WO2023187673A1

WO2023187673A1 - Compounds, process of preparation and a method for fluoride removal from wastewater

Info

Publication number: WO2023187673A1
Application number: PCT/IB2023/053127
Authority: WO
Inventors: Tina CHAKRABARTY; Pinakpani Biswas; Saroj Kumar Banerjee; Amar BOUNTRA; Maitreyee DEB
Original assignee: Tata Steel Limited
Priority date: 2022-03-30
Filing date: 2023-03-29
Publication date: 2023-10-05

Abstract

The present invention relates to an aluminium based inorganic polymeric compound [i.e. compound of formula (I)] useful for removal of fluoride from coke oven wastewater in steel industry. The present disclosure provides a simple, scalable and economical and an efficient method for the preparation of said inorganic polymeric compound and a method for removal of fluoride from coke oven wastewater in steel industry.

Description

COMPOUNDS, PROCESS OF PREPARATION AND A METHOD

FOR FLUORIDE REMOVAL FROM WASTEWATER

TECHNICAL FIELD

The present disclosure relates to the field of chemical sciences, particularly to wastewater treatment, to the disclosure specifically relates to an aluminium based inorganic polymeric compound [i.e. compound of formula 1] usefill for removal of fluoride from wastewater in steel industry. The present disclosure also provides a simple, scalable and economical and an efficient method for the preparation of said inorganic polymeric compound and a method for removal of fluoride from wastewater in steel industry.

BACKGROUND AND PRIOR ART

Fluoride is a naturally occurring element, found in rock minerals and ground water, and its intake at a fairly low level is beneficial for our body for the prevention of dental decay and to maintain bone health. However, fluoride intake that is more than its limit is harmful to human health. Excessive fluoride intake can lead to fluorosis, dental damage, liver damage, thyroid damage, etc. It can even lead to severe cancer. Other complications, like infertility, Alzheimer’s disease, and tumors, may also arise if the fluoride concentration increases in daily water intake (1).

Due to its electronegativity, fluoride ions get attracted to calcium ions, causing calcium decay in bones and teeth. As per World Health Organization (WHO) guidelines, the acceptable fluoride concentration for living entities in drinking water should be < 1.5 mg/L. By Indian standards, the acceptable limit is 1.0 mg/L and, according to drinking water specifications from 1992, the limit is up to 1 .5 mg/L (2). Depending on chemical properties, the existence of fluoride varies in nature. When it dissolves in water, it exists in the form of fluoride and, primarily, it exists as calcium fluoride in soil. Due to its strong electronegativity, it exists in the form of an inorganic compound. Fluoride elements exist in different forms in waste water as well as in ground water due to the negligence of different industries and their direct discharge into drains. If the fluoride concentration exceeds 10 mg/L in wastewater, it becomes severely lethal for the health of humans, plants, and the animal kingdom (3). Thus, focusing on wastewater treatment plants, recently, the US EPA has established that the limit for effluent discharge is 4 mg/L (4). Fluoride discharge limits for common effluent treatment plants in India were set at 2 mg/L for discharge into surflice water and 15 mg/L for discharge into marine coastal areas and public sewers in 2010 (5). There are several technologies available for fluoride separation from industrial wastewater, like ion-exchange (6), adsorption (7), membrane process (8,9), etc. Apart from these technologies, some electrochemical technologies like electroflotation (10), electrocoagulation (11), electrochemical oxidation, EC using aluminium electrode, and constant current capatitive deionization (12) have been established for fluoride removal. The Nalgonda technique is a conventional process, first implemented in Andhra Pradesh, India for fluoride removal. This process is based on the adsorption of fluoride on flocs of aluminium hydroxide. But these techniques have their limitations due to scalability issues or high cost. It may be noted that coke oven discharge water from the steel industry has fluoride concentration levels of up to 90 ppm. Although prior art describes a process for fluoride removal, there is a constant need for developing improved product/process which is scalable at industrial level and economical and the present disclosure is able to achieve the same.

STATEMENT OF THE DISCLOSURE

The present disclosure provides an aluminium based inorganic polymeric compound [i.e., a compound of formula I].

Compound of formula I wherein ‘X’ is a halide;

'n' represents the degree of polymerization wherein l<n<5.

Said compound is useful for the removal of fluoride from wastewater in the steel industry.

In accordance with one aspect, the present invention provides aluminium based inorganic polymeric compound of formula I

Compound of formula I wherein ‘X’ is a halide;

‘n’ represents the degree of polymerization wherein l<n<5.

In accordance with another aspect, the present invention provides a process for preparing an aluminium based inorganic polymeric compound of formula I as defined above, wherein said process comprising steps of

> grinding polyaluminium chloride (IPC), aluminium sulphate-based compound (AS 1) and poly vinyl alcohol (PVA) followed by heating to obtain mixture 1;

> adding water to the mixture 1, above the saturation point to obtain mixture 2; and

> adding aluminium halide (AlCb) to the mixture 2 followed by heating and cooling to obtain the aluminium based inorganic polymeric compound of formula I. In accordance with yet another aspect, the present invention provides a method for removal of fluorides from fluoride rich water/wastewater, said method comprising steps of:

> treating the fluoride rich water/wastewater with aluminium based inorganic polymeric compound of formula I as defined in claim 1 to precipitate or adsorb fluorides as aluminium based inorganic polymeric compound -fluoride complexes; and

> separating the aluminium based inorganic polymeric compound- fluoride complexes.

In accordance with yet another aspect, the present invention provides a simple, scalable, and economical method for removal of fluorides from fluoride rich water/wastewater.

BRIEF DESCRIPTION OF ACCOMPANYING FIGURES

In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a detailed description below form a part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure wherein:

Figure 1 depicts image of Coke-plant wastewater sample before treatment and after treatment with the compound of formula I - dose 1.5 gm/L.

Figure 2 depicts reduction in F- concentration in presence of different coagulants (Inset pic: F- concentration of treated water @1.5 gm/L dosing of different coagulants)

Figure 3 depicts (a) Effect on pH and TDS with the variation of the compound of formula I dosing (gm/L). (b) % reduction in fluoride concentration with different dosing of the compound of formula I. Figure 4 depicts the effect of dose of the compound of formula I on reaction kinetics for fluoride removal.

Figure 5 depicts settling time and settling velocity measurement for fluoride removal method by employing compound of formula I.

Figure 6 depicts flow diagram of treatment process, using compound of formula I.

DETAILED DESCRIPTION OF THE DISCLOSURE

The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to the compound and methods, together with further objects and advantages will be better understood ftom the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises... a” does not, without more constraints, preclude the existence of other acts or additional acts in tire method.

Throughout the specification, the term ‘compound of formula 1 ’ or ‘compound’ are used interchangeably and are intended to have the same meaning, as regularly known in the field of patents disclosure.

Throughout the specification, tire term ‘coagulants’ or ‘compounds employed for fluoride removal’ or ‘Ammonium aluminum sulphate (AAS)’ or ‘aluminum sulphate-based compound (AS1)’ or ‘Ammonium iron (iii) sulphate (AS2)’ or ‘Poly aluminum chloride (IPC)’ or ‘Poly vinyl alcohol’ or ‘Aluminum based inorganic polymeric liquid compound’ or ‘aluminium based inorganic polymeric compounds’ are used interchangeably and are intended to have the same meaning, as regularly known in the field of patents disclosure.

The present disclosure relates to an aluminium based inorganic polymeric compound of formula I

Compound of formula I wherein ‘X’ is a halide; ‘n’ represents the degree of polymerization wherein l<n<5.

In an embodiment of the present disclosure, the halide is chloride, bromide, fluoride or iodide. In another embodiment of the present disclosure, the halide is chloride.

In another embodiment of the present disclosure, the aluminum based inoiganic polymeric compound of formula IA

Compound of formula IA wherein ‘X’ is chloride;

‘n’ represents the degree of polymerization wherein l<n<5.

In an embodiment of the present disclosure, the aluminium-based inoiganic polymeric compound reduces fluoride concentration < 2 ppm from any fluoride containing water.

The present invention also relates to a process for preparing an aluminum based inorganic polymeric compound of formula I, wherein said process comprising steps of

> grinding polyaluminium chloride (IPC), aluminum sulphate-based compound (AS1) and poly vinyl alcohol (PVA) fallowed by heating to obtain mixture 1 ;

> adding aluminum halide (AlX₃) to the mixture 2 followed by heating and cooling to obtain the aluminum based inorganic polymeric compound of formula I. In an embodiment of the present disclosure, the aluminium halide is aluminium chloride, aluminium bromide, aluminium fluoride or aluminium iodide.

The present disclosure also provides a process for preparing an aluminium based inorganic polymeric compound of formula IA, wherein said process comprising steps of

> grinding polyaluminium chloride (IPC), aluminium sulphate-based compound (AS1) and poly vinyl alcohol (PVA) followed by heating to obtain mixture 1A;

> adding water to the mixture 1A, above the saturation point to obtain mixture 2A; and

> adding aluminium chloride (AlCh) to the mixture 2A followed by heating and cooling to obtain the aluminium based inorganic polymeric compound of formula IA.

In an embodiment of the present disclosure, the heating in step (a) is carried out at a temperature ranging from about 30-50 °C.

In another embodiment of the present disclosure, the heating in step (a) is carried out at a temperature ranging from about 40-45 °C.

In yet another embodiment of the present disclosure, the heating in step (c) is carried out at a temperature ranging from about 40-80°C.

In still another embodiment of the present disclosure, the heating in step (c) is carried out at a temperature ranging from about 40-70°C.

In an embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration ranging from about 25 to 32% wt/wt, aluminium sulphate-based compound at a concentration ranging from about 65 to 72% wt/wt, poly vinyl alcohol at a concentration ranging from about 1 to 1.5% wt/wt and aluminium chloride at a concentration ranging from about 1 to 1.5% wt/wt, wherein the total parts of the components in the process add up to 100 % wt/wt.

In another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30% wt/wt, aluminuium sulphate-based compound at a concentration of about 68 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In an embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 25% wt/wt, about 26% wt/wt, about 27% wt/wt, about 28% wt/wt, about 29% wt/wt, about 30% wt/wt, about 31% wt/wt or about 32% wt/wt.

In another embodiment of the present disclosure, the process employs aluminuium sulphate-based compound at a concentration of about 65 % wt/wt, about 66 % wt/wt, about 67 % wt/wt, about 68 % wt/wt, about 69 % wt/wt, about 70 % wt/wt, about 71 % wt/wt or about 72% wt/wt.

In yet another embodiment of the present disclosure, the process employs poly vinyl alcohol at a concentration of about 1 % wt/wt, about 1.05 % wt/wt, about 1.1 % wt/wt, about 1.15 % wt/wt, about 1.2 % wt/wt, about 1.25 % wt/wt, about 1.3 % wt/wt, about 1.35 % wt/wt, about 1.4 % wt/wt, about 1.45 % wt/wt, or about 1.5 % wt/wt.

In still another embodiment of the present disclosure, the process employs aluminium chloride at a concentration of about 1 % wt/wt, about 1.05 % wt/wt, about 1.1 % wt/wt, about 1.15 % wt/wt, about 1.2 % wt/wt, about 1.25 % wt/wt, about 1.3 % wt/wt, about 1.35 % wt/wt, about 1.4 % wt/wt, about 1.45 % wt/wt, or about 1.5 % wt/wt. In an embodiment of the present disclosure, the total parts of the components in the process add up to 100 % wt/wt.

In yet another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 32% wt/wt, aluminium sulphate-based compound at a concentration of about 66 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 31% wt/wt, aluminium sulphate-based compound at a concentration of about 67 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 29% wt/wt, aluminium sulphate-based compound at a concentration of about 69 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 28% wt/wt, aluminium sulphate-based compound at a concentration of about 70 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 27% wt/wt, aluminium sulphate-based compound at a concentration of about 71 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 26% wt/wt, aluminium sulphate-based compound at a concentration of about 72 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 29 % wt/wt, aluminium sulphate-based compound at a concentration of about 68 % wt/wt, poly vinyl alcohol at a concentration of about 1.5 % wt/wt and aluminium chloride at a concentration of about 1.5 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30 % wt/wt, aluminium sulphate-based compound at a concentration of about 67 % wt/wt, poly vinyl alcohol at a concentration of about 1.5 % wt/wt and aluminium chloride at a concentration of about 1.5 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 29.5 % wt/wt, aluminium sulphate-based compound at a concentration of about 67.5 % wt/wt, poly vinyl alcohol at a concentration of about 1.5 % wt/wt and aluminium chloride at a concentration of about 1.5 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30 % wt/wt, aluminium sulphate-based compound at a concentration of about 67.5 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1.5 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30.5 % wt/wt, aluminium sulphate-based compound at a concentration of about 67 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminium chloride at a concentration of about 1.5 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30.5 % wt/wt, aluminium sulphate-based compound at a concentration of about 67 % wt/wt, poly vinyl alcohol at a concentration of about 1.5 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30 % wt/wt, aluminium sulphate-based compound at a concentration of about 67.5 % wt/wt, poly vinyl alcohol at a concentration of about 1.5 % wt/wt and aluminium chloride at a concentration of about 1 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30 % wt/wt, aluminium sulphate-based compound at a concentration of about 67.5 % wt/wt, poly vinyl alcohol at a concentration of about 1.25 % wt/wt and aluminium chloride at a concentration of about 1.25 % wt/wt.

In still another embodiment of the present disclosure, the process employs polyaluminium chloride at a concentration of about 30.5 % wt/wt, aluminium sulphate-based compound at a concentration of about 67 % wt/wt, poly vinyl alcohol at a concentration of about 1.25 % wt/wt and aluminium chloride at a concentration of about 1.25 % wt/wt.

The present disclosure also relates to a process for preparing an aluminum based inorganic polymeric compound of formula, wherein said process comprising steps of a) grinding polyaluminium chloride (IPC) at a concentration ranging from about 25 to 32% wt/wt, aluminum sulphate-based compound (AS1) at a concentration ranging from about 65 to 72% wt/wt and poly vinyl alcohol (PVA) at a concentration ranging from about 1 to 1.5% wt/wt followed by heating to obtain mixture 1; b) adding water to the mixture 1, above the saturation point to obtain mixture 2; and c) adding aluminum chloride (AlCh) at a concentration ranging from about 1 to 1.5% wt/wt to the mixture 2 followed by heating and cooling to obtain the aluminum based inorganic polymeric compound of formula I.

In another embodiment, the present disclosure provides a process for preparing an aluminum based inorganic polymeric compound of formula LA, wherein said process comprising steps of

> grinding polyaluminium chloride (IPC) at a concentration of about 30 % wt/wt, aluminum sulphate-based compound (AS1) at a concentration of about 68% wt/wt and poly vinyl alcohol (PVA) at a concentration of about 1 % wt/wt followed by heating to obtain mixture 1A;

> adding water to the mixture 1 A, above the saturation point to obtain mixture 2A; and

> adding aluminum chloride (AlCh) at a concentration of about 1 % wt/wt to the mixture 2A followed by heating and cooling to obtain the aluminum based inorganic polymeric compound of formula LA. In an embodiment, the aluminium-based polymeric liquid coagulant was developed in the laboratory to treat fluoride rich water. Experiments for fluoride removal was carried out with different complex compounds (i) Ammonium aluminium sulphate (AAS) (ii) aluminium sulphate-based compound (AS1) (iii) Ammonium iron (iii) sulphate (AS2) (iv) Poly aluminium chloride (IPC) (v) 1% Poly vinyl alcohol (vi) Aluminium based inorganic polymeric liquid compound, which was prepared by using 30% of IPC, 68% of AS1, 1% of PVA and 1% of aluminum chloride.

In another embodiment, the solubility in water of an individual compound was checked. IPC, ASI, and PVA were ground into fine powder and mixed together by following the above-mentioned proportions. The temperature was set at 40-45°C and water was added very slowly above the saturation point, continuing the heating at 40°C till the salts became soluble completely. At this point, a pinch of AlCh was added to avoid crystal formation. Further, heating was increased to 70°C so that the solution became clear. After that, the solution was cooled down to 40°C to obtain a light yellow, slightly viscous liquid compound. Scheme 1 depicts a schematic pathway for the preparation of a compound of formula I.

Scheme 1. Schematic representation of a process for preparing compounds of formula I The present disclosure also provides a method for removal of fluorides from fluoride rich water/wastewater, said method comprising steps of:

> treating the fluoride rich water/wastewater with the aluminium based inorganic polymeric compound of formula I to precipitate or adsorb fluorides as aluminium based inorganic polymeric compound -fluoride complexes; and

In an embodiment of the present disclosure, the fluoride rich wastewater is coke- oven wastewater.

In an embodiment of the present disclosure, wherein the above method further comprises step of mixing (after treating) the fluoride rich water/wastewater with the aluminium based inorganic polymeric compound of formula I, wherein mixing is carried out by maintaining a speed of around 90-100 ppm, followed by slow mixing. The method requires approximately 2-hours for completion.

In an embodiment of the present disclosure, the inorganic polymeric compound- fluoride complex is aluminium fluoride hydroxide complex.

The present disclosure also provides a method for removal of fluorides from coke- oven wastewater, said method comprising steps of:

> treating the coke-oven wastewater with the aluminium based inorganic polymeric compound of formula I to precipitate or adsorb fluorides as aluminium based inorganic polymeric compound - fluoride complexes; and

High fluoride containing wastewater is generated during coke manufacturing. Due to the presence of other cations and anions, removal of fluoride becomes more difficult from such wastewater. Particularly, high sulphate concentration is generally the main constituent interfering with fluoride removal. The characteristics of coke-plant wastewater are mentioned in Table 1.

The present disclosure provides an aluminium-based polymeric coagulant capable of removing fluoride from wastewater by up to 95% using five coagulants as mentioned above.

Experiments for fluoride removal were carried out with different complex compounds (i) Ammonium aluminum sulphate (AAS) (ii) aluminum sulphate- based compound (AS1) (iii) Ammonium iron (iii) sulphate (AS2) (iv) Poly aluminum chloride (IPC) (v) 1% Poly vinyl alcohol (vi) Aluminum based inorganic polymeric liquid compound, which was prepared by using 30% of IPC, 68% of AS 1 and 1% of PVA and 1% of aluminum chloride.

Among these five coagulants, fluoride removal efficiency was higher for IPC in comparison to AAS, AS1 and AS2 and maximum efficiency was observed with the compound of formula I, which performs best without much change in parameters like pH, TDS, etc. The compound of formula I has the capacity of > 95% fluoride removal at a very minimum dose. With increasing doses of the compound, fluoride removal capacity of up to 2 mg/L increases by more than 99%.

In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing fiom the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

EXAMPLES

EXAMPLE 1:

MATERIALS AND METHODS

Preparation of coagulants

AAS, AS 1 , AS2, IPC, and PVA were ground into powder form and stored in airtight containers. Without any further treatment, all coagulants were used for fluoride removal experiments.

Composition/Compound/Coagulant of the present disclosure

Composition for preparing compound of formula I of the present disclosure: 30wt% IPC+ 68% AS1+1% PVA+1% AlCh. The mechanistic route for fluoride removal is depicted in Scheme 2.

Scheme 2. Probable Mechanisms for fluoride removal reaction

Floc formation and fluoride concentration measurement

All coagulants (AAS, AS1, AS2, 1PC, and compound of formula I) were ground finely into powder and used without further treatment or purification. Whenever required, stock solution was prepared first as per desired concentration. All laboratory' tests were carried out using Multiple Stirring Device (Jar Tester) equipped with stirring paddles and provision for controlled mixing. Experiments were carried out using all individual coagulants, which were added directly to the wastewater sample in varying concentrations: 0.5 gm/L, 1 gm/L, 1.5 gm/L, and 2 gm/L with continuous stirring. The floc size and its settleability were observed in each case with the illuminating device. As time progressed, changes in floc size with time were also noted for all experiments.

Using a 1000 ml beaker, the measured volume of sample solution was flocculated using the Jar test apparatus. After placing the beaker in the jar tester, the motor of the stirrer paddles was started. AAS, AS1, AS2, 1PC, and compound of formula I were added, maintaining a speed of around 90-100 ppm, followed by slow mixing. Time and floc formation progress were noted continuously to check reaction kinetics. The experimentation lasted up to 2 hours. The fluoride concentration of treated water was measured using an ion selective method using a Fluoride electrode and an Advanced Electrochemistry metre from Thermo Scientific as the reaction progressed. All analytical detections were carried out following the standard method. The colour changes after treatment with coagulants were noted carefully. The coke plant's discharged water before and after treatment with a compound of formula I (dose 1.5 gm/L) has been depicted in Figure 1. Also, Figure 6 depicts flow diagram of treatment process, using compound of formula I.

TDS AND pH MEASUREMENT

Real coke-plant discharged water was collected from Tata Steel BSL, Angul for all the studies. Total dissolved solids (TDS) and pH of wastewater samples were measured at the beginning of each experiment by using a standard method of measurement with the help of a TDS meter from Thermo Scientific and a pH meter, Thermo Scientific, respectively. TDS and pH were checked as the reaction progressed, and the pattern of change in their values was noted. EXPERIMENTAL RESULTS FOR FLUORIDE REMOVAL

Fluoride removal experiments were carried out by varying the dosing concentration, ranging from 0.5 gm/L to 2 gm/L for all coagulants. Figure 2 describes the pattern of fluoride concentration changes with varying dosages for the cases of all coagulants. It was found that at dosing concentration of 2 gm/L, the fluoride concentration level came down to < Ippm, which was 0.35 ppm in the case of the compound of formula I. A clear difference in fluoride concentration level was observed when a 1.5 gm/L dose was charged for each coagulant (Inset Picture, Figure2). In the presence of compound of formula I, the fluoride concentration of coke-plant discharged water reached 3 ppm, up from 105 ppm, which is within the permissible range.

During industrial application, dosing is carried out in 500 m² dosing tank after 1.5 hrs of settling.

An experimental study was carried out for pH and TDS variation with variation in dosing of the compound of formula I. The initial pH for the tested water was 7.8. Real coke-plant discharge water was collected from Tata Steel BSL, Angul, and an Orion Star bench top pH meter, made by Thermoscientific, was used for pH measurements throughout the experiments. The TDS value for raw water collected from the same coke plant discharge stream was 3000-3030 ppm range, which was tested by using Thermoscientific Orion Star A200. When the compound's (Formula I) dosing was increased from 0.5 gm/L to 2 gm/L, a variation in pH value was observed. The pH of the treated water decreased to 5.7 when the compound of formula I was dosed at l.gm/L, which is allowable, and there were no further changes in pH value even with increased dosing of the compound of formula I (Figure 3(a)).In the case of TDS value, it was observed that with increasing the dosing of the compound of formula I up to 1.5 gm/L, it decreases slightly, which is 2950 ppm. So, the impact of this coagulant on the TDS value of coke-plant discharge water, is very negligible [Figure 3(a)]. Figure 3(b) shows the percentage decrease in fluoride concentration in treated water as the dose of compound in formula I is increased. Furthermore, Figure 3(b) shows that at 0.5 gm/L of compound of formula I dosing, fluoride concentration decreases by 97% and reaches a steady state, while at 2 gm/L of compound of formula I dosing, fluoride concentration decreases by 99% and reaches a steady state.

Further, the effect of the amount of coagulant added has also been studied. In the case of a compound of formula I, dose concentration varied from 0.5 gm/L to 2 gm/L and kinetics was observed up to 90 min from the point of the first addition of coagulant. The first fluoride concentration in treated water, was measured after 10 min from the start of the reaction, and as the reaction progressed, at every 10 min interval, the fluoride concentration was checked up to 90 min. Fluoride concentration was checked using the same ion selective electrode. A sharp fell in concentration was noticed after 10 min from the start of the reaction, and a steady state was achieved afterwards. The fluoride concentration change value followed the same pattern for all three doses applied. Figure 4 is a graph illustrating the effect of coagulant dose on the kinetics of fluoride uptake from Coke-plant wastewater utilising the compound of formula I.

SETTLING TIME AND SETTLING VELOCITY

Settling time experiments were carried out using a compound of formula I only. In this case, it was observed that 20 min of settling was sufficient, and it has been sufficient to render the upper 3/4^th portion of the total volume partially clear even without filtering. It was also found that the first 10 min of settling was fast and was the maximum. Even when operating on a laboratory' scale, drawing-off the treated water can be done easily, after solid is settled or by adding a filtration connection at the bottom of the separation tube. Figure 5 shows images taken during laboratory scale performances of fluoride removal with the compound of formula I, maintaining the concentration of 1.5 gm/L. Table 2 below provides details of settling experiments. Settling time and settling velocity were checked during these experiments. The setting velocity was determined to be 1.15 cm/min.

Table 2. Experimental data for settling velocity measurement

REACTION PATHWAY FOR FLUORIDE REMOVAL BY ALUMINA

BASED ADSORBENT

As the affinity towards cations increases with cationic charges, the order of affinity for Pions towards various cations follows this order: Al³⁺ > Fe²⁺ > Ca²⁺. The different ways F reacts with Al³⁺ are mentioned below:

In water, the solubility of AIF₃ is very poor. However, it becomes completely soluble after its conversion to AIF4-. Formation of various fluoride complexes with Al³⁺is an important reason for the adsorption of fluoride from water samples. Ionic forms of aluminium fluoride complexes are soluble in water. Thus, to precipitate the AIF4-, a cation is essential. Ca2+ plays an important role following the reaction Ca2+ + A1F4 — > Ca(AlF4)2, where Ca(AlF4)2 is the precipitate. Additionally, Ca²⁺ also has a high affinity towards F- . Therefore, the presence of both Ca²⁺ and Al³⁺ in the system, can enhance the F- removal from wastewater.

An aqueous solution of fluoride ions containing activated alumina may not be clearly soluble and form various aluminium species, including several fluoride and hydroxyl-aluminium complexes. The spontaneous interaction of Al³⁺ ions with hydroxide ions (OH-) in water results in the formation of insoluble Al(OH)₃ flocs. Adsorption of the fluoride ion by Al(OH)₃ results in the formation of aluminium fluoride hydroxide complexes [Al_nF_m(OH)3_n-m]. In addition, fluoride ions can also be removed from water by co-precipitation of Al³⁺, F and OH- ions. Both the approaches, like adsorption and co-precipitation reaction mechanisms, have been mentioned in Scheme 1. In tire present disclosure, aluminium based inorganic polymeric compounds were used along with aluminium sulphate in 70:30 percentage ratio. During investigation, it was found that IPC has greater efficiency than other aluminium sulphate salts used. To optimise the cost of the total process for industrial use, an optimum composition of IPC and AS1 has been established, which works very effectively in F removal from coke-plant discharge water.

Scheme 3: Probable reaction scheme for F- adsorption and co-precipitation

Reversibility of reaction

In the case of compound of formula I, reversibility of 5-10% was observed during the compound's formation reaction. Reversibility can be avoided by adding more IPC.

Equivalents

With respect to the use of substantially plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. Various singular/plural permutations may be expressly set forth herein for the sake of clarity. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as ‘open" terms (e.g., the term incl‘uding" should be interpreted as including‘ but not limited to," the term h‘aving" should be interpreted as hav‘ing at least," the term ‘includes" should be interpreted as incl‘udes but is not limited to," etc.).

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

We claim:

1. An aluminum based inorganic polymeric compound of formula I

Compound of formula I wherein ‘X’ is a halide;

‘n’ represents the degree of polymerization wherein l<n<5.

2. The compound as claimed in claim 1, wherein the halide is chloride, bromide, fluoride or iodide.

3. The compound as claimed in claim 1, wherein the halide is chloride.

4. A process for preparing an aluminum based inorganic polymeric compound of formula I as defined in claim 1, wherein said process comprising steps of

> grinding polyaluminium chloride (IPC), aluminum sulphate-based compound (AS1) and poly vinyl alcohol (PVA) followed by heating to obtain mixture 1;

> adding aluminum halide (AIX₃) to the mixture 2 followed by heating and cooling to obtain the aluminum based inorganic polymeric compound of formula I.

5. The process as claimed in claim 4, wherein the aluminium halide is aluminium chloride, aluminium bromide, aluminium fluoride or aluminium iodide.

6. The process as claimed in claim 4, wherein the heating in step (a) is carried out at a temperature ranging from about 30-50 °C.

7. The process as claimed in claim 4, wherein the heating in step (a) is carried out at a temperature ranging from about 40-45 °C.

8. The process as claimed in claim 4, wherein the heating in step (c) is carried out at a temperature ranging from about 40-80°C.

9. The process as claimed in claim 4, wherein the heating in step (c) is carried out at a temperature ranging from about 40-70°C.

10. The process as claimed in claim 4, wherein the process employs polyaluminium chloride at a concentration ranging from about 25 to 32% wt/wt, aluminum sulphate-based compound at a concentration ranging from about 65 to 72% wt/wt, poly vinyl alcohol at a concentration ranging from about 1 to 1.5% wt/wt and aluminum chloride at a concentration ranging from about 1 to 1.5% wt/wt, wherein the total parts of the components in the process add up to 100 % wt/wt.

11. The process as claimed in claim 4, wherein the process employs polyaluminium chloride at a concentration of about 30% wt/wt, aluminum sulphate-based compound at a concentration of about 68 % wt/wt, poly vinyl alcohol at a concentration of about 1 % wt/wt and aluminum chloride at a concentration of about 1 % wt/wt.

12. A method for removal of fluorides from fluoride rich water/wastewater, said method comprising steps of:

> treating the fluoride rich water/wastewater with aluminum based inorganic polymeric compound of formula I as defined in claim 1 to precipitate or adsorb fluorides as aluminum based inorganic polymeric compound -fluoride complexes; and

> separating the aluminum based inorganic polymeric compound- fluoride complexes.

13. A method as claimed in claim 12, wherein the inorganic polymeric compound-fluoride complex is aluminum fluoride hydroxide complex.