This invention relates to a method of treating wastewater containing fluoride.

BACKGROUND

Hydrofluoric acid (HF) is commonly used as an etchant and a cleaner, such as in the semiconductor industry to etch and clean silicon wafers, to etch, polish and frost glass, and as a component in household rust/water stain removers. However, although it is regarded as a weak acid in dilute aqueous solution, it remains highly corrosive and is a strong contact poison. Free fluoride ions from HF or other sources have a very high affinity for calcium, and are therefore damaging to bones and to the balance of electrolytes in the body, which can be fatal. There is therefore a need for an apparatus and method to treat industrial wastewater containing fluoride in order for such wastewater to be safely disposed of.

SUMMARY

According to a first aspect, there is provided a method of treating wastewater containing fluoride (F ), the method comprising the steps of: a) providing calcium ions (Ca ²⁺ ) in a reaction tank; b) flowing a quantity of the wastewater containing fluoride from a wastewater supply into the reaction tank provided with the calcium ions to form a reaction mixture and holding the reaction mixture in the reaction tank for a predetermined duration; c) adding a flocculent and a coagulant into the reaction tank and holding the reaction mixture with the flocculent and the coagulant in the reaction tank for a predetermined period; and d) flowing liquid from the reaction tank to a discharge tank.

The method may further comprise, after step c), flowing the liquid from the reaction tank through water treatment media prior to flowing the liquid to the discharge tank.

The water treatment media comprise a first medium and a second medium.

The method may comprise flowing the liquid through the first medium followed by flowing the liquid through the second medium.

The first medium may comprise Katalox Light® and the second medium may comprise Catalytic-Carbon™.

The method may further comprise, in step b) adding sodium hydroxide (NaOH) into the reaction tank to maintain pH of the reaction mixture at a pH ranging from 7 to 13.

The calcium ions may be provided by providing at least one of: calcium hydroxide (Ca(OH)2), calcium carbonate (CaCCb), calcium bicarbonate (Ca(HCC>3)2) and calcium chloride (CaCh) in the reaction tank.

Providing the calcium hydroxide in the reaction tank may comprise pumping the calcium hydroxide from a preparation tank into the reaction tank.

The method may further comprise determining concentration of fluoride in the liquid in the discharge tank, and discharging the liquid in the discharge tank when the concentration of fluoride is less than or equal to 15 ppm, and flowing the liquid in the discharge tank back to one of: the reaction tank and the wastewater supply when the concentration of fluoride is greater than 15 ppm.

The method may further comprise, after step c), pumping sludge in the reaction tank into a de watering system to remove wastewater before disposal of the sludge.

BRIEF DESCRIPTION OF FIGURES

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings. FIG. 1 is a schematic illustration of a method of treating wastewater containing fluoride.

DETAILED DESCRIPTION

Exemplary embodiments of a method 200 of treating wastewater containing fluoride will be described below with reference to FIG. 1.

As shown in FIG. 1, in the method 200, a reaction tank 10 is provided in which calcium ions (Ca ²⁺ ) are provided (210). In an exemplary embodiment, the calcium ions may be provided by providing calcium hydroxide (Ca(OH)2) in the reaction tank 10, largely in suspension. Use of calcium hydroxide which is basic is advantageous in providing a neutralizing effect on acidic hydrogen fluoride (HF) present in the wastewater. In an exemplary embodiment, the reaction tank 10 may have a volume of two cubic metres, although size of the reaction tank 10 can be varied depending on the scale of treatment required. Calcium hydroxide in the reaction tank 10 may be provided by pumping the calcium hydroxide from a preparation tank 20 into the reaction tank 10 (210). Alternatively, the calcium ions in the reaction tank 10 may be provided by reacting sodium hydroxide (NaOH) with calcium chloride (CaC h) in the reaction tank 10 to form calcium hydroxide in the reaction tank 10 (not shown). The calcium ions may further alternatively or additionally be provided by providing other calcium-containing compounds such as calcium carbonate (CaCCb), calcium bicarbonate (Ca(HC03)2) and/or calcium chloride (CaCh) in the reaction tank 10 (not shown).

Subsequent to providing the calcium ions in the reaction tank 10 (210), the method 200 comprises the step of flowing wastewater 30 containing fluoride (F, such as from used hydrofluoric acid (HF)) from a wastewater supply 32 into the reaction tank 10 that already contains the calcium ions, to form a reaction mixture 40 in the reaction tank 10. It should be noted that this step of flowing wastewater 30 containing fluoride into the reaction tank 10 that already contains the calcium ions is not equivalent to adding calcium ions into a reaction tank that already contains wastewater. Instead, in the present method 200, wastewater 30 is added into a reaction tank 10 that already contains calcium ions such that the amount of calcium ions in the reaction tank 10 is greater than the amount of fluoride added to the reaction tank 10.

The reaction mixture 40 is then held for a predetermined duration in the reaction tank 10 (235) to allow the calcium ions already in the reaction tank 10 to react with the fluoride in the wastewater 30 to form calcium fluoride (CaF2) as precipitate in the reaction mixture 40. In some embodiments, the predetermined duration may range from 5 minutes to 20 minutes, and in an exemplary embodiment, the predetermined duration may be 15 minutes.

The method 200 may optionally comprise adding sodium hydroxide into the reaction tank 10 (230) together with the wastewater 30 in order to control pH of the reaction mixture 40. The amount of sodium hydroxide added is an amount needed to maintain the pH at a pH ranging from 7 to 13. In an exemplary embodiment, the pH of the reaction mixture 40 may be kept at 10. Increase in pH of the reaction mixture 40 increases precipitation of calcium in the form of calcium fluoride, thereby accordingly increasing precipitation of fluoride out of the reaction mixture 40 to increase efficiency of fluoride removal.

After the predetermined duration has elapsed, the method 200 further comprises adding a coagulant 12 and a flocculent 11 into the reaction tank 10 (240). The reaction mixture 40 with the added coagulant 12 and flocculent 11 are then held for a predetermined period in the reaction tank 10 (245). In an exemplary embodiment, 6 kg of coagulant 12 and 0.2 kg of a polymer flocculent 11 may be added into the reaction tank 10 to form larger floe containing the calcium fluoride precipitate that settle in the reaction tank 10 as sludge 42. In some embodiments, the predetermined period may range from 5 minutes to 15 minutes, and in an exemplary embodiment, the predetermined period may be 10 minutes. Sludge 42 in the reaction tank 10 may be pumped into a de-watering system 43 for removal of excess water before going into a dry sludge holding tank 44 for disposal, in order to reduce the weight and accordingly the cost of sludge removal.

After the predetermined period has elapsed, liquid 41 from the reaction tank 10 may optionally be flowed from the reaction tank 10 through water treatment media 50 (250) before being flowed into a discharge tank 60 (260). For example, the water treatment media 50 may comprise a first commercially available medium 51 such as Katalox Light® that is a granular filtration medium comprising zeolite particles coated with manganese oxide (MnCh) and a second commercially available medium 52 such as Catalytic-Carbon™ that comprises granular activated carbon coated with feroxyhyte (FeOOH). In an exemplary embodiment as shown in FIG. 1, the liquid 41 from the reaction mixture 40 is flowed through the first medium 51 followed by flowing the liquid 41 through the second medium 52.

A fluoride sensor 61 may be provided at the discharge tank 60 to determine concentration of fluoride in the liquid 41 in the discharge tank 60 (261). In an exemplary embodiment, if the sensor 61 detects that concentration of fluoride in the liquid 41 in the discharge tank 60 is less than or equal to 15 ppm, the liquid 41 is allowed to be discharged (270), for example, into a sewer. If it is detected that concentration of fluoride in the liquid 41 in the discharge tank 60 is greater than 15 ppm, the liquid 41 in the discharge tank 60 may be flowed back to the reaction tank 10 (280) for further treatment, or may alternatively be flowed back to the wastewater supply 32 (not shown).

From experiments conducted using the above disclosed method, hydrogen fluoride at 11 wt% concentration was treated to result in discharge liquid 41 containing only 0.95 ppm of fluoride. The above disclosed method is therefore an efficient and cost effective way to treat wastewater containing fluoride.

Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations and combination in details of design, construction and/or operation may be made without departing from the present invention.