AU634504B2 - Foam control - Google Patents

Description

63450 4 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class In Application Nurber: Lodged: Form it. Class Complete Specification Lodged: Accepted: Published: S ority Priority o 0 o Related Art 00 Soo 0 %Name of Applicant INDUSTRIAL 0 0 00 MINERALS RESEARCH AND DEVELOPMENT PTY. LTD.

Address of ApplicantCurtin University, School of S- o 6102, Australia Applied Chemistry, Bentley, Western Australia 00 Actual Inventol: MARK KEENEY ard GARY CONSTANTINE 0 0co0 Qoco Address for Service 0 Aldress for Service WATERMARK PATENT TRADEMARK ATTORNEYS.

290 Burwood Road, Hawthorn, Victoria, Australia Complete Specification for the invention entitled: FOAM CONTROL The following statement is a full description of this invention, including the best method of performing it known to -2 FOAM CONTROL This invention is concerned with reducing or substantially eliminating the formation of foam in alumina processing circuits.

Foaming in the alumina industry in Bayer liquors poses a severe problem both to productivity and to equipment used throughout the process. In particular, Australian alumina refineries have foaming problems far in excess of their European or American counterparts owing to the nature and origins of the bauxite. Foam can cause large hydrate scale build-up on vessels which can lead to process problems, including mechanical failure to equipment, and it can affect the agglomeration of alumina particles in precipitation.

°0°15 The manufacture of alumina is generally 0 accomplished by the Bayer process. This operation involves 0 eo 0 0 the digestion of bauxite by sodium hydroxide solutions at "000 150 200 C to convert aluminium hydroxides to sodium o o0 aluminate.

0ooo020 After decantation or filtration of solid residues, the sodium aluminate liquor undergoes a decomposition according to: AlO Na 2HO0 Al(OH), NaOH 00 0 2 2 3 0 00 0 O '0 This precipitation reaction is triggered by the addition of significant quantities of alumina trihydrate three to 0 0 five times the quantity to be precipitated) of suitable size 000000 distribution to act as seed.

.0.30 The size distribution of these seed particles must a satisfy two conflicting requirements; 0o To increase precipitation rate, a small particle size is required to maximise the total surface area in the precipitators.

To enhance solid/liquid separation, a coarse particle size is required.

7 3 During the extraction of alumina fronr. bauxite using the Bayer process a number of organic reagents may be added to the liquors to facilitate processing, e.g. crystal modifiers, drainage aids and antifoams. The effect of these reagents is frequently a result of interaction between the organic compound and the surface of the alumina. The adsorption by the mineral of the reagents added alters the surface chemistry of the alumina and can create a hydrophobic surface.

In separate processing operations the association between these reagents and the alumina can have undesirable consequences. A hydrophobic coating is formed over the surface of the particle and where there is sufficient air present to form a foam, the particle can be carried up into the foam in a manner similar to the technique of froth o flotation.

,The froth formed from this reaction is persistent and does not respond to treatment with conventional to, antifoams, some antifoams, because of their affinity for the hydrate surface can in fact exacerbate the problem by contributing to the stability of the hydrate froth.

The control of foaming of Bayer liquors is of great importance to alumina producers. The conditions under which the alumina is processed give rise to severe foaming problems when the bauxite contains impurities, particularly St organic impurities. Thus, European bauxites, which contain little or no organic matter, cause little or no foam, whereas the laterite bauxites of Australia and Africa contain high levels of organics leading to the formation of significantly stable foams. With the increased use of Australian and African bauxites, which are up to 60% of the known world reserves, a distinct process problem has arisen and must be dealt with in the easiest and cheapest way possible.

This presently involves foam breakage by chemical or mechanical means, or by modification to existing unit operations to ensure that foam production is minimised.

4 Indeed, the air introduced or entrained in the suspension is stabilised by the action of organic matter and fine particles of hydrate and forms a stable foam which can occupy a large volume of the holding vessel if not treated.

This air is introduced mainly in the precipitation stage by the need to agitate the seed slurry and pregnant liquor to effect the agglomeration of the fine particles of hydrate. The method of agitation used is either (1) mechanical or air-lift. Although both of these can cause foaming, the most severe problems occur where entrained air is present.

Foaming also occurs due to other operations but primarily the entrainment of air is the first step to the formation of the foam, with the organics and fine particles of alumina playing a large role in foam stabilisation found in Bayer liquors.

The organic impurities and reagents present in the liquors act to render the surface of the fine particles hydrophobic. Such impurities include low molecular weight organic acids, humic matter, benzene carboxylic acids and phenolic acids.

Bayer liquors in precipitation contain a high proportion of solids, most of which is fine particles being trtt Svigorously agitated to enhance the effects of agglomeration.

I! E 25 It is necessary for the solids, if they are to be carried up C Ir by the foam, to have a hydrophobic nature very much like the mechanism of flotation. However, highly stable foams are capable of entraining many hydrophilic particles.

Thus the stabilisation of the foams by solids as well as the presence of high levels of organics leads to a foaming problem, which is commonly contained by the addition of chemical antifoamers and defoamers.

Antifoams adsorb at the air-water interface in preference to film stabilising surfactants. The molecule has a strong affinity for the air-water interface and does not have the capacity to stabilise the foam.

Defoamers are added to existing foams and act in small droplets which spread on lamellae, carrying the liquid comprising the foam lamellae thus thinning the film and breaking it. Some defoamers can also act as antifoams, but this is not always the case.

The alumina industry use both antifoams and defoamers within their process. It has been found that compounds which exhibited antifoam characteristics in concentrated alkaline aluminate solutions at concentrations of 1-15 parts per million were camphor oil, C5 C7 ketones, C2 -C14 aminoparaffins, dibutyl pthalate and an 1 0 organosilicon liquid.

Other reagents such as petroleum solvents, neutral oils, C8 -C11 alcohols, fatty acids, ethylene amides and mixtures of these reagents have found usage as defoamers in other chemical systems. Organosilicon liquids must be disregarded however, as most alumina producers wish to keep silica levels as low as possible.

1 5 Commonly these reagents are added either upstream of the problem (antifoams) or directly to the already foaming solution (defoamers).

A significant and greatly detrimental effect of the antifoamers and defoamers presently in use is that, although they are added for the purpose of destroying a foam, high level addition intended to reduce foaming in fact exacerbates the problem, leading to the production of a stable froth.

There is therefore a need to eliminate or substantially ameliorate the S0 deficiencies of the prior art defoaming and antifoaming practices.

The present invention therefore provides a method of reducing foam formation in alumina processing circuits which comprises injecting into the slurry 25 stream of the circuit, at any point where aeration in the presence of solids occurs, a caustic stable agent selected from the group consisting of ethoxylated alcohols, polyethylene glycols, polypropylene glycols and mixtures thereof which renders the slurry particles hydrophilic.

Preferably, the caustic stable agent is added to the slurry stream upstream of the precipitators in the alumina processing circuit. Advantageously, the caustic stable agent is sprayed directly onto the surface of an already foaming solution in the slurry stream.

The invention is predicated on the discovery that the froth generated by the addition of defoamers is a solids-stabilised foam, which cannot be broken down by the 3,5 addition of further conventional defoamer.

'o i-\ 4:k 1 -6 The invention is further predicated on the realisation that solids entrained in the Bayer process convert the gas-liquid emulsion into a froth the froth being stabilised by the action of these solids. Any antifrothing agent is suitable for use in the present invention when added prior to or in the precipitation stage, or any unit operation where aeration in the presence of solids occurs.

It has additionally been found that the tendency for foaming can be further reduced when the caustic stable agent is used in combination with conventional anti-foams.

A distinction must be drawn here between anti-foaming which concerns the air-liquid interface, and anti-frothing which concerns the solid-air interface.

The solids in Bayer processing will be fine alumina trihydrate and sodium oxalate and must be hydrophobic on _their surfaces to be carried up into the froth by the entrained air present.

However, both alumina trihydrate and sodium oxalate are naturally hydrophilic and the hydrophobicity is imparted to their surfaces as high MW humic acids or as other organic chemicals present in the liquor stream to assist the process elsewhere, e.g. antifoams.

Adsorption of these chemicals onto the solids I I,; 25 surface renders them as "collectors" in a broad sense and thus the solids are carried up into the froth.

The purpose of the invention is to destabilise such Sfroths by the addition of anti-frothing agents to break down the bubbles and render the surfaces of the fine solids j 30 hydrophilic, to allow them to be discharged back to the bulk liquid phase, thus giving the froth no backbone for stability.

SPreferably the caustic stable agent is a wetting agent which, in addition to preventing frothing, also emulsifies the "collector" type chemicals present in the liquor thereby preventing them from further adsorption onto the surfaces of the fine particles.

1~ Advantageously the wetting agent may also be a surfactant, which will have a further destabilising effect on the froth.

The caustic stable agents may typically be added to the slurry stream in a dosag9,e range of 1 to 100 ppm.

The invention will now be further illustrated with reference to the accompanying examples.

Apparatus In the production of alumina foams and froths are usually associated with the precipitation of alumina trihydrate from solution. Air is introduced into the 1 0 precipitators for the purpose of mixing the slurry and allowing the hydrate particles to agglomerate. The conditions used to test the antifoams were designed to simulate these conditions in the laboratory whilst also allowing for the measurement of foam characteristics to be made.

The foam rig consists essentially of a stainless steel cylinder column 1 5 connected to an air supply through the base. The column is approximately Im high x 0.lm diameter with a perspex window section at the front to allow viewing of the foam S and measuring of foam heights. The cylinder was surrounded by a stainless steel water jacket which was connected to a thermostat controlled water bath to provide temperature i control for the solutions.

S 20 At the base of the cylinder is a stainless steel mesh of 200 micron aperture. Compressed air from a cylinder was fed through the mesh. This was found to be the most effective method of dispersing the gas to produce even air flow. The air flow St was measured by means of a pressure gauge placed between the gas cylinder and the foam S column. Valves at the base and side of the column allow for emptying and cleaning of the column or for taking samples of liquor as required.

4 2 io €,21 -8 Generation of Froth As stated above, the froth is thought to occur as a result of the carrying into the foam of particles with a hydrophobic surface. Further to this, previous reports have also found that the froth is caused by the fine fraction of hydrate in the slurry, that is, the fraction with a particle size of less than 60 micron diameter. Whilst larger particles will also be entrained in a foam, where the fine particles of hydrate are involved the froth produced will be more voluminous and persistent.

The first stage in the generation of the froth was to create a sample of fine aluminium trihydrate. Samples obtained from Alcoa Kwinana were pulverised to produce samples with a particle size distribution where 50 of the o 15 particles have a diameter of 50 microns or less. This o hydrate was used for all the experimental work.

0 To ensure that the surfaces were hydrophobic the 4o particles were coated in a film of a hydroph6bic reagent.

This was achieved by firstly dissolving the reagent in a 004 20 volatile solvent to form a 1% solution and then pouring this onto the hydrate. Enough solvent was used to allow mixing of the solution and the hydrate ensuring that each particle came into contact with the solution. The hydrate was spread ooo o, g out and the solvent allowed to evaporate leaving the oo 25 particles each coated in a thin film of reagent creating a 0 0o hydrophobic surface.

.0ooo: Table 1. shows the range of compounds and the concentrations 0 used to form a froth. All these reagents are used to 0 0 improve the process efficiency in alulinr'a refineries.

004° 9 o 04 00 0 0 0 00 00 0 o 06 0 0 0 00 0 4 0 O 0 04 00 4 0 t t If 40 0 04 4 Reagent Function Conc. on hydrate (ppm) JMT 656 Crystal modifier 25-50 Ryco Antifoam 25-100 Alcan modifier Crystal modifier 25-50 Priolene Drainage Aid Priplus Crystal Agglomerator 25-100 Table 1. Compounds used for creating hydrate froth.

Testing of De-frothing Agents To enable a quantitative assessment to be made of 15 the de-frothing abilities of a number of reagents a standard froth had to be generated. The fine pulverised hydrate was coated in a thin film of Priplus, a fatty aci.d distillation residue, by dissolving the Priplus in 40-70 C petroleum spirit. A concentration of 0.2 w/w Priplus on hydrate was used. This concentration is vastly in excess of the quantity found in the plant but was useful in the 'aboratory for producing a stable froth very rapidly.

The water jacket around the column was heated to 60 0 C and 1.5 L of spent liquor introduced to the column.

25 The air pressure entering the column was maintained between 150-200 KPa. This caused the liquor to foam, and as the foam height increased, the hydrate was added to the foam through the top of the column. The froth was allowed to climb to 450mm at which point the air pressure was decreased 30 until a steady foam height was achieved.

The reagent to be tested was added to the top of the column using a microlitre syringe and the response of the froth to the chemical was noted by monitoring the foam height against time. The lowest height recorded by the foam was also recorded.

10 Testing of Antifrothing Agents The conditions used for the testing of possible antifrothing agents were as for the testing of the de-frothing compounds with the exception that the hydrate and de-frothing agents were all present in the liquor before any air was introduced into the column. The air was introduced at 150-200 KPa and the maximum foam height achieved in 10 minutes was monitored.

Results All the compounds tested were from the ICI "Teric" range of surfactants. The compound present in each reagent as the active ingredient is shown in Table 2. Enquiries have been made concerning a more specific description of the reagents PE 61, PE 62, BL 8 and 160.

Commercial Name Chemical Teric N 4 nonylphenol 4 moles ethoxylate Teric N 5 nonylphenol 5 moles ethoxylate N 9 nonylphenol 9 moles ethoxylate N 12 nonylphenol 12 moles ethoxylate N 20 nonylphenol 20 moles ethoxylate N 30 nonylphenol 30 moles ethoxylate N 100 nonylphenol 100 moles ethoxylate 25 PE 61 alkyl aryl polyoxyethylene ethers 160 polyoxypropylene ethoxylate ethers Table 2. Chemistry of reagents tested.

Example 1 This example illustrates how a conventional antifoam is not only ineffective against a foam containing solids but also contributes to the froth stability.

of aluminium trihydrate hydrate was coated with 0.1% w/w Ryco A, a commercial antifoam, by dissolving the Ryco A in kerosene to form a 1% solution, pouring this on the hydrate and mixing thoroughly. The hydrate was spread out and the kerosene evaporated until only the Ryco was

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11 coating the particles. A 2L measuring cylinder was filled with 1200ml of Bayer liquor heated to 60 0 C, and coated aluminium trihydrate was added to the liquor. A sintered glass frit was connected by tubing to an air supply and placed at the bottom of the cylinder. A pressure gauge placed between the air supply and the frit allowed measurement of the pressure. The slurry was aerated at 100 KPa. As the air bubbles rose through the slurry particles of hydrate were collected and brought to the surface forming a froth which rose to the top of the cylinder. The addition of 20 ppm of a tridecyl ethoxylated alcohol (approximate formula R 1 2 _1 3 -[O-CH CH 7 OH) as a 1% solution to the top of the froth caused the foam to begin to collapse. Eventually the foam disappeared altogether.

Example 2 1 g of priplus pitch, a fatty acid distillation residue which can be used in alumina refineries for a variety of purposes, was dissolved in 40 ml of 40-70 petroleum spirit and poured on to 500g of fine aluminium trihydrate. The slurry was mixed thoroughly and the petroleum spirit evaporated. 1 litre of spent liquor from an alumina refinery was heated in the foam apparatus (described in the report) to 60 0 C and air was introduced through the stainless steel mesh at the base of the column at 150 KPa. As the foam created by the aeration rose through the column 50g of the pitch coated hydrate prepared above was added to the foam. The particles rather than sinking through the foam were colleteftd by the foam and Sformed a froth. The air was turned off at this point and 30 the froth decayed slightly but 120mm of persistent froth remai sed after 60 s. Re-aerating the liquor caused the froth to rise again and it. rapidly achieved a height of

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450mm. The addition of 5 ppm of a nonyl phenol polyethoxylate of th- formula RgC 6

H

4

-[O-CH

2

CH

2 (N 4) caused the foam to collapse to 30mm in 12 The addition of reagent to an existing froth measures the abili 4 of the chemical to destroy the froth, i.e. its de-frothi,.j capacity; the addition of reagent before the introduction of air alls the measurement of the antifroth capacities of the reagent.

Air was introduced to column of liquor containing the coated aluminium trihydrate and five parts per million of the reagent and the maximum height achieved the froth recorded as Example 3 A slurry of aluminium trihydrate coated in pitch and spent liquor were used to form a froth as prepared in example 2. Five parts per million of a nonyl phenol polyethoxylate containing five ethoxylate units (N 5) in the 15 chain were added to the top of the froth. The froth height before the addition of the reagent was 450mm, following the addition of the reagent the froth height collapsed to The maximum height recorded in antifroth tests was Example 4 A froth was created as in example 2. Five parts per million of a nonyl phenol polyethoxylate containing nine ethoxylate units (N 9) were added to the froth. The initial froth height of 450mm was reduced to 50mm. The maximum

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height recorded in antifroth tests was 25 Example A froth was created as in example 2. Five parts per million of a nonyl phenol polyethoxylate containing twelve ethoxylate units (N 12) were added to the froth. The initial froth height of 450mm was reduced to 40mm. The maximum height recorded in antifroth tests was Example 6 A froth was created as in example 2. Five parts per million of a nonyl phenol polyethoxylate containing twenty ethoxylate units (N 20) were added to the froth. The initial froth height of 450mm was reduced to 50mm. The maximum height recorded in antifroth tests was 30 mm.

I I I 13 Example 7 A froth was created as in example 2. Five parts per million of a nonyl phenol polyethoxylate containing thirty ethoxylate units (N 30) were added to the froth. The initial froth height of 450mm was reduced to 400mm. The maximum height recorded in antifroth tests was Example 8 A froth was created as in example 2. Ten parts per million of a nonyl phenol polyethoxylate containing thirty ethoxylate units (N 30) were added to the froth. The initial froth height of 450mm was reduced to 150mm. The maximum height recorded in antifroth tests was Example 9 A froth was created as in example 2. Five parts per million of a nonyl phenol polyethoxylate containing one hundred ethoxylate units (N 100) were added to the froth.

The initial froth height of 450mm was reduced to 300mm.

Example A froth was created as in example 2. Ten parts per million of a nonyl phenol polyethoxylate containing one hundred ethoxylate units (N 100) were added to the froth.

The initial froth height of 450mm was reduced to 100mm.

Example 11 A froth was created as in example 2. Five parts per million of an alkyl aryl polyoxyethylene ether (PE 61) were added to the froth. The initial froth height was reduced to 30mm. The maximum antifroth height recorded in antifroth tests was Example 12 A froth was created as in example 2. Five parts per million of a polyoxypropylene ethoxylate ether with six ethoxylate units (N 160) were added. The initial froth height of 450mm was reduced to 60mm. The maximum froth height recorded in antifroth tests was The minimum froth height recorded for each of the reagents together with the dosage used and the time required to reach this height is summarised in Table 3.

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Reagent Dosage Min Froth Height Time Max Froth Height ppm mm s mm N 4 5 30 40 N 5 5 40 20 N 9 5 50 60 N 12 5 40 60 N 20 5 50 60 N 30 5 400 60 N 30 10 150 60 N 100 5 300 N 100 10 100 PE 61 5 30 100 160 5 60 50 initial froth height 450mm Table 3. Results of de-frothing tests.

The maximum height recorded for the antifroth tests is also shown. The concentration of the reagents used reflects the range which would be considered economically acceptable by the alumina industry rather than any limit 25 imposed by the reagents. It is expected that the antifrothing agents would operate successfully up to and above concentrations in the range of 100ppm. In semiquantitative tests 150ppm of N 4, N 9 and 160 were used to destroy a foam created using 200 g/l trihydrate and 50 ppm Ryco.

The Teric series of compounds incorporates ethoxylate units onto a hydrophobic end section; with an increasing number of ethoxylate units the molecules become increasingly soluble in water and also have higher melting points, so that, N 4 is a water insoluble liquid, N 20 is a very viscous liquid and N 100 is a waxy water soluble solid.

As the number of ethoxylate units is increased in the molecule the antifrothing abilities of the liquids begins to u i

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t I I I 15 diminish. This has been attributed to the solubility of the compounds as it is thought that complete miscibility with the liquor would prevent the reagent acting as an efficient emulsifier. It is also possible that the effect of the antifroths is due to their ability to reduce interfacial tension and thereby prevent the particles being carried into the froth.

From the above, the most presently preferred method appears to be the addition of 1-5 ppm of an alkyl aryl ethoxylated alcohol with 4-10 ethoxylate units Teric N 4, N 5, N 9).

Antifroths can be introduced directly into the liquor upstream of the precipitators. Sufficient movement of the liquor takes place in the precipitators to allow o o, 15 adequate mixing of the reagent and the liquor. Where the 0o 0 compounds are used as de-frothing agents spraying of the o0 liquid directly on to the surface of the foam would be the o o o o0 preferred method of addition, although injection into the liquor stream would also be acceptable.

o, 20 The invention thus provides significant economic benefits to alumina producers in overcoming the problem of frothing.

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Claims (8)

1. A method of reducing foam formation in alumina processing circuits which comprises injecting into the slurry stream of the circuit, at any point where aeration in the presence of solids occurs, a caustic stable agent selected from the group consisting of ethoxylated alcohols, polyethylene glycols, polypropylene glycols and mixtures thereof w!iich renders the slurry particles hydrophilic.

2. A method as claimed in claim 1, wherein the caustic stable agent is a wetting agent.

3. A method as claimed in claim 2, wherein the caustic stable wetting agent is also a surfactant.

4. A method as claimed in any one of claims 1 to 3 wherein the caustic stable ,0 agent is added to the slurry stream in an amount of from 1 to 100 parts per million. o°

5. A method as claimed in any one of the preceding claims, wherein the caustic stable agent is used in combination with conventional anti-foams.

6. A method as claimed in any one of claims 1 to 5 wherein the caustic stable agent is added to the slurry stream upstream of the precipitators in the alumina :I .processing circuit.

7. A method as claimed in any one of claims 1 to 6, wherein the caustic stable agent is sprayed directly onto the surface of an already foaming solution in the slurry Sstream. i p^ 17

8. A method as claimed in any one of the preceding claims, wherein the caustic stable agent is an alkyl aryl ethoxylated alcohol having 4 to 10 ethoxylate units, and is added to the slurry stream in an amount of from 1 to 5 parts per million. DATED this 1st day of October, 1992. INDUSTRIAL MINERALS RESEARCH AND DEVELOPMENT PTY LTD WATERMARK PATENT TRADEMARK ATTORNEYS 00 THE ATRIUM 04 0 °4 ,290 BURWOOD ROAD 40 HAWTHORN VICTORIA 3122 "AUSTRALIA B 0 a a 9 o 0 f 9