GB1560674A - Process for the rmoval of fluoride ion from water - Google Patents

Description

(54) PROCESS FOR THE REMOVAL OF FLUORIDE ION FROM WATER (71) I, KOHEI DEGUCHI, a Japanese citizen, of 4-7-3, Yamadanishi, Suitashi, Osakafu, Japan, do hereby declare the invention for which I pray that a patent may be granted to me, 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 a method of removing fluorine from water, thereby adapting same to a drinkable water.

A common practice to remove fluorine content from waste water is to use calcium to produce insoluble calcium fluoride. However, under this method it is found that the remaining water still contains as much as from 15 to 50 ppm of fluorine, which is unacceptable if the water is to be discharged into a river. A more effective method of fluoride removal employs activated alumina as an adsorbent for the fluoride and another uses aluminium sulfate as an aggregating agent. But these methods require labour and skill in regenerating the agents used and additionally they are difficult to control. This prevents them from being widely applied.

It is commonly accepted that low levels of fluorine in water do not constitute a health hazard, and indeed low levels are known to prevent tooth decay. Even so if the fluorine content exceeds 0.5 ppm there is a risk of damage to tooth enamel and above 0.8 ppm it can be considered no longer fit for drinking water. Therefore, to make water drinkable the fluorine content must be kept below 0.8 ppm, and more preferably, below 0.5 ppm.

Depending on locality, river water may contain as much as from 1.0 to 2 ppm of fluorine, whilst underground water may contain as much as from 3.0 to 10.0 ppm or even more. In such cases known methods of precipitation, filtration and pasteurization are generally ineffective for lowering the fluoride content to an acceptable level and consequently, the water remains unfit for drinking.

Accordingly the present invention seeks to provide an improved method for removing fluorine from water. This method comprises treating the water with a source of aluminium ions, thereby to produce aluminium fluoride and removing the aluminium fluoride by adsorption onto flocs of aluminium hydroxide formed in situ in said water.

According to one technique the aluminium ions are provided in solution by electrolysing the fluorine-containing water feed using a copper or iron cathode and an aluminium anode.

Aluminium sulfate or a mixture of aluminium chloride and sodium aluminate are then added, preferably as aqueous solutions, and the pH of the treated solution adjusted to 6.0 to 7.0 and stirred, thereby to form flocs of aluminium hydroxides containing adsorbed fluoride.

In another technique, the water feed is treated with polyaluminium chloride and sodium aluminate, preferably in aqueous solution. The treated solution is stirred while adjusting the pH to a value in the range 6.5 to 6.8. and a further quantity of polyaluminium chloride and sodium aluminate preferably in weight ratio 3:1, is added to the solution while maintaining the pH value in the same range, thereby to produce aluminium fluoride absorbed onto flocs of aluminium hydroxide which are formed in situ.

The present invention will be further described by way of example, with reference to the accompanying drawings in which: - Figure 1 is a schematic view of an apparatus for use in demonstrating a first embodiment of the present invention and Figure 2 is a schematic view of an apparatus for use in demonstrating a second embodiment of the present invention.

Referring to the first embodiment, i.e. Figure 1, the water n to be treated is placed in a 500 mi beaker 11 provided with an aluminium anode 13 and a copper (or iron) cathode. A DC voltage of 5 to 10 volts is applied across the electrodes to pass a 30 mA d.c. current. At the cathode 14 hydrogen is liberated whilst at the anode 13 aluminium ions pass into solution and a small amount of oxygen gas is liberated. At this stage the pH value of the water is adjusted to a value in the range of 6.0 to 7,0, at which value the aluminium ions readily form aluminium fluoride hydrates by reaction with the fluoride ions. The aluminium fluoride hydrates are insoluble in the water and disperse in the form of colloid, which is adsorbed by the insoluble aluminium hydroxide floc additionally produced from the aluminium ions. Finally by removing the aluminium hydroxide floc from the water the adsorbed fluoride is removed. In this way 60% to 90% of the fluoride ions can be removed, as indicated in Table 1. In order to ensure an excess of aluminium ions against fluoride ions, aluminium sulfate or aluminium chloride and sodium aluminate are added in amounts of from 30 to 200 ppm. Normally as the electrolysis proceeds the pH value rises, and the aluminium sulfate or aluminium chloride are added to maintain the pH value in the range & 7 thereby maintaining an efficient operation. In addition, this reduces the amount of aluminium ions ionized from the anode. But the added aluminium ions are less active, and react with only a small part of fluoride ions.

Table 2 indicates the comparative data obtained from the experiments in which an aluminium or carbon anode is used with a copper cathode, so as to demonstrate that the aluminium ions ionized from the anode are more effective than those from the added aluminium compound.

TABLE 1 (Added agents) (**) (*) F- before F- after Aluminium Sodium Assistant Specimen treatment treatment sulfate aluminates (Bentonite) (PPM) (PPM) no. 1 26.1 9.6 - - 2 26.1 6.8 200 + 50 ppm 30 ppm 3 26.1 2.9 200 + 50 ppm 30 ppm 100 ppm 4 2.88 1.21 - - 5 2.88 0.66 50 + 20 ppm 10 ppm 6 2.88 0.28 50 + 20 ppm 10 ppm 50 ppm (*) The specimens No. 1 to No. 3 were waste water samples from a factory whilst specimens No. 4 to No. 6 were samples from a well. In the case of speciments 3 to 6 the treatment was carried out for 2 hours, and to the others one hour.

(**) The bentonite was used to facilitate the aggregation of the aluminium hydroxide floc thereby strengthening the adsorbtive effect thereof upon the aluminium fluoride hydrates.

Thus the removal of fluorine ions proceeds efficiently. Alternatively diatomaceous earth can be used.

TABLE 2 (*) F- before F- after Material Aluminium Sodium Calcium Specimen treatment treatment of Anode sulfate aluminium carbonate (PPM) (PPM) No. 7 20.0 7.0 Aluminium - - 8 20.0 10.4 Carbon 200 + 50ppm 20 ppm 9 20.0 17.8 Carbon - - 10 2.0 0.62 Aluminium 50 + 20ppm 8 ppm 11 2.0 1.78 Carbon - - 12 2.0 1.94 Carbon - - 20 ppm (*) Each specimen was prepared by putting equal amounts of sodium fluoride in purified water.

Specimens Nos. 7 to 9 were electrolysed by a 50 mA current for 50 minutes, and specimens Nos. 10 to 12 were electrolysed by a 20 mA current for 2 hours.

The dissociation of aluminium ions from the anode can be controlled by adjusting the flow of current through the cell. This prevents the waste of electricity, thereby leading to economy of operation. Since no addition of calcium is required, the cathode is made safe from a possible formation of scale. The aluminium ions ionized from the anode are active enough to combine with hydroxyl radicals, thereby promoting the formation of aluminium hydroxides, which, as described above, adsorb the fluorine ions in the water.

Referring to Figure 2, the water to be treated is introduced into a tank 22 through an inlet passage 21. In the tank the water is stirred by means of stirrers, with the addition of polyaluminium chloride and sodium aluminate, which are supplied from respective containers 24. The fluorine ions in the water combine with the aluminium ions from the added agents, thereby forming aluminium fluoride in a colloidal state. In this case the aluminium ions are mostly exhausted in forming aluminium hydroxides, and accordingly, it is necessary to provide aluminium ions in excess so as to secure an adequate amount for the fluoride ions. The colloidal aluminium fluorides are adsorbed by the aluminium hydroxide flocs and the treated water is led into a settling tank 23 for separation of the flocculated aluminium hydroxide. In this way the fluorine content in the water is removed together with a sludge taken out of the settling tank.

Table 3 shows the data obtained from the demonstrations employing the apparatus mentioned above, in which the treatment was divided into three phases, as shown in Figure 2. These phases will be referred to as the first chamber, the second chamber and the third chamber.

In the first chamber a given amount of polyaluminium chloride and sodium aluminate were simultaneously poured in the treating water, and the pH value thereof was adjusted to 6.5 to 6.8. During the passing of the water from the first chamber to the third chamber via the second chamber polyaluminium chloride and sodium aluminate were added by 3 parts to 1 part while maintaining the pH value to 6.5 to 6.8. In this way the water was made suitable for drinking.

TABLE 3 (*) First chamber Second chamber Third chamber Treated water Specimen Added agents amount pH amount pH amount pH pH F-ppm No. 1 Polyaluminium chloride 200ppm 50ppm 50ppm 6.60 6.67 6.70 6.78 0.50 Sodium aluminate 50ppm 15ppm 15ppm Polyaluminium chloride 200ppm 100ppm 100ppm No. 2 6.60 6.72 6.63 6.75 0.39 Sodium aluminate 50ppm 30ppm 35ppm (*) The specimens were both samples of river water having a pH of 7.86 and fluoride ion content of 1.23 ppm.

Table 4 shows the data obtained from experiments with two water specimens, No. 3 being obtained from a deep well and No. 4 being from the underground water, each having a pH of 7.52 and 4.40 ppm of fluorine.

TABLE 4 First chamber Second chamber Third chamber Treated water Specimen Added agents amount pH amount pH amount pH pH F-ppm Polyaluminium chloride 600ppm 100ppm 100ppm No. 3 6.65 6.71 6.66 6.81 0.46 Sodium aluminate 180ppm 30ppm 30ppm Polyaluminium chloride 300ppm 30ppm 30ppm No. 4 7.88 7.68 7.55 7.80 2.26 Sodium aluminate 120ppm 10ppm 10ppm During the treatment of the specimen No. 3 the pH value was maintained in the range of 6.5 to 6.8, which, as mentioned above, is required to secure a constant reactive power of aluminium ions. In contrast the pH value of the specimen No. 4 was not controlled, thereby causing it to rise above the desired limit. As a result, this specimen remained unfit for a drinking water.

Claims (5)

WHAT I CLAIM IS:

1. A method of removing fluoride ions from water which comprises treating the water with a source of aluminium ions, thereby to produce aluminium fluoride and removing the aluminium fluoride by adsorption onto flocs of aluminium hydroxide formed in situ in said water.

2. A method according to claim 1, wherein the fluoride containing water is first of all electrolysed using an aluminium anode and an iron or copper cathode, and then treated with sodium aluminate and either aluminium sulphate or aluminium chloride, the treated water thereafter being stirred, adjusted to a pH in the range 6.0 to 7.0, to form said flocculated aluminium hydroxide in situ, and thereafter removing the flocculated aluminium hydroxide containing absorbed fluoride.

3. A method according to claim 2, wherein said sodium aluminate and aluminium sulphate or chloride are added as aqueous solutions.

4. A method according to claim 1, wherein the fluoride ion-containing water is first of all treated simultaneously with polyaluminium chloride and sodium aluminate and adjusted with stirring to a pH in the range 6.5-6.8, the treated water thereafter being treated with additional quantities of polyaluminium chloride and sodium aluminate in a weight ratio of 3:1, thereby to form flocs of aluminium hydroxide containing absorbed aluminium fluoride and thereafter removing said aluminium fluoride-containing, flocculated aluminium hydroxide.

5. A method according to claim 4, wherein said polyaluminium chloride and sodium aluminate are added as aqueous solutions.