GB1560644A

GB1560644A - Treatment of waste water to remove compounds of heavy metals

Info

Publication number: GB1560644A
Application number: GB3269076A
Authority: GB
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1975-08-08
Filing date: 1976-08-05
Publication date: 1980-02-06
Also published as: JPS599237B2; CA1078083A; CH632979A5; NL7608552A; JPS5220664A; DE2635672A1

Abstract

A process for treating waste water containing heavy metals is described, the heavy metals being present in the form of solid sparingly soluble compounds and in dissolved form in the said waste water. The heavy metals are removed by adding ferrous ions to the waste water in a 2 to 100-fold molar excess as iron with respect to the total amount of other heavy metals present and further adding to the waste water 0.9 to 1.2 equivalents, with respect to acid radicals present in the waste water, of an alkaline substance and then stirring the waste water without aeration, insoluble ferrite crystals precipitating out which have incorporated the heavy metals which were present either in the form of solid sparingly soluble compounds or dissolved in the waste water.

Description

(54) IMPROVEMENTS IN OR RELATING TO THE TREATMENT OF WASTE WATER TO REMOVE COMPOUNDS OF HEAVY METALS (71) We, NIPPON ELECTRIC COMPANY LIMITED, a Company duly organized and existing under the laws of Japan and having its executive office at 33-1, Shiba Gochome, Minato-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to methods of treating waste waters containing solid-phase inorganic compounds of heavy metals other than iron, the compounds having a low solubility in water.

Waste waters containing such compounds may result from washing the residual ashes after incineration of municipal refuse, from a wash-out of smoke from furnaces or incinerators, and from a wash-out of fly-ashes discharged from electric precipitators or multicyclones. Such waste waters also include waters containing hydroxides precipitated by carrying out a conventional neutralization precipitation process on waste waters containing heavy metal ions, and wet sludges of hydroxides of heavy metals such as are separated from waters subjected to the neutralization precipitation process.

The above-mentioned waste waters from the wash-out of residual ashes, smoke, or fly-ashes contain suspended and/or precipitated solid-phase substances including silica, carbon and its derivatives, low solubility inorganic compounds of heavy metals, as well as dissolved heavy metal ions. The low solubility inorganic compounds suspended and/or precipitated in 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 sulfate, and chlorides such as mercury chloride. These waste waters have heretofore been treated by the neutralization precipitation method, whereby dissolved heavy metal ions are precipitated as hydroxides, together with the foregoing solid-phase substances. and then separated and removed from the waste water.

The treated waste water is then discharged. Heavy metal hydroxides are solid and hardly soluble in water in general, but heavy metals may often be redissolved from the hydroxide sludges at the specific pH values of the environment, involving great possibilities of causing secondary pollution due to redissolved heavy metals. Secondary pollution may also be caused by a similar redissolution of heavy metals from solid-phase, low solubility heavy metal salts which have been originally suspended and/or precipitated in the waste water and then separated therefrom together with the hydroxides.

It is usual to treat waste water containing heavy metal ions and with little or no solid substances suspended or precipitated, by adjusting the pH values thereof to precipitate hydroxides of the heavy metals, which are then separated from the waste water. 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, low solubility heavy metal compounds have been solidified with cement before discharge. This approach, however, is not perfect as yet. At present recycling of heavy metal hydroxides is used very little on an industrial basis, and they are thrown away without being recycled.

According to the present invention there is provided a method of treating a waste water containing solid-phase inorganic compounds of heavy metals other than iron, the compounds having a low solubility in water, comprising the steps of adding ferrous ions to waste water containing said compounds in an amount in mols of 2 to 100 times the amount in total mols of the heavy metals other than iron present in said waste water, adjusting the amount of alkaline substances in said waste water to an amount corresponding to approximately 0.9 to 1.2 equivalent of acid radicals present in said waste water, and stirring the waste wate without aeration at a temperature above 40"C, whereby insoluble ferrite crystals incorporating the heavy metals other than iron which were present in said waste water are precipitated.

In carrying out a preferred method according to the invention, ferrous ions are added to a waste water containing low solubility heavy metal inorganic compounds present as solid-phase substances, the amount of ferrous ions being 2 to 100 times, in mols, the amount of the heavy metals other than iron 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 approximately one chemical equivalent, or preferably 0.9 to 1.2 chemical equivalents, of the free acid radicals present in the waste water. Then, the waste water is heated to a temperature above 40"C and stirred, without passing air or without oxidizing agents, at a temperature above 40"C. The heavy metal ions present in the low solubility compounds are incorporated into the crystal lattice of the ferrite crystals, eventually forming a precipitate of crystal ferrite particles incorporating the heavy metal ions. Where heavy metal ions are present together with solid-phase heavy metal compounds in the waste water, these free heavy metal ions are also incorporated into the precipitated ferrite crystals and hence extracted from the waste water. The low solubility heavy metal compounds present as solid substances in the waste water may be oxides, carbonates, sulfates, 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 water containing dissolved heavy metal ions with or without solid-phase, hard-soluble heavy metal compounds, by adjusting the pH values of the waste water to form precipitates of heavy metal hydroxides, adding ferrous ions in an amount of 2 to 100 times, in mols, the amount of all heavy metal ions present in any form in the waste water, adding alkali in an amount of 0.9 to 1.2 equivalents of the free acid present in the waste water, and stirring the waste water without aeration at a temperature above 40"C to form precipitates 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 processes of adding thereto a ferrous salt, such as ferrous sulfate or ferrous chloride, suitably 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 an alkali suitably in the form of an aqueous solution in an amount of 0.9 to 1.2 equivalents of the free acid present, and stirring the mixture at a temperature of 40"C or more without aeration to convert the hydroxides into ferrite crystals incorporating heavy metals.

The ferrites incorporating toxic heavy metals produced in a method according to the invention are not soluble into water, and hence there is only the slightest possibility of causing secondary pollution due to heavy metals liberated in water. Furthermore, because a ferrite is a ferromagnetic substance, the ferrites precipitated from 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 can be highly advantageous in recycling wastes. Among heavy metals which can readily be incorporated into ferrites according to the invention are Mn and Zn, in a ratio of about 33% to Fe. Therefore, by adding Fe ions to a waste water in an amount in mols equal to twice the amount of 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 or Pb, into ferrites, but these elements can still be incorporated into ferrites in a ratio of up to 1% to Fe. Accordingly, to extract 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, since the foregoing heavy metal compounds have small in solubility, the rate at which ferrites are formed from them is slow. Therefore, the time necessary for treating waste water is prolonged. To reduce the treatment time, it is desirable to enhance the solubility of the compounds. To this end, the temperature of the waste water is raised during the ferrite formation to promote the reaction. At a temperature above 40"C, a sufficient effect is obtained. Preferably, the waste waste is kept at a temperature below 95"C, or in the range of 60 to 80"C. If the waste water is stirred with aeration during the process of ferrite formation, the ferrous ions are oxidized excessively into ferric ions, hampering the normal ferrite formation. Accordingly, to form ferrites from low solubility compounds, the waste water must be stirred under non-aerated conditions.

The invention will now be described in more detail by way of examples.

Example 1 In this example, the waste water from from a wash-out of the residual ashes discharged from a municipal refuse disposal plant and of fly-ashes from electric precipitators and multicyclones. The waste water contained dissolved heavy metals at a total concentration of about 50 ppm and suspended compounds of heavy metals excluding suspended compounds 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, which was stirred and then neutralized by adding sodium hydroxide thereto.

The waste water was then heated to a temperature of 60"C and stirred by a low speed stirrer at 60"C for 6 hours, resulting in black ferromagnetic precipitates of ferrite. After the reaction process, the waste water was forced to pass through a magnetic separator to separate the precipitate from the liquid. The resultant water was a colorless, transparent liquid in which no solid-phase substances, such as suspended substances, were observed.

The initial waste water and the treated water were sampled and analyzed. The analyzed results are shown in the following table. Also shown are the results of tests on the dissolution of heavy metals from the precipitates. The analysis of metal ions contents or concentration in the water was carried out by an atomic absorption spectrophotometric method.

Metal Contents in Waste Water Kind of Before After Metal Dissolution Heavy Metal Treatment Treatment from Precipitates (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 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 1 Example 2 The waste water described in Example 1 was treated by the neutralizing method, and precipitated heavy metal hydroxixdes were sedimented together with suspended substances, and separated. The water content in the resultant sludge was 97%. The sludge was sampled in an amount of 1 kg, which was placed in a ball mill of 3-liter capacity. Then 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 65"C, and the sample was stirred for 10 hours at this temperature. As a result of this process, black, ferromagnetic ferrite precipitates were contained. After the reaction, the sludge containing water was passed through a press filter and solidified sludge was thus obtained. The test on 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 sludge (ppm) Cd less than 0.001 Pb less than 0.03 Zn 0.05 Ni 0.03 Cu less than 0.02 Mn 0.008 As described above, low solubility compounds are converted into ferrite compounds by a chemical reaction, whereby the low solubility compounds can be transformed into stabilized insoluble compounds and can thus be effectively recycled. Although the disclosed method needs a longer treatment time than known methods, a method according to the invention permits the sludge to be readily recycled in many ways, as opposed to other prior art methods.

In the disclosed examples, sodium hydroxide is used as the alkali. According to the invention, basic substances such as hydroxides of alkali metals and hydroxides of alkaline earth metals may be used instead of sodium hydroxides, depending on the reaction conditions chosen and economical considerations. In the disclosed examples, ferrous sulfate is used as ferrous salt. Instead, other ferrous salts may be used. From the viewpoint of economy and effective recycling of industrial wastes, it is desirable that ferrous chloride or ferrous sulfate be used as the ferrous salt because these materials are available as by-products produced in quantities in the pickling of iron sheets or in the production of titanium oxide. For treating waste water containing a sufficient amount of ferrous salt, the amount of ferrous salt added is suitably adjusted or reduced to zero. For weak alkaline waste water the amount of alkali added may be adjusted according to the amount of ferrous salt added. For strong alkaline waste waters, the addition of an alkali may be omitted.

WHAT WE CLAIM IS: 1. A method of treating a waste water containing solid-phase inorganic compounds of heavy metals other than iron, the compounds having a low solubility in water, comprising the steps of adding ferrous ions to waste water containing said compounds in an amount in mols of 2 to 100 times the amount in total mols of the heavy metals other than iron present in said waste water, adjusting the amount of alkaline substances in said waste water to an amount corresponding to approximately 0.9 to 1.2 equivalent of acid radicals 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 other than iron which were present in said waste water are precipitated.

2. A method of treating a waste water containing precipitated hydroxides of heavy metals other than iron, comprising the steps of adding ferrous ions to said waste water in an amount of 2 to 100 times the amount in total mols of heavy metals other than iron present in said waste water, adjusting the amount of alkaline substance in said waste water to an amount corresponding to approxmately 0.9 to 1.2 equivalent of acid radicals present in said waste water, and stirring said waste water without aeration at a temperature above 40"C, whereby insoluble ferrite crystals incorporating said heavy metals other than iron which were present in said waste water are precipitated.

3. A method of treating a waste water containing heavy metal ions other than iron, comprising the steps of adjusting the pH of said waste water to precipitate hydroxides of said heavy metals other than iron, adding ferrous ions to said waste water in an amount of 2 to 100 times, the amount in total mols of said heavy metals other than iron present in said waste water, adding alkali to said waste water in a amount corresponding to approximately 0.9 to 1.2 equivalent of acid radials present in said waste water, and stirring said waste water without aeration at a temperature of at least 400c, whereby insoluble ferrite crystals incorporating the heavy metals other than iron are precipitated.

4. A method of treating sludge of hydroxides, comprising the steps of adding ferrous salt to wet sludge of hydroxides of heavy metals other than iron, the amount of ferrous ions in said ferrous salt being 2 to 100 times the amount in total mols of said heavy metals other than iron present in said sludge, adjusting the amount of alkali in the mixture to an amount corresponding to approximately 0.9 to 1.2 equivalent of acid radicals present in the mixture, and stirring said mixture without aeration at a temperature of at least 40"C, whereby insoluble ferrite crystals incorporating the heavy metals other than iron are precipitated.

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

6. The method of claim 2, in which said waste water is kept at a temperature of 60"C to 80"C during said stirring step.

7. A method of treating waste water according to claim 1 substantially as hereinbefore described and exemplified.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In the disclosed examples, sodium hydroxide is used as the alkali. According to the invention, basic substances such as hydroxides of alkali metals and hydroxides of alkaline earth metals may be used instead of sodium hydroxides, depending on the reaction conditions chosen and economical considerations. In the disclosed examples, ferrous sulfate is used as ferrous salt. Instead, other ferrous salts may be used. From the viewpoint of economy and effective recycling of industrial wastes, it is desirable that ferrous chloride or ferrous sulfate be used as the ferrous salt because these materials are available as by-products produced in quantities in the pickling of iron sheets or in the production of titanium oxide. For treating waste water containing a sufficient amount of ferrous salt, the amount of ferrous salt added is suitably adjusted or reduced to zero. For weak alkaline waste water the amount of alkali added may be adjusted according to the amount of ferrous salt added. For strong alkaline waste waters, the addition of an alkali may be omitted. WHAT WE CLAIM IS:

1. A method of treating a waste water containing solid-phase inorganic compounds of heavy metals other than iron, the compounds having a low solubility in water, comprising the steps of adding ferrous ions to waste water containing said compounds in an amount in mols of 2 to 100 times the amount in total mols of the heavy metals other than iron present in said waste water, adjusting the amount of alkaline substances in said waste water to an amount corresponding to approximately 0.9 to 1.2 equivalent of acid radicals 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 other than iron which were present in said waste water are precipitated.