy metal compound may, in some cases, be an organometallic compound, such as Pb alkyl, and the organic portion of the compound may compete for the oxidizer ions and it is desired that there be at least an amount of oxidizer present to satisfy all the competing oxidation reactions.

The oxidation rate and extent of oxidation is largely temperature dependent, thus it is preferred to heat the aqueous solution to effect feasible rates of oxidation. Temperatures above ambient, preferably above about 40.degree. C, most preferably above about 60.degree. C are employed. Temperature should be kept at less than boiling point to avoid use of pressurized equipment. If the oxidation step is being done in a location where speed of oxidation is not as necessary as conservation of heat energy, then ample time of oxidation must be provided in order to effect a sufficient or substantial amount of heavy metal removal.

After the oxidation step, the aqueous solution is brought to a pH of about 7.0 or more by addition of OH.sup.- ions, thereby causing precipitation of the heavy metal chromate. As stated previously, the pH is preferably adjusted to about 7.2 .+-. 0.1.

The precipitated heavy metal chromate, such as Pb chromate, is separated from the aqueous medium by any convenient means, such as settling, filtration, centrifugation or flocculation/settling. Preferably the precipitate obtained in a large scale operation is separated by settling with a polishing filtration step.

In those instances in which excess chromate is employed, some chromium values remain in the aqueous medium after the heavy metal chromate has been removed. The chromium values may require substantial removal and this is accomplished by adding Fe.sup.++ ions, e.g., ferrous sulphate or ferrous chloride such as FeCl.sub.2.4H.sub.2 O, to the solution; this drops the pH into the acid range and reduces the chromium to the Cr.sup.3+ state. In some cases it may be advisable to avoid the use of a sulphate because it may be an interfering ion. Following this, the pH is again raised to alkaline and the chromium and iron separated as insoluble hydrates. Here again, if the aqueous medium contains alkaline earth metal values (such as Mg or Ca) which are desired to be kept in solution, then the pH should be kept below about 7.4, preferably about 7.2 .+-. 0.1.

The following examples demonstrate a preferred application of the invention. A practitioner of the relevant arts, having learned of this invention, will be able to apply the invention in other embodiments without departing from the spirit and scope of the present invention.

To an acidic 25% MgCl.sub.2 solution, which contained 400 ppm Pb (100 gram solution), was added 0.058 gram (twice equivalent to Pb as needed to form PbCrO.sub.4) of K.sub.2 Cr.sub.2 O.sub.7. The solution was heated to 60.degree. C for 15 minutes with stirring, the pH adjusted up to 7.2 by adding aqueous NaOH, with stirring, and the precipitate (analyzed as PbCrO.sub.4) which formed was filtered out using a fritted glass filter. To the filtrate was added 0.32 gram of FeCl.sub.2.4H.sub.2 O; this dropped the pH to acidic range. It was stirred and the pH was raised to 7.2 using aqueous NaOH and the ppt which formed (Cr hydrate and Fe hydrate) was filtered out. On a 100% MgCl.sub.2 basis, the MgCl.sub.2 solution now contained 150 ppm Pb, less than 10 ppm Cr, and about 80 ppm Fe.

In this set of runs the MgCl.sub.2 brine is at a pH of about 0.5 (very acid); at this low pH most of the chromium is present as dichromate (literature equilibria data) regardless of whether a chromate or dichromate is introduced. The chromate or dichromate (i.e., MgCrO.sub.4 or K.sub.2 Cr.sub.2 O.sub.7) dissolves readily. After the chromate is added, the solution is heated to about 90.degree. C for about 0.5 hour to assure substantially complete oxidation of organic Pb which is present in the MgCl.sub.2 brine. The pH is raised to 7.1-7.2 by adding OH.sup.- ion (NaOH) with stirring. At this pH PbCrO.sub.4 precipitates and is removed by filtering or settling. The efficiency of Pb removal is a function of chromate/Pb concentration, thus and excess of chromate is ordinarily preferred even though the excess chromium may then need to be substantially removed by adding Fe.sup.+2 and more OH.sup.-. Stating it another way, it can be said that since the dichromate-chromate equilibrium is pH dependent, excess is necessary to have enough precipitate a substantial portion of the Pb at a pH of about 7.2. (Precipitation of PbCrO.sub.4 may be aided by adding seed crystals at the precipitation pH.)

The following Table I gives data for the runs of this example. The brine employed is 100 grams of 25% MgCl.sub.2 solution containing 400 ppm Pb. This calculates to be 1600 ppm Pb on 100% MgCl.sub.2 bases. Analyses is made of the brine after treatment.

The data in following Table II were obtained by adding various amounts of K.sub.2 Cr.sub.2 O.sub.7 to 100 gms of a MgCl.sub.2 brine, heating to 95.degree. C for 11/2 hours, cooling, adjusting the pH to 7.2 by adding NaOH, stirring for 2 hours and then, after standing overnight, filtering through a medium glass frit. The MgCl.sub.2 brine at the start was acidic and contained 25% by weight of MgCl.sub.2 and 400 ppm Pb. This amount of Pb is equal to 1600 ppm Pb on 100% MgCl.sub.2 basis. Analysis is obtained by emission spectroscopy.

The invention is limited only by the following claims.