GB2184716A - The production of metal halohydrates - Google Patents

Abstract

Metal halohydrates, for example aluminium chlorohydrates e.g. having a % basicity of from 50 to 85 are produced by suitably controlled heating of a metal halide, for example aluminium chloride in the form of an atomised aqueous solution. Suitably aluminium chloride liquor is dried in finely divided form at an inlet temperature of from about 500 DEG C to about 1200 DEG C and so as to avoid complete loss of chlorine. The product is useful for water treatment or for other known uses of metal halohydrates such as sewage conditioning. <IMAGE>

Description

SPECIFICATION The production of metal halohydrates This invention relates to the production of metal hal ohydrates the term "metal halohydrate" being used to denote compounds having a general formula corresponding to a metal halide in which a deficiency in halogen atoms is counterbalanced, at least partly by a content of hydroxyl ions.

Some halohydrates, for example, are used asfloc culantsforthe removal of suspended matter andior colourfrom water and particularly where the water is intended for human consumption it is importantthat impurities should not be introduced into it by vi rtue of the use of impure treating agents. Metal halohydrates are also commonly used for sewage conditioning and fortanning.

United States Patent No. 3544476, discloses that aluminium, chromium or iron chlorohydrates (des cribedtherein by the synonym basic aluminium, chromium or iron chloride) are usable asflocculants in water treatment. Aluminium chlorohydrate is acknolwedged as being a most effective flocculant and this is confirmed by a substantial body of published literature relating to its production and usefor this purpose.

There is a requirement, therefore, for pure metal halohydrates, more particularly for pure iron, chromium or, pre-eminently, aluminium chlorohydrates.

Metal chlorohydratesfor use asflocculants have been produced from pure starting materials. In United States Patent No. 3544476 for example, aluminium chlorohydrate is disclosed to be produced from starting materials such as aluminium powder (purity 99.98%) and hydrochloric acid (2N) which are inherently of ow impurity content.

USSR Patent No. 747816 discloses the production of aluminium chlorohydrate from aluminium chloride hexahydrate crystals by exposing the crystals to a stream of air having a temperature, critically, of between 1 7Q3C and 1 90 C, there being a sharp drop of productivity at temperatures below 1 700C and a complete loss of chlorine attemperatures above 1900. The precrystallisation of the aluminium chloride hexahydrate reduces the inherent impurity content of the product.

There are available aluminium chloride aqueous liquors having concentrations of upto about 10% or 11% by weight, calculated as Al203, which liquors may be produced as by products from, for example, organic syntheses using Friedel-Crafts/cata lysts.

Such liquors often contain organic compounds, often #'n#dentified or only partly identified, which may amount to a total quantity, in terms oftotal organiccarbon (TOC) of up to over 2000 parts permillion (0.2%) TOC calculated as C. Such an impurity contentwould normally eliminate such a liquorfrom consideration as a raw material for the direct production of a water treatment chemical and intervening pun#cation techniques such as, for example, mult- iple recrystallisation, would be considered necescry.

Tha present invention provides a process for the direct production of a metal halohydratefrom a liquorcontaining organic impurities, usually ata level above 500 parts per million and often even above 2000 parts per million calculated as C, to produce a relatively pure product.

According to the present invention there is provided a processforthe production of a metal halohydrate by the heat decomposition ofthecor- responding metal halidecharacterised inthatan aqueous liquor containing the metal halide is contacted in atomised form with a gas having a temperature, immediately before contact with the liquor, of above 1900C, thereby to evaporate liquid from the liquor and to at least partially decompose the metal halide to produce a particulate product, the residence time ofthe liquor in contact with the gas being controlled to avoid complete loss of halidefrom the particulate product, and in thatthe particulate product is recovered.

it is recognised that there are a number of operating parameters relevant to the direct contact heating of an atomised spray liquid,forexample in the form of a spray of droplets of the liquid, with a gas. Due to the effect of evaporation of water present, the effect of normal heat losses there is a downward temperature gradient between the point of contact ofthe liquid by the hot gas and the point at which there sulting product is separated from the hot gas.This temperature gradient is affected by the relative quantity of liquor being sprayed which will usually, given a particular nozzle conformation, be a fuction of spray velocity, as well as by the velocity of the hot gas and whether the hot gas is directed concurrently with or countercurrently to the spray of liquid. It is within the normal skill of those in the art of spray drying to combinethese and other operational parameters to achieve a desired combination of inlettemperature and residence time to achieve a desired degree of heating.

In order to achieve a satisfactory degree of control overthe heating process it is highly desirable to use a reactorwhich allows the establishment ofashort and measurable residence time. Particularly prefer redforthispurposeisaspraydrierora pneumatic conveyor drier designed for contact times of less than 60 seconds and capable of achieving contact times of below 15 seconds for example from 4 seconds to below 15 seconds, since such contact times are generally, although not necessarily invariably, required.

The inlet temperature, of the hot gas is preferably at least 5000C particularly preferably at least 7000 and is, further, preferably less than about 1 200 C, part icularly preferably less than 1 0000C. Attemperatures above 1 0000C there is an increasing tendency for some alumina to be formed although useful results may be obtained at up to about 1 200 C.

The outlet tem peratu re of the hot gas, now mixed with water vapour and hydrochloric acid vapour, is preferably above 15000. It will generally be found de- sirableforthe outlettemperatureto exceed this and to be at least 17500. It is pointed out however, that the majority ofthe residence time of the metal halide in the reactorwill be at a considerably highertemperature certainly above 19000 as a result of the inlet temperature used.

The characteristics ofthe metal halohydrate product may be controlled at least to an extent by control of the heating process. The general formula of such a product may be expressed as Me(OH)mXn m.yH2O, or a multiple thereof, where Me is a metal atom of valency n, X is a halogen atom, mis a number than n andy is a number at least equal toO.Thusthecompositionofaluminiumchlorohyd- rate, forexample, may be expressed as Al(OH)rnCl3 rn.yH2O where m is from 1 to below 3. The hydroxyl content of the product, its "basicity", is expressed as a % ofthetheoretical content of the corresponding metal hydroxide.A commercially acceptable solid aluminiumchorohydrateshould have an aluminium content, expressed as Awl203, of at least 40% by weight and the number ofmolecules of water of crystallisation in a commercial chlorohydrate product is preferably restricted accordingly.

The performance of metal halohydrates intended forwatertreatmentuses,forexample iron oraluminium halo hydrates, particularly chlorohydrates, is dependentonthe basicityofthe product. In general basicities of from about 30% to about 85% are contemplated bythis invention. Above about85%the product tends to become unstable although products having basicities up to about 90% would still be regarded as within the present invention. Below about 30% the product becomes little more effective in use, than aluminium chloride. The preferred range of basicities is from about 50% to about 80%.

In controlling the heating process to achieve the required basicity, which may be required to be iimi- ted narrowlyto achieve consistency of product between successive production batches, it is preferred to perform one or more "sighting" runs followed by the analysis ofthe productfor its content of halogen and aluminium and the adjustmentofthe parameters of operation ofthe reactor, for example, its inlettemperature, or the rate of gas flow through the reactor, therefore controlling the residence time of the metal halide in the reactor, to identify the appropriate parametersforthe production of a product having a given basicity.It is feature of the present invention thatthe process parameters, e.g.the inlet temperature and/orthe residence time may be con trolledin response to the halogen and aluminium contents ofthe product to achieve a desired basicity andtoavoidtheformationofan undue quantity of metal oxide or indeed, to avoid the complete conversion ofthe metal chloride into metal oxide which can otherwise readily occur atthe temperatures involved.

The metal halide aqueous liquor used as a raw material for the present process is preferably re .actively concentrated to reduce the heat load on the process and enable a suitably short residence time to be achieved. Preferably such solutions contain metal halide in at least 1%,forexample upto about 10% parxicularly suitably in from about 5% to about 10% byv sei~ht calculated as A1203. Aqueous metal halide liquors may contain a proportion of the corresponding acid. Aluminium chloride solutions as described shave may typically contain from 1% to 3% of hydro choloric. acid.This is not detrimental to the use of the liquor as a raw material in the practice ofthis invention.

A major benefit of the invention isthatorganicim- purities present in the liquor are reduced in quantity in the metal halohydrate product. Such impurities may be presentfor, examplewherethe aluminium chloride is recovered from waste Friedel-Crafts catalyst.

The metal halohydrate productofthe invention is in solid finely divided form and can have an average particle size less than 10 microns, which product may be separated from the gases leaving the reactor by means of a cyclone. Due to the cooling effect of vaporisation the temperature of such solids and of the gases in which they are, or have been, in contact will generally be within the range suitable for pro- cessing byavailablehightemperaturecycloneequi- pment having, for example, stainless steel dust collectors.

The residual gases may contain a preponderance ofwatervapour, a substantial quantity of hydrogen halide, combustion gases ifthe reactor has been heated by the direct use of combustion gases, inert gases derived from air used for combustion if this was the case, and organic compounds derived from impurities contained in the metal chloride liquor.

Halide gas may be recovered in the form of a liquor ofthe acid by the use of a wet scrubber and this liquormay, if requiredforparticularend uses, be further purified by known means.

The product of the invention is a dry free-flowing finely divided easily water-soluble metal halohydrate suitable for water treatment or other known applications such as sewage conditioning and is par tics marly suitable as a raw material for the production of highly pure alumina according to the process of our copending patent application of even date.

The invention will now be illustrated by means of the following specific Examples.

The raw material used forthe Examples was an aqueous aluminium chloride liquor having a concentration equivalent to 10.76% expressed as A1203 an acidity of 1.69% calculated as HCI and a total organic carbon content, due to the presence of unidentified organic impurities, of 2035 parts per million.

The reactor was a spray drier produced by Drytec Limited comprising a vertical chamberwith an inlet for liquor comprising a spray head at the top and an outlet for gases and entrained solids at the bottom.

The furnace was fired by passing into it gases from the combustion of propane at a level just below the spray head. The spray drier was equipped with normal control means to enable the temperature and the quantity ofthe combustion gas, the pressure of the spray and therefore the feed rate of liquorto be varied.

The runs described below by way of Examples were performed each followed by examination ofthe solid reactor product recovered by the cyclone. Example 2 is according to the invention but Examples 1 and 3 are not according to the invention and are in sertedforcomparative purposes only.

Example 1 The inlettemperature of the spray drierwas controlled at 460"C and the residence time at 8 seconds.

The outlettemperature was 1 60"C. The solid product which had an averageparticlesizeof40 micronswas an aluminium chlorohydrate containing 25% by weightofaluminiumexpressedasAl2O3and 15.8% by weight of chlorine. This analysis corresponds to the general formula Al(OH)2.1Clc.s.6.1 H20 Although this solid product had a basicity of 70% it was, due to its low aluminium content, unsuitable as a commercial product.

Example2 Using the same flow rates and therefore residence time the inlet temperature was raised to 7750C. The outlettemperature was 1 94"C. The solid product was free flowing and had an average panticle size of less than 10 microns, a composition analysis corresponding to the formula Al(OH)2.07C10.93.0.4H2O corresponding to aluminium content, expressed as Awl203 of 49.8% and a basicity of 69% and a total organic carbon content of 177 parts per million. The product dissolved readily in water.

Example 3 To illustrate the effect of an increased residence time the product of Example 2 was fed back into the spray drier using a powderfeeder, the supply of liquor being stopped. The residence time of the pow derinthe drierwas 8 seconds making a total ex posuretime of 1 seconds. The drier inlettem- perature was 830 C and the drier outlettemperature was 31 0 C. The product was insoluble in both water and dilute nitric acid and gave an analysis of 89.1% byweightofaluminium expressed as A1203 and 10.8% ofchloride. This product was effectively an alumina.

Claims (12)

1. A process for the production of a metal halohydrate by the heat decomposition ofthecorresponding metal halide characterised in that an aqueous liquor containing the metal halide is contacted in atomised form with a gas having a temperature, immediately before contact with the liquor, of above 1 90#C, thereby to evaporate liquid from the liquor and to at least partially decompose the metal halide to produce a particulate product, the resid encetimeofthe liquor in contact with the gas being controlled to avoid complete loss of halide from the particulate product, and in thatthe particulate product is recovered.

2. A process as claimed in claim 1 wherein the metal halide is aluminium chloride and the metal hal ohydrate is aluminium chlorohydrate.

3. Aprocessasclaimedinclaim 1 or 2 wherein the metal halide liquor is contacted with the gas having a temperature, immediately before contact, of from 500into 12000C.

4. A process as claimed in claim 3 wherein the gas has a temperature, immediately before contact with the liquor, of from 700 C to 1 000 C.

5. A process as claimed in any preceding claim wherein the residence time of the liquor in contact with the gas is from 4 seconds to 60 seconds.

6. A process as claimed in claim 5 wherein the said residence time is from 4 seconds to 15 seconds.

7. A process as claimed in any preceding claim wherein the metal halide liquor has a concentration of up to 10% by weight, calculated as A1203 and contains at least 500 parts per million by weight oftotal organic carbon.

8. A process as claimed in claim 7 wherein the metal halide liquor contains at least 2000 parts per million oftotal organic carbon.

9. A process as claimed in any preceding claim wherein the particulate product is a halohydrate having a basicityoffrom 50%to80%.

10. A process as claimed in any preceding claim conducted in a spray drier.

11. A process as claimed in claim 1 and substantially as described herein with reference to the Examples.

12. The product of a process as claimed in any preceding claim in the form of particulate aluminium chlorohydrate having a total organic carbon content below 500 parts per million.