AU2012205190B2 - Waste Treatment Process - Google Patents

Abstract

James & Wells Ref: 13152ORF1AU/3 This invention relates to a method of treating spent hot liner (SPL) material or materials having a similar compositions thereto, the method including the steps of: a) placing a SPL mixture into a kiln, wherein the mixture includes SPL material, a silicon source, a calcium source and an iron source , and b) heating the SPL mixture to sufficient temperature to cause combustion of the carbon in the SPL material and melting of the SPL mixture to form a slag, and c) discharging the resulted slag from the kiln. the method characterised by e) ensuring the percentage of iron within the net SPL mixture is in the range of 4 - 20% w/w.

Description

James & Wells Ref: 13152ORF1AU/3 WASTE TREATMENT PROCESS TECHNICAL FIELD This invention relates to the process of treating waste. BACKGROUND ART 5 In particular, the present invention relates to a process of treating waste such as spent pot liners or other materials having similar properties thereto. Reference to the present invention will now be made to treating spent pot liners used in the manufacture of aluminium. It should be appreciated however that the principles of the present invention can apply to similar waste materials. o In the process of aluminium production, alumina is dissolved in electrolytic cells or pots. Typically the pots contain a molten electrolyte having a number of materials including fluoride salts. Each pot has a lining which includes an insulation layer and carbon which acts as a cathode in the electrolytic process. 5 When a lining cracks, or the infusion of fluoride and sodium salts affects performance, the cell is taken offline and the cathode lining material is removed. This material (now called a Spent Pot Liner or SPL) is impregnated with fluoride salts as well as cyanides. Because of this, SPL is considered as a hazardous material. Considerable effort has been exerted in devising waste treatment processes to recycle SPL without fluoride, cyanide or other significant waste o emissions. Some of these processes are described in the following patent specifications US4,497,464, US 4,993,323, US 5,286,274, US 5,245,115, US 4,113,832, US 4,735,784, US 5,164,174, US 4,993,323, and US 5,711,018. 1 James & Wells Ref: 13152ORFIAU/3 A particularly relevant example of SPL processing is discussed in WO 2006/086874. All of these processes are concerned with forming a slag from the SPLs within a kiln and reducing emissions to atmosphere from the process and reducing leachate levels in the slag. These levels give an indication of the amount of leachable fluoride remaining in the slag 5 created by the recycling process. However, each of these processes have problems associated with them. One significant problem is that high temperatures are required to heat the fluoride and cyanide components sufficiently so that they can then be converted to nontoxic compounds. These high temperatures require a considerable amount of energy to generate, this making the o recycling process highly expensive and not environmentally friendly. Another problem associated with these processes is the fluidity of the slag produced. It should be appreciated that slag is a very viscous material and, as a consequence of its poor fluidity, slag can build up in the kilns. Eventually, the build up of slag is such that the kiln can be blocked off, impede gas flow and the conversion process. 5 Yet another problem associated with previous processes is that although levels of toxic emissions are under regulatory guidelines, they are still uncomfortably high. A further problem is that slag produced can only be used in limited applications as it tends to have a low crush strength and cannot be used in load bearing surfaces such as roading. It is an object the present invention to address the foregoing problems or at least provide the o public with useful choice. All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly 9 James & Wells Ref: 13152ORF1AU/3 understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. Throughout this specification, the word "comprise", or variations thereof such as "comprises" or 5 "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. DISCLOSURE OF THE INVENTION According to one aspect of the present invention there is provided a method of treating spent : pot liner (SPL) material or materials having a similar composition thereto, the method including the steps of: a) placing a SPL mixture into a kiln, wherein the mixture includes SPL material, a silicon source, a calcium source and an iron source, and b) heating the SPL mixture to a sufficient temperature to cause combustion of the carbon 5 in the SPL material and melting of the SPL mixture to form a slag, and c) discharging the resultant slag from the kiln the method characterised by the percentage of iron with the net SPL mixture is in the range of 4 - 20% w/w. In particular the range of 7-15% w/w has been found to work particularly well. o A typical composition of SPL material is given in the table below. C Al Ca Mg Na Si F Fe CN S Weight 10- 5- 1- 0.2- 7- 1- 10- 0.3- 0.2- 0.01 50 22 8 1 20 20 19 5 0.5 0.3 James & Wells Ref: 13152ORFIAU/3 It should be appreciated however that the term "spent pot liner material" is one is which is well understood in the art including the nature of its composition. The term "spent pot liner material" includes, but is not limited to a material containing carbon and / or inorganic material derived from a receptacle that has been used in the production of 5 primary aluminium in electrolysis process. The spent pot liner material is essentially a aluminium bi-product. The term "Inorganic material" includes, but is not limited to refractory material such as silicon and/or alumina. It should be appreciated that the term "kiln" is also well appreciated by those in the industry. > It is a preferred embodiment of the present invention, that the kiln is a counter current fired rotary kiln. It should be appreciated however that the present invention can be used in relation to other types of kilns, furnaces and heat sources. The use of silicon sources in the production of slag from SPL material is well known. The silicon in general imparts a solid glassy composition to the slag, as well as forming an 5 impervious matrix with the other SPL materials to physically lock in fluoride into the slag matrix in an unleachable form. Typical silicon sources that can be used with the present invention include silicon oxide, waste glass (cullet), silica sand and any material which has a high silica content. Likewise, a calcium source is most likely to be one already known in the industry such as o calcium carbonate, calcium oxide, calcium hydroxide (lime) or other calcium rich material where the other elements present are compatible with the process. Calcium is necessary to lock up the fluoride into stable calcium fluoride (CaF 2 ). However, the greater the proportion of calcium, the greater the viscosity of the resultant slag.

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James & Wells Ref: 13152ORF1AU/3 It is envisioned that calcium will be added automatically according to the amount of fluoride detected in the gas. Deliberate introduction of an iron source having a significant presence in the SPL mixture is a key feature of the present invention. 5 Preferably, the iron is added in one of its oxide forms or its pure elemental form. However, in some embodiments, iron may be added in the form of an iron rich material where the other elements present are compatible with the process. It should be appreciated that SPL material has a significant variation of carbon content depending on many factors, including the age of the liner. Thus, the percentage of iron added D to the SPL mixture is best defined by reference to the net SPL mixture net SPL mixture has had the carbon removed through combustion. The percentage of iron in the net SPL mixture is preferably in the range of 4 - 20% w/w. Although a range of 7-15% w/w has been found to work particularly well. Typically fluoride leachate levels in the end product can be as high as 31 mg/ without the 5 addition of iron. However, addition of iron of 5.1% gives far better results as can be seen in the table below. % Iron w/w TCLP Fluoride 0 31 mg/I 5.1 24 mg/I 15 3.9 mg/I 17 1.9 mg/I If iron levels are above 20% and in most cases 15% this will affect the mix and cause a reduction in the proportion of SPL that can be added. Iron is an expensive additive in this o context.

James & Wells Ref: 13152ORF1AU/3 The temperature to which the SPL mixture is heated is critical. While this will be discussed in greater detail later on in the specification, it should be noted that the temperature will in preferred embodiments rise to be in the range of 10500C - 11500C. This ensures that the carbon in the SPL mixture begins to combust, that the cyanide is dissociated, fluoride gases will 5 begin to evolve into the gas stream, and most importantly, reactions will be taking place in the mixture with free calcium to lock up the fluoride. In this situation fluoride converts to stable CaF 2 which has a higher melting point of 1250 0 c or higher. As the mix temperature has an upper limit of 1150 0 c, CaF 2 added from the scrubber combines with the mix. The CaF2 from the scrubber is combined with the mix and passes o through the kiln without being dissociated. This acts to concentrate the amount of the fluoride in the end product with no other waste streams. This has not been possible with other processes. According to a further aspect of the present invention there is provided a combustion source for heating a kiln for use in the treatment of spent pot liner material, characterised in that 5 the source is first cut spent pot liner material. Preferably first cut SPL material is the combustion source for the method previously described. Those skilled in the art know that first cut SPL material is basically the carbon cathode from the reduction cell which is used to conduct electricity from the anode above through the alumina powder reducing it to aluminium metal. Second cut SPL is the refractory of material which o surrounds the cathode and insulates it from electricity leakage and prevents heat escaping from the cell. Typically first cut material has a composition of around 60% carbon. This is in contrast to second cut SPL which has around 4 to 10% carbon. In preferred embodiment of the present invention, the SPL material is finely ground to form a pulverised fuel that is blown into the kiln and combusts providing the thermal energy for the 5 process (preheats the gas for the kiln.) The inventor has found that grinding this material so

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James & Wells Ref: 13152ORF1AU/3 that approximately 90% of it is in the order of 70 microns or less works particularly well. The inventor has recognised that the use of this material can only be used in processes where there is scrubbing and/or collection of the fluoride gases that evolve as a consequence of the material being burnt. Thus, this aspect of the present invention is particularly elegant in that the 5 material being treated is also used to fuel additional treatment of the material in an environment that captures waste emissions. It should also be noted that the present invention provides a solution to a number of the problems associated with the treatment of SPL mixtures. For example, * The additional concentration of iron in the mix acts as a flux in lowering the viscosity of 0 the mix when molten promoting the evolution of Fluoride gases from the mix. It appears that the fluoride, calcium, and iron bind in a matrix with the aluminium and silicon. * The inclusion of iron also appears to assist the critical transition of the mix from solid to liquid as it passes through the kiln by changing the character of the mix when in transition from solid to liquid so that it does not adhere to the kiln wall causing 'Build-up' 5 * Iron appears to give the resultant slag a greater crush strength. This enables the slag to be used in more load bearing applications such as roading, construction and hard fill. BEST MODES FOR CARRYING OUT THE INVENTION A preferred means by which the present invention operates is given as follows. SPL material, silica, iron and calcium are crushed to a size of approximately 2mm or less. o The SPL material is then proportioned by weight with the other raw materials and thoroughly mixed together. A counter current fired rotary kiln is preheated. When the temperature at the infeed end of the kiln reaches approximately 5000C, the SPL mixture is fed into the kiln at an even and controlled 7 James & Wells Ref: 13152ORF1AU/3 rate. The kiln is heated by PF firing (pulverised fuel) in which a stream of finely ground carbon dust made from first cut SPL is combusted to provide the heat energy needed. The atmosphere in the kiln will be oxidising and as the SPL mixture heats to 500 0 C or more, the 5 carbon in the SPL begins to combust and cyanides will begin to dissociate. The oxygen level in the exhaust gases is monitored and the kiln atmosphere is kept to be slightly oxidising. As the SPL mixture progresses through the kiln towards the exit end (approximately %- 2/3 of the way through) its transits from a solid state to a liquid state. At this point, the temperature will be approximately 800"c or greater and various changes will D be occurring. For example, fluoride gases will begin to evolve into the exhaust gas stream and reactions will be taking place in the mixture between the SPL, the calcium, iron and silca. At this point most of the carbon will have combusted and cyanide will have dissociated. The temperature of the mixture will continue to rise as it progresses through the kiln and reach a temperature in the range of 1050 to 11 50'C. After which, the liquid mixture will emerge from 5 the kiln into the cooler as a slag. The cooler operates as follows: The liquid mix falls on to a cooled plate feeder and is spread 25 to 50 mm thick. This provides rapid cooling from 1100 C to approx. 800 C which prevents crystal formation and provides an impervious matrix to better protect CaF2 against acid leaching. o The solidified slag is then passed through a fluid bed type cooler where it is further cooled and the hot air produced is used as secondary combustion air. As the exhaust gases leave the kiln they are subject to a reduction in temperature by either atmospheric bleed air, or water spray to lower the temperature of the gas so that it is at a suitable temperature for the following process.

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James & Wells Ref: 13152ORF1AU/3 The gases pass next to a ceramic lined cyclone which removes most of the entrained dust which is recycled back into the kiln feed material. From here the gases are scrubbed with either a solution of calcium hydroxide or by dry powder calcium oxide or hydroxide or fine calcium carbonate which combines with the Fluoride gases to 5 form CaF2. At the same time Sodium salts are formed from the reaction of NaF and Ca(OH). Water with these compounds is caused to evaporate leaving them as a dry powder. This is collected at the bag filter and is recycled back to the kiln feed material. Environmental tests for air emissions have shown levels of fluorine, cyanide, and particulate in the exhaust gases to be at low or undetectable levels on the process test kiln. > Chemical analysis ( TCLP )on the processed SPL mix have shown that: Cyanide is reduced to harmless levels less than 0.004 mg/Itr Fluoride is reduced below the 15mg/Itr target ( lowest is 1.9 mg/ltr) Chemical reactions in the Process The key chemical reactions of this process are as follows: 5 Cyanide is dissociated by temperature within the kiln into Nitrogen and Carbon dioxide. 2CN+202> 2CO2+N2 Most of the Fluoride in the SPL that is not fixed in a stable compound within the SPL is evolved into a gaseous state mainly as HF and NaF in the kiln and passes to the Scrubber where it is converted to CaF2 by the reaction with Calcium (hydroxide) after which it is returned to the o process with the feed material. Or alternatively some Fluoride may combine with free Calcium oxide in the mix within the kiln to form CaF2 within the mix.

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James & Wells Ref: 13152ORF1AU/3 CaO + 2NaF> CaF2 + Na20 CaO + 2 HF > CaF2 + H20 It is important that the mix temperature is kept below 11 50C (1 200C) as above this CaF2 could be subject to dissociation and evolution of Fluoride gases. 5 Mix Proportions The quantity of SPL in the mix is calculated after subtracting the amount of carbon in the SPL. This is done because the amount of carbon varies and it will combust to C02 in the process and be lost to the mix. The net SPL will be 40 - 60% of the mix. > Additives include: Cullet ( waste glass) 20 - 40% w/w Lime as CaO or Ca C03 7 - 30% w/w Iron 7 - 15% w/w Aspects of the present invention have been described by way of example only and it should be 5 appreciated that modifications and additions may be made thereto without departing from the scope of the appended claims. 1n

Claims (11)

1. A method of treating spent pot liner (SPL) material or materials having a similar compositions thereto, the method including the steps of: a) placing a SPL mixture into a kiln, wherein the mixture includes SPL material, a silicon source, a calcium source and an iron source, and b) heating the SPL mixture to a temperature in the range of 10500C to 11 500C to cause combustion of the carbon in the SPL material and melting of the SPL mixture to form a slag, and c) discharging the resulted slag from the kiln the method characterised by d) ensuring the percentage of iron within the net SPL mixture is in the range of 5 - 20% w/w.

2. The method as claimed in claim 1 wherein the percentage of iron whether the net SPL mixture is in the range of 5 - 15% w/w.

3. A method as claimed in either claim 1 or claim 2 wherein the kiln is a counter current fired rotary kiln.

4. A method as claimed in any one of claims 1 to 3 wherein the silicon source includes one or more of the following: silicon oxide, waste glass, silicon sand. 11 James & Wells Ref: 13152ORF1AU

5. A method as claimed in any one of claims 1 to 4 wherein the calcium source includes one or more of the following: calcium carbonate, calcium oxide, calcium hydroxide.

6. A method as claimed in any one of claims 1 to 5 wherein the calcium is added automatically according to the amount of fluoride detected in the gas.

7. A method as claimed in any one of claims 1 to 6 wherein the iron is added in one of its oxide forms.

8. A method as claimed in any one of claims 1 to 7 wherein the percentage of iron added to the SPL mixture is determined by the net SPL mixture.

9. A method as claimed in any one of claims 1 to 8 wherein the SPL mixture is heated to a temperature in the range of 10500C to 11500C.

10. A method as claimed in anyone of claims 1 to 9 wherein first cut SPL material is used as a combustion source in the kiln.

11. A method of treating spent hot liner (SPL) material or materials, substantially as herein described and illustrated by the examples in the Best Modes section excluding the prior art disclosed within the specification. 12