CA1078083A - Method of treating waste waters containing solid-phase hard-soluble compounds - Google Patents

Abstract

Abstract of the Disclosure It is known to treat waste waters containing heavy metal ions or compounds having a low solubility with a base to form a heavy metal sludge.
Such sludges are not recycled and present an environmental hazard. The present invention overcomes the drawback in the prior art by providing a process for treating waste water which contains heavy metal ion or compounds having a low solubility, which comprises adding ferric ions to the waste water in an amount from 2 to 100 mols per mole of all heavy ion present in the waste water and adding an alkaline substance to said waste water in an amount from 0.9 to 2 equivalents per equivalent of acid radical present in said waste water and stirring the waste water without aeration at a temperature above 40°C whereby insoluble ferrite crystals incorporating the heavy metals are precipitated.

Description

1~78083 The present invention relates to methods of treating waste waters containing solid phase compounds having a low solubility, particularly to methods of removing heavy metals contained in such solid compounds or con-tained in both the solid compounds and the waste waters therefrom. Such waste waters include those resulting from washing residual ashes after inci-neration of municipal refuse sr the like> those resulting from the washing-out of smoke from furnaces or incinerators, and those resulting from the washing-out of fly-ashes discharged from electric precipitators, multicyclones or the like. Such waste waters also include those containing hydroxides pre-cipitated through the conventional neutralization precipitation process ap-plied to waste waters containing heavy metal ions, and wet sludges of hydro-xides of heavy metals such as are separated from the waters subjected to the neutralization precipitating process.
The mentioned waste waters from the washing-out of residual ashes, smoke, or fly-ashes contain suspended and/or precipitated solid-phase sub-stances including silica, carbon and its derivatives, solid phase inorganic compounds of heavy metals having a low solubility, as well as dissolved heavy metal ions. The solid phase inorganic compounds having a low solubility sus-pended in and~or precipitated from such waste waters include metallic oxides such as copper oxide, lead oxide and mercury oxide, carbonates such as copper carbonate, lead carbonate and manganese carbonate, sulfates such as lead sul-fate, and chlorides such as mercury chloride. These waste waters have hereto-fore ~een treated by the neutralization precipitating method whereby dissolved he~vy metal ions are precipitated as hydroxides, together with the foregoing solid_phase substances, and then separated and removed from the waste water.
The treated was~e water is then discharged. Heavy metal hydroxides are solid and have a low solubility in water in general but heavy metals may often be redissolved from the hydroxide sludges at the specific pH values of the en-vironment, involving great possibilities of causing secondary pollution due to redissolved heavy metals. Secondary pollution may also be caused by similar redissolution of heavy metals from solid-phase, heavy metal salts having a low solubility which have been originally suspended in and/or precipitated from the waste water and then separated therefrom together with the hydroxides.
It is usual to treat waste waters containing heavy metal ions with little or no solid substances suspended or precipitated therein, by adjusting the pH values thereof to precipitate hydroxides of the heavy metals which are then separated from the waste waters. Such separated heavy metal hydroxides may similarly cause secondary pollution by redissolution of the heavy metals.
To solve this problem, sludges of heavy metal hydroxides or other solid-phase, heavy metal compounds having a low solubility have been solidified with cement before discharge. This approach, however, is not perfect as yet. At present, recycling of heavy metal hydroxides is generally not promoted on an industrial basis, and they are thrown away without being recycled.
Therefore the present invention seeks to provide a method of effi-cîently treating waste waters containing solid phase compounds having a low solubility.
The present invention also seeks to provide a method of treating heavy metal hydroxides in the form of precipitates in waste waters or in the form of sludges separated from waters to prevent redissolution of heavy metals The method in accordance with the invention is characterized in that heavy metal hydroxides or other, solid-phase heavy metal compounds having a low solubility are converted into ferrites which are insoluble compounds, thus making the heavy metals insoluble and permitting the resultant compounds to be recycled.
More specifically, according to the invention, ferrous ions are added to a waste water containing heavy metal compounds having a low solubili-ty present as solid-phase substances, the amount of ferrous ions being 2 to 100 times, in mols, the amount of the heavy metals present in the waste water, and an alkaline substance or its aqueous solution is added to the waste water, the amount of alkaline substance or its aqueous solution corresponding to 1~78083 approximately one chemical equivalent, or preferably from 0.9 to 1.2 chemical equivalents for each chemical eqivalent of free acid radicals present in the waste water. Then, the waste water is heated to a temperature above 40C and stirred without passing air therethrough orwithout adding oxidizing agents at a temperature kept above 40C, and thus the heavy metal ions present in the compounds having a low solubility are incorporated into the crystal lattice o the ferrite crystals, thereby, eventually forming ferrite precipitate crystal par-ticles associating the heavy metal ions. Where heavy metal ions are present together with solid-phase heavy metal compounds in the waste water to be trea-ted according to the invention, these free heavy metal ions are also incorpora-ted into the precipitated ferrite crystals and hence extracted from the waste water. Heavy metal compounds having a low solubility contained in the waste water to be treated according to the invention may be oxides, carbonates, sul-fates, chlorides, or hydroxides of heavy metals such as Pb, Cu, Ti, Cr, Mn, Co, Ni, Zn, Cd, Sn, Hg, or Bi. It is also possible according to this invention to treat waste waters containing dissolved heavy metal ions with or without solid-phase, heavy metal compounds having a low solubility by adjusting the pH values of the waste water to form precipitation of heavy metal hydroxides, adding fer-fous ions in an amount of 2 to 100 times, in mols, the amount of all heavy me-tal ions present in any form in the waste water, adding alkali in an amount from 0.9 to 1.2 equivalents for each chemical equivalent of free acid present in the waste water, and stirring thewaste water without aeration at a tempera-ture above 40C to form precipitation of ferrite crystals incorporating heavy metal ions which have been originally present in the waste water. Sludges of heavy metal hydroxides can be treated by the invention through the processes of adding thereto ferrous salt such as ferrous sulfate or ferrous chloride, favor-ably in the form of an aqueous solution, in an amount of 2 to 100 times in moles that of heavy metals in the sludge, adding thereto alkali favorably in the form of an aqueous solution in an amount from 0.9 to 1.2 equivalents for each chemi-cal equivalent of free acid present, and stirring the mixture at a temperature 107~0~3 of 40C OT more without aera~ion to convert the hydroxides into ferrite crys-tals incorporating heavy metals.
The ferrites incorporating toxic heavy metals according to the inven-tion are not soluble in water and hence there is a slight possibility of caus-ing secondary pollution due to heavy metals liberated in water. Furthermore, because ferrite is a ferromagnetic substance, the ferrite precipitate associat-ing toxic wastes can be recycled for use as the material of ferrite magnets, electromagnetic wave absorbing materials, and other magnetic materials. In other words, the method of this invention is highly advantageous in recycling wastes. Among heavy metals which can readily be incorporated into ferrites ac-cording to the invention are Mn and Zn, in a ratio of about 33% of Fe. There-fore, by adding Fe ions to a waste water to an amount in mols twice the heavy metals contained in the waste water, these heavy metals are duly taken in the ferrite crystals, permitting the heavy metals to be extracted from the waste water. On the other hand, it is somewhat difficult to incorporate a large amount of Hg, Pb, or the like into ferrites, but these elements can still be incorporated into ferrites in a ratio of up to 1% of Fe. Accordingly, to ex-tract these heavy metals, the amount of Fe ions to be added to the waste water is 100 times, in mols the amount of heavy metals contained therein.
In general, the foregoing heavy metal compounds have a low solubili-ty, and hence reaction for the formation of their ferrites is slow. Therefore, the time necessary for treating the waste waters is prolonged. To reduce the treatment time, it is desirable to enhance the solubility of the compounds having a low solubility. To this end, the temperature of the waste water is raised during the ferrite formation reaction. At a temperature above 40~C, sufficient solubility is obtained. Favorably, the waste water is kept at a temperature below 95C, or in the range of 60 to ~0C. If the waste water is stirred with aeration in the process of ferrite formation, the ferrous ions are oxidized excessively into ferric ions, hampering normal ferrite formation reaction. For ferrite formation of compounds having a low solubility, the waste water must be stirred under non-aeration condition.
The invention will be described in more detail by way of examples.
Example The waste water was obtained from the washing-out of residual ashes discharged from a municipal refuse disposal plant and of fly-ashes from elec-tric precipitators and multicyclones. The waste water contained dissolved heavy metals at a total concentration of about 50 ppm and suspended substances of heavy metal compounds, excluding suspended substances of sulfuric acid and its derivatives and carbon and its derivatives, at a concentration of about 1200 ppm. Ferrous sulfate was added to the waste water in a ratio of 15 kg per 1 m3 of the waste water. The waste water was stirred and then neutralized by adding sodium hydroxide thereto. Then the waste water was heated to a tem-perature of 60C and stirred using a low speed stirrer at 60C for 6 hours, resulting in blac~ ferromagnetic precipitates of ferrite. After the reaction was complete, the waste water was passed through a magnetic separator to sepa-rate the precipitate from liquid. The resultant water was a colorless, trans-parent liquid in which no solid-phase substances were observed. The initial waste water and the treated one were sampled and analyzed. The analysed re-sults are shown in the following table. Also shown are results of tests for the dissolution of heavy metals from the precipitates. The analysis of metal ions contents or concentration in the water was carried out by the atomic absorption spectrophotometric method, Metal Contents in Waste Water Kind of Before After Metal Dissolution Heavy Metal lreatment Treatment from Preci~itates (ppm) (ppm) (ppm~ ~

Cd 1.12 0.001 less than 0.001 Pb 5.38 less than less than 0.03 0.03 Zn 41.0 less than less than 0.005 0.005 Ni 0.34 0.03 0.016 cont'd.

la7sos3 Kind of Before After Metal Dissolution Heavy Metal Treatment Treatment from Precipitates tppm) lPpm) tppm5 Cu 2,01 0.006 less than 0.006 Mn 2.21 less than less than 0.005 0.005 Suspended 1200 less than Substances Example 2 The waste water described in Example 1 was treated by the neutraliz-ing method, and precipitated heavy metal hydroxides were sedimented together with suspended substances, and separated. The water content in the resultant sludge was 97%. A 1 kg sample of sludge was placed in a ball mill having a 3-liter capacity. 0.36 kg of ferrous sulfate and 0.12 kg of sodium hydroxide were added to the sample. The ball mill was heated to a temperature of about 65C, and the sample was stirred for 10 hours at this temperature. As the result of this process, black, ferromagnetic ferrite precipitates were ob-tained. After the reaction, the sludge containing water was passed through a press filter and thus solidified sludge was obtained. The test for the dissolution of heavy metals from the sludge was carried out by the same method as used in Example 1. The results are tabulated below.

Kind of Metal Dissolution Heavy Metal from Slud~e tppm) Cd less than 0.001 Pb less than 0.03 Zn 0.05 Ni 0.03 Cu less than 0.02 Mn 0.008 According to the invention, as described above, heavy metal compounds are converted into ferrite compounds by chemical reaction, whereby the compounds can be transformed into a stabilized insoluble compound which can be effective-ly recycled. Although the disclosed method needs a longer treatment time than known methodsin general, the method of the invention permits the sludge to be readily recycled in many ways, as opposed to other prior art methods.
In the disclosed examples, a sodium hydroxide is used for ~he pur-pose of alkali. According to the invention, basic subs~ances such as alkaline metals and hydroxides of alkaline earth may be used instead of sodium hydro-xides, depending on reaction condi~ions chosen and in economy point of view.
In the disclosed examples, ferrous sulfate is used as ferrous salt. Instead, other ferrous salt may be used. From the viewpoint of economy and effective recycling of industrial wastes, it is preferred that ferrous chloride or fer-rous sulfate be used as the ferrous salt because these materials are available as by-products produced in a large amount in pickling of iron sheets or in the production of titanium oxide. For treating a waste water containing a suffi-cient amount of ferrous salt, the amount of addition of ferrous salt is to be suitably adjusted or reduced to zero. For weak alkaline waste waters, the amount of alkali addition may be adjusted according to the amount of ferrous salt addition. For strong alkaline waste waters, the alkali addition may be omitted.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of treating a waste water containing solid-phase compounds, having a low solubility comprising the steps of adding ferrous ions to a waste water containing solid phase hard-soluble compounds in an amount in mols of 2 to 100 times the amount in mols of whole heavy metals present in said waste water, adding an alkaline substance to said waste water in an amount from 0.9 to 1.2 chemical equivalents for each chemical equivalent of acid radicals pre-sent in said waste water, and stirring the waste water without aeration at a temperature above 40°C, whereby insoluble ferrite crystals incorporating the heavy metals are precipitated.

2. A method of treating a waste water containing precipitated hydroxides of heavy metals, comprising the steps of adding ferrous ions to said waste wa-ter in an amount of 2 to 100 times, in mols, the amount of whole heavy metals present in said waste water, adding an alkali to said waste water in an amount from 0.9 to 1.2 chemical equivalent for each chemical equivalent of acid radi-cals present in said waste water, and stirring said waste water without aera-tion at a temperature above 40°C, whereby insoluble ferrite crystals incorporat-ing said heavy metals are precipitated.

3. A method of treating a waste water containing heavy metal ions, com-prising the steps of adjusting the pH of said waste water to precipitate hydroxides of said heavy metals, adding ferrous ions to said waste water in an amount of 2 to 100 times, in mols, the amount of whole heavy metals present in said waste water, adding alkali to said waste water in an amount from 0.9 to 1.2 chemical equivalents for each chemical equivalent of acid radicals present in said waste water, and stirring said waste water without aeration at a tem-perature of at least 40°C, whereby insoluble ferrite crystals incorporating the heavy metals are precipitated.

4. A method of treating hydroxide sludge, comprising the steps of adding ferrous salt to wet sludge of hydroxides of heavy metals, the amount of ferrous ions in said ferrous salt being 2 to 100 times in mols the amount of whole heavy metals present in said sludge, adding an alkali to the mixture in an amount from 0.9 to 1.2 chemical equivalents for each chemical equivalent of acid radicals present in the mixture, and stirring said mixture without aera-tion at a temperature of at least 40°C, whereby insoluble ferrite crystals incorporating the heavy metals are precipitated.

5. The method of claim 1 or 2 in which said waste water is kept at a temperature between 60°C and 80°C during said stirring step.