WO2006123284A2

WO2006123284A2 - Production of hydrotalcite

Info

Publication number: WO2006123284A2
Application number: PCT/IB2006/051514
Authority: WO
Inventors: Frederik Johannes Willem Jacobus Labuschagne; Ernst Werner Giesekke; Johannes Daniel Van Schalkwyk
Original assignee: Labuschagne Frederik Johannes; Ernst Werner Giesekke; Johannes Daniel Van Schalkwyk
Priority date: 2005-05-19
Filing date: 2006-05-15
Publication date: 2006-11-23
Also published as: ZA200709947B; WO2006123284A3

Abstract

This invention relates to a process for producing hydrotaicite of the general formula: AwBx(OH)yCz.nH2O wherein A is a bivalent metal cation; B is a trivalent cation; and C is a mono or multivalent anion and: 0 < z ≤ x ≤ 4 ≤ w ≤ 1/2y and 0 ≤ n ≤ 12. The process includes the steps of providing a particulate compound selected from the group consisting of bivalent and trivalent metal oxides, hydroxides and carbonates; mixing the particulate compound with a water source and a carbonate source in the form of an alkali metal bicarbonate, to form a sludge, and subjecting the sludge to hydro/thermal treatment to induce crystallisation and formation of hydrøtaldte. in the case that the bivalent meÊal is Mg, run- of-roine mineral rrtagnesrte, is used as raw material for the production of hydrotafcfe.

Description

PRODUCTION OF HYDROTALCITE

INTRODUCTION TO THE INVENTION

This invention relates to a process for producing hydrotalcite, More particularly, but not exclusively, this invention relates to a process for producing hydrotalcite having virtually zero effluent and utilising mainly run-of-mine mineral magnesite.

DESCRIPTION OF THE STATE OF THE ART

Three different methods for the synthesis of synthetic hydrotalcite have been described in literature and there are several patents describing these preparation methods.

The oldest known method follows the so-called co-precipitation route. Water soluble salts of bivalent and trivalent metals are used and co-precipitated in the presence of the chosen anion in an alkaline medium to form hydrotalcite. A hydrotalcite of high purity and quality is produced in this way. The disadvantage of this method is the targe volumes of salt-containing effluent that must be disposed of and the cost of raw materials and equipment. Preparing hydrotalcite from metal sails is an ineffective and much more expensive route than preparing it from metal oxides. Furthermore, the most soluble, most common and cheapest metal salts are typically metal chlorides, Using metal chlorides require highly corrosion resistant equipment; which is very costly,

A second known method for the preparation of hydrotalcite is the so-called salt-oxide method, in this method, typically the bivalent metal salt is replaced by a metal oxide such as MgO, and reacted with a metal salt of a trivalent metal. Using a metal oxide of the bivalent metal reduces the cost of raw materials and the salt concentration of the effluent. However, a disadvantage of this method is that if the chloride salt of the trivalent metal is used, expensive corrosion resistant equipment must still be used.

The third known method refers to the use of oxides, hydroxides or carbonates of both the bivalent and trivalent metals, and is the so-called two powder method. This method further reduces the raw material costs. The raw materials should however be of highly reactive grades. An effluent with tow salt concentration is also produced.

However, one of the problems associated with this method is the solubilisation of the trivalent metal if the oxide, hydroxide or carbonate of aluminium is used. This could be overcome by using amorphous alumina (e.g. rehydratable or activated alumina, or pseudoboehmite); or by treating the oxide or hydroxide of aluminium with sodium hydroxide (to produce soluble sodium aluninate). Other disadvantages of this method are the tow conversion rates and reduced purity and quality of the produced hydrotalcite.

Known methods for producing hydrotalcite are disclosed in the following USA patents/applications: US2004/0141S07A1 ; US5,578,286: US5,250,279 and US4,904,457; the first two being the most relevant.

US 2004/0141907A1 discloses a process for the production of a fine hydrotalcite according to the two powder method. The alleged novelty of the mentioned process is the in situ milling of either or both the feed oxides, hydroxides or carbonates of the bivalent- and/or trivalent metals in either the presence or absence of the soda or bicarbonate solution. This milling leads to the formation of a reactive raw material mixture (including the aluminium source) which in turn leads to increased yields. There is thus no need to solubilise the aluminium oxide or -hydroxide.

Some of the disadvantages of the above process are that soms of the costs saved by using the oxides, hydroxides or carbonates are tost in the cost of milling; and a salt containing effluent (that needs to be treated and disposed of) is still produced. US 5,578,286 discloses a similar method as disclosed in US2004/014/907 A1 , with the exception that no milling is undertaken and thus reactive grades of raw materials must be used. As is evident from the examples, a disadvantage of this method is that hydrotalcite is produced only when rehydratable or amorphous alumina (e.g. pseudoboehmite) is used, Another disadvantage of this method is the salt containing effluent produced as by product, which needs to be disposed of.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide a process for the production of hydrotalcite with which the aforesaid disadvantages can be overcome or at least minimised.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a process for producing hydrotalcite of the genaral formula:

A_wB_x(OH)_yC₂nH₂O

wherein A is a bivalent metal cation: B is a trivalent cation; and C is a mono or multivalent anion and: 0 < z ≤ x ≤ 4 ≤ w ≤ ½y and 0 ≤ n ≤ 12,

the process including the steps of: - providing a particulate compound selected from the group consisting of bivalent and trivalent metal oxides, hydroxides and carbonates; mixing the particulate compound with a water source and a carbonate source in the form of an alkali metal bicarbonate, to form a sludge; and subjecting the sludge to hydrothermal treatment to induce crystallisation and formation of hydrotalcite.

Further according to the invention, the process includes the steps of; - subjecting the sludge to filtration, to form a filter cake, with the filtrate contributing to the said source of water; washing the filter cake, with the wash effluent further contributing to the said water source; and drying the filter cake, the arrangement being such that the filtrate and the wash effluent are recirculated to the said mixing step to limit effluent from the process. Further according to the invention the process includes the step of calcining the filter cake to drive off water of crystallisation.

The bivalent metal (A) may be selected from the group consisting of Mg²⁺, Zn²⁺, Ca²⁺ _, Co²⁺, Ni²⁺, Fe²⁺ and Cu²⁺.

Preferably the bivalent metal is selected from the group consisting of Mg²⁺ and Zn²⁺.

The bivalent metal (B) may be selected from the group consisting of Al³⁺, Co³⁺, Ni³+, Fe³⁺ and B³⁺,

Preferably the trivalent metal is Al³⁺.

The anion (C) may be selected from the group consisting of borates, OH , Cl , and CO₃ ^2-.

Preferably the anion (C) is CO₃ ^2-.

The process may include the further step of, in the case that the bivalent metal is Mg, calcining run-of- mine mineral magnesite, to produce the magnesium oxide, -hydroxide or -basic carbonate and CO₂. The process may include the even further step of producing the alkali metal bicarbonate from an alkali metal carbonate, with the CO₂ produced in the step of calcining the run-of-mine mineral magnesite being used in the step of carbonating the alkali metal carbonate to form the alkali metal bicarbonate.

More particularly, during the calcination step, run-of-mine mineral magnesite may be calcined at a temperature of from 800°C to 1000°C, preferably 900°C for a period of from 1 minute to 20 minutes, preferably 10 minutes to produce reactive magnesium oxide (MgO) and carbon dioxide (CO₂).

Further particularly, CO₂ may be bubbled through the filtrate and wash effluent of the previous batch, forming a solution containing sodium carbonate (Na₂CO₃), until the solution's pH is between 7 and 7.5

Even further particularly, the carbonation step may include the step of adding make-up water to the solution.

The step of providing MgO may include the step of simultaneously providing aluminium trihydrate (ATH),

This sludge may be charged to an autoclave for a period of from 4 hours to 6 hours, preferably 5 hours of hydrothermal treatment at a temperature of from 140°C to 200°C, preferably 170°C and at a pressure of from 10 bar to 14 bar, preferably 12 bar, to form a hydrotalcite.

After the hydrothermal treatment step, the process may include the further steps of discharging the solution from the autoclave, filtering the solution, and washing the filter cake.

According to a second aspect of the invention there is provided hydrotalcite produced by the process of the first aspect of the invention.

8RIEF DESCRIPTION OF THE DRAWING

The invention will now be described further by way of a non-limiting example with reference to the accompanying drawing which is a schematic flow chart illustrating a method according to a preferred embodiment of the invention for producing hydrotalcite,

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In a preferred embodiment of the invention, as illustrated in the drawing, a process for producing hydrotalcite of the general formula:

A_wB_x(OH)_y C_z,nH₂O 0

The process incudes tns further step of, in the case that the bivalent metal is Mg, calcining run-of-mine mineral magnesite, to produce the magnesium oxide, hydroxide or basic carbonate and CO₂.

The process includes the even further step of producing the alkali metal bicarbonate from an alkali metal carbonate, witn the CO₂ produced in the step of calcining the run-of-mine mineral magnesite being used in the step of carbonating the alkali metal carbonate to form the alkali metal bicarbonate.

More particularly, during the calcination step. 3054 g run-of-mine mineral magnesite is calcined at 900°C for approximately 10 minutes to produce 1460 g reactive magnesium oxide (MgO) and 1605 g carbon dioxide (CO₂).

Further particularly, approximately 642 g of CO₂ is bubbled through the flitrate and wash effluent of the previous batch, forming a solution containing approximately 985 g of sodium carbonate (Na₂CO₃), until the solution's pH is between 7 and 7.5.

Even further particularly, during the carbonatlon step, make-up water is added to the solution until the solution volume is approximately 12 C, 1460 g of MgO is added to the solution along with 1413 g aluminium trihydrate (ATH). This sludge is charged to an autoclave for a period of 5 hours of hydrothermal treatment at a temperature of 17O°C and at a pressure of -12 bar. After the hydrothermal treatment, the hydrotalcite is discharged from the autoclave, filtered and the filter cake washed. The Na₂CO₃ containing filtrate and wash effluent is recycled as described above. The produced hydrotalcite is dried and optionally calcined. Yields of 95% hydrotalcite were obtained and these compare highly favourably with the prior art methods.

As an alternative, it is possible to replace some or all of the MgO with zinc oxide (ZnO).

Advantages

The method according to the present invention uses run-of-mine magnesite (MgCO₃) as raw material for the production of hydrotalcite. This is the relatively cheapest magnesium raw material that is generally available. The CO₂ produced as a by-product in the preparation of a reactive magnesium oxide by heating the magnesite is used further down in the process,

Furthermore, the described method lor the production of hydrotalcite is effluent free. The salt containing effluent stream is fully recycled. The sodium carbonate (Na₂CO₃) in the recycled effluent stream is reacted with the CO₂ produced from calcination of the magnesite to produce sodium bicarbonate (NaHCO₃). This is fed into the autoclave with the other raw materials and make-up water.

Thi is a clean, zero effluent, environmentally friendly method for the production of hydrotalcite using cheap, rur-of-mine raw materials.

It will be appreciated that variations in detail are possible with a process for producing hydrotalcite according to the invention without departing from the scope of the appended claims,

Claims

1. A process for producing hydrotalcite of the genera! formula:

wherein A is a bivalent metal cation; B is a trivalent cation; and C is a mono or multivalent anion and:

the process including the steps of: providing a particulate compound selected from the group consisting of bivalent and trivalent metal oxides, hydroxides and carbonates; mixing the particulate compound with a water source and a carbonate source in the form of an alkali metal bicarbonate, to form a sludge; and subjecting the sludge to hydrothermal treatment to induce crystallisation and formation of hydrotalcite.

2. A process according to claim 1 which includes the steps of: subjecting the sludge to filtration, to form a filter cake, with the filtrate contributing to the said source of water; washing the filter cake, with the wash effluent further contributing to the said water source; and drying the filter cake, the arrangement being such that the filtrate and the wash effluent are recirculated to the said mixing step to limit effluent from the process.

3. A process according to claim 2 which includes the step of calcining the filter cake to drive off water of crystallisation.

4. A process according to any one of the preceding claims wherein the bivalent metal (A) is selected from the group consisting of Mg²⁺, Zn²⁺, Ca²⁺, Co²⁺, Nl^2+, Fo²⁺ and Cu²⁺.

5. A process according to claim 4 wherein the bivalent metal is selected from the group consisting of Mg²⁺ and Zn²⁺.

6. A process according to any one of the preceding claims wherein the trivalent metal (B) is selected from the group consisting of Al³⁺, Co³⁺, Nl³⁺, Fe³⁺ and B³⁺.

7. A process according to claim 8 wherein the trivalent metal is Al³⁺.

8. A process according to any one of tho preceding claims wherein the anion (C) is selected from the group consisting of borates, OH , Cl , and CO₃ ².

9. A process according to claim 8 wherein the anion (C) is CO₃ ²,

10. A process according to claim 5 or any one of claims 6 to 9 insofar as they are dependant on claim 5 which includes the further step of, in the case that the bivalent metal is Mg, calcining run-of-mine mineral magnesite, to produce the magnesium oxide, -hydroxide or -basic carbonate and CO₂.

11. A process according to claim 10 which includes the even further step of producing the alkali metal bicarbonate from an alkali metal carbonate, with the CO₂ produced in the step of calcining the run-of- mine mineral magnesite being used in the step of carbonating the alkali metal carbonate to form the alkali metal bicarbonate.

12. A process according to claim 11 wherein the run-of-mine mineral magnesite is calcined at a temperature of from 800°C to 1000°C for a period of from 1 minute to 20 minutes to produce reactive magnesium oxide (MgO) and carbon dioxide CCO₂).

13.A process according to claim 12 wherein the run-of-mine mineral magnesite is calcined at a temperature of 900°C for a period of 10 minutes to produce reactive magnesium oxide (MgO) and carbon dioxide (CO₂).

14. A process according to claim 12 or claim 13 wherein the CO₂ is bubbled through the filtrate and wash effluent of the previous batch, forming a solution containing sodium carbonate (Na₂CO₃), until the solution's pH is between 7 and 7,5

15. A process according to any one of cfalms 11 to 14 wherein the carbonation step includes the step of adding make-up water to the solution.

16- A process according to any one of the preceding claims wherein the step of providing MgO includes the step of simultaneously providing aluminium trihydrate (ATH).

17, A process according to any one of the preceding claims wherein the sludge is charged to an autoclave for a period of from 4 hours to 6 hours of hydrothermal treatment at a temperature of from 140°C to 200°C, and at a pressure of from 10 bar to 14 bar, to form a hydrotalcite.

18. A process according to claim 17 wherein the sludge ts charged to the autoclave for a period of 5 hours of hydrothermal treatment at a temperature of 170°C and at a pressure of 12 bar, to form a hydrotalcite.

19. A process according to claim 17 or claim 18 wherein after the hydrothermal treatment step, includes the further steps of discharging the solution from the autoclave, filtering the solution, and washing the filter cake.

20. Hydrotalcite produced by the process of claims 1 to 19.

21. A process substantially as herein described with reference to tie accompanying figure.

22. Hydrotalcite produced by the process substantially as herein described with reference to the accompanying figure.