AU2013257463B2

AU2013257463B2 - A method of recovering sulfur dioxide and heavy metals from metallurgical flue gas

Info

Publication number: AU2013257463B2
Application number: AU2013257463A
Authority: AU
Inventors: Ping NING; Xueqian Wang; Zaifei YIN
Original assignee: Univ Kunming Science & Technology; Kunming University of Science and Technology
Current assignee: Kunming University of Science and Technology
Priority date: 2012-11-16
Filing date: 2013-11-14
Publication date: 2015-09-10
Anticipated expiration: 2033-11-14
Also published as: WO2014075390A1; CN102949926B; CN102949926A

Abstract

Abstract The present discloses a method of recovering sulphur dioxide and heavy metals from smelting flue gas. The method uses an ammonium sulfide solution to simultaneously remove sulfur dioxide and heavy metals from the flue gas after pre-treatment of the metallurgical flue gas and also to recover and reuse the heavy metals and the sulfur products. The method is simple and easy to operate, is highly efficient in the desulfurisation and the method is also low cost and is suitable for use in industry. Figure 1 Figure 1

Description

- 1 A METHOD OF RECOVERING SULFUR DIOXIDE AND HEAVY METALS FROM METALLURGICAL FLUE GAS [0001] The present application claims the benefit of Chinese utility model application number CN 20121046119.6, filed on 16 November 2012, the disclosure of which is incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention pertains to the technical field of environmental protection, and in particular, relates to a method of simultaneously recovering sulfur dioxide and heavy metal pollutants from metallurgical flue gas. BACKGROUND TO THE INVENTION [0003] The non-ferrous metallurgy industry is one that has experienced dramatic growth in recent times. During the duration of the Chinese social and economic development initiative known as the "Eleventh Five-Year Guideline", the country experienced an average annual growth of 13.8% in the production of 10 types of non-ferrous metals. Nowadays, China is the largest global producer of these non-ferrous metals. On the back of consumer demand, China now produces more than 40% of the world's supply of zinc and lead, and the non-ferrous metallurgy industries have contributed greatly to the economic development in China. [0004] Non-ferrous metallurgic flue gas is generated from metallurgical processes such as concentrated dry ore roasting, sintering, smelting, and refining. The volume of flue gas produced in these processes depends on specific process variations and the type of metallurgic furnace employed. [0005] Currently, there are two types of processing technologies used in the treatment of non-ferrous metallurgic flue gas, namely, flue gas dust removal and flue gas desulfurization by sulfuric acid generation. Flue gas dust/ash removal [0006] Flue gas dust removal methods are can be divided into dry or wet flue gas dust removal/scrubbing technologies.

-2 [0007] In the case of dry flue dust removal technology, the entire process is carried out under conditions in which the flue gas temperature is greater than the gas dew point and thus allows removal of dust from the dry flue gas. Currently, 90% of dust removal in heavy metal non-ferrous metallurgy is carried out using dry flue gas dust removal technology. [0008] Wet flue gas dust removal technology is suitable for the removal of dust from flue gases having higher moisture content. Wet flue dust removal technology is typically used in the treatment of flue gas dust produced in ore refining and discharge drying processes. Wet flue dust removal technology is also used in the purification of sulfur. The wet flue gas process involves contacting the flue gas dust with water thus to generate water droplets, liquid films and bubbles to separate the gas dust from the flue gas. Metallurgic flue gas sulfur removal by sulfuric acid generation [0009] Sulfur dioxide is the major gas pollutant in metallurgic flue gas. Flue gas having a sulfur dioxide content of 3.5% or more can be used to make sulfuric acid through utilisation of contact methods. If the flue gas also contains mercury, a mercury removal step needs to be utilised during the purification process. After sulphuric acid generation, the remaining flue gas has sulfur dioxide content of less than 3.5%, which is sufficiently low to allow exhaust gas to be further treated with absorption, adsorption and catalytic oxidisation methods. Problems associated with flue gas treatment in the non-ferrous metallurgy industry [0010] Typically, non-ferrous metal ores contain heavy metal impurities, such as, Hg, Pb, As and Cd. These heavy metal impurities can be emitted in the form of particles or volatile components in the flue gas. In China, non-ferrous metallurgic furnace flue gas is one of the major emission sources of atmospheric Hg, Pb, As and Cd heavy metal pollutants, and this is particularly true in the case of mercury emissions. Flue gas mercury emissions account for 45% of the gross annual atmospheric mercury emissions in China and account for 15% of the world's mercury emissions, and of course, have a negative impact on the environment. [0011] Sulfur compounds are a major component of non-ferrous metal ores. Non-ferrous metallurgic processing generates large volumes (0.05% - 25%) of sulfur dioxide gas and 8% of sulfur dioxide emissions arise from non-ferrous metallurgic flue gases. To date, there are no efficient recycling technologies capable of reducing sulfur dioxide emission in the non-ferrous metallurgic industry and so it would be desirable to develop suitable recycling technologies.

-3 [0012] Increasing controls on sulfur dioxide emission levels in exhaust gas from metallurgic flue gas sulfuric acid generation processing, and as well as more stringent standards for the emission of heavy metal pollutants, for example, mercury and lead emissions arising from mercury or zinc metallurgic engineering industries, means there is a need in these industries for technologies that enable better control of non-ferrous metallurgic flue gas pollutants in flue gases, particularly, heavy metal content and sulfur dioxide levels. [0013] Existing methods have a number of problems. Firstly, the current recycling technologies are only suitable for the treatment of flue gas having higher concentrations of sulfur dioxide, and are not suitable for removal of sulfur dioxide and heavy metals at lower concentrations. Second, current technologies use sodium sulphide solutions to remove sulfur dioxide by absorption, and do not provide means to separate the sulfur from the heavy metals. Heavy metal recycling is difficult and secondary pollution by secondary heavy metals can easily occur. [0014] To provide a solution to the above mentioned problems, the present invention provides to a method of simultaneously removing and recycling sulfur dioxide and heavy metals from non-ferrous metallurgic flue gas. The method of the invention can handle high concentrations of sulfur dioxide in the non-ferrous metallurgic flue gas, associated volume fluctuations, as well multiple heavy metals in the flue gas, for example, Hg, As, Cd and Pb. The invention utilises (NH4) 2 S absorption to simultaneously remove SO 2 and Hg, As, Cd, and Pd pollutants, and allows the sulfur and heavy metals in the flue gas to be recovered for reuse and recycling. The method of invention thus provides options for controlling several pollutants from non-ferrous metallurgic flue gas. [0015] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. [0016] It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative. SUMMARY OF THE INVENTION [0017] The invention provides means for simultaneously removing sulfur dioxide and heavy metal pollutants from a flue gas stream and for producing recoverable forms of these -4 pollutants. Advantageously, the method extracts flue gas pollutants with very high efficiency without generation of any significant secondary pollutants. The recoverable outputs of the method are high purity elemental sulfur, a mixture of heavy metal sulfides and sulfate salts, and crystalline ammonium sulfate salt. The method is superior to prior art methods since the recovered sulfur is separated from the heavy metal recoverable. Furthermore, since the initial ammonium sulfide treatment solution is recovered and recycled in the process for reuse, the method is very cost efficient and with little raw material waste. The removal efficiency is such that the flue gas easily achieves compliance with current emission standards. The method is operable even with low level of pollutants, making it a very attractive technology for industry. [0018] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". [0019] In a first aspect, the invention provides a method of purifying a non-ferrous metallurgic gas, comprising the steps of: (i) cooling a dust free non-ferrous metallurgic flue gas to a temperature of below 98 'C; (ii) contacting the flue gas with an atomised ammonium sulfide solution to form a contaminant solution comprising one or more inorganic sulfur compounds and at least one inorganic heavy metal oxide; (iii) converting the at least one inorganic heavy metal oxide in the contaminant solution into at least one insoluble inorganic sulfidised heavy metal compound; (iv) subjecting the inorganic sulfur compounds in the contaminant solution to conditions that form elemental sulfur and regenerate a solution of ammonium sulfide. [0020] Preferably, the method of the invention further comprises the step of reusing/recycling the ammonium sulfide solution generated in step (iv) in step (ii). [0021] The ammonium sulfide treatment solution for absorbing the pollutants from the flue gas is recycled at the end of the first process cycle and then is returned to the start of the process for the next round of purification.

-5 [0022] Preferably, the flue gas is cooled to a temperature of 950C, more preferably, to a temperature of less than 90C, less than 80C, less than 70C, less than 60C, less than 50C, or less than 400C. Suitably, the gas is cooled to a temperature of less than about 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, or less than about 400C. It will be appreciated that the term "about" means ±1% of the specified temperature. [0023] Preferably, during converting step (iii) the one or more inorganic sulfur compounds are converted to inorganic ammonium polysulfide in the contaminant solution. [0024] Preferably, the inorganic sulfur compounds in the contaminant solution are selected from one or more of sulfur dioxide (SO 2 ), ammonium sulfide ((NH4) 2 S), sulfuric acid (H 2 SO4), ammonium hydrogen sulfate ((NH4)HSO 3 ), hydrogen sulfide (H 2 S), and ammonium hydrosulfide ((NH4)HS). [0025] Ammonium hydrosulfide ((NH4)HS) is generated in the atomised solution from the reaction of ammonium sulfide ((NH4) 2 S) with hydrogen sulfide (H 2 S) in the contacting step (i) of the method of the invention. Hydrogen sulfide (H 2 S) and ammonium hydrogen sulfate ((NH4)HSO 3 ) are formed by reaction of sulfuric acid (H 2 SO4), and ammonium sulfide ((NH4)2S) in the contacting step (i) and/or the reaction of sulfur dioxide (SO 2 ) and ammonium hydroxide

(NH

4 0H), wherein the ammonium hydroxide (NH 4 0H) is formed as a byproduct of converting step (iii). The sulfuric acid (H 2

SO

4 ) is formed from the reaction of sulfur dioxide (SO 2 ) and water in the contacting step (i). [0026] Preferably, the method of the invention further comprises the step of recovering the elemental sulfur formed in step (iv). The sulfur produced in this step is very pure and advantageously is not contaminated with any heavy metal compounds. [0027] Preferably, the concentration of the ammonium sulfide solution used in step (i) is from about 0.5 to about 10%, more preferably from about 1 to about 8%, and more preferably still is from about 3 to about 5% by weight. Suitably, the concentration of the ammonium sulfide solution can range from about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, to about 10%. It will be appreciated that the term "about" means ±0.5% of the specified amount. As ammonium sulfide solution is recovered later in the process and returned for reuse in step (i), use of lower concentrations is possible.

-6 [0028] Preferably, in step (i) of the method of the flue gas is contacted with the atomised ammonium sulfide solution by passing the flue gas through the atomised solution, thus generating a contaminant solution. [0029] Preferably, prior to step (iii), the contaminant solution is separated from the flue gas, for example, washing, rinsing, gas scrubbing, by filtration or centrifugation or other suitable separation means known to those skilled in the art. [0030] Preferably, in the method of the first aspect of the invention, the converting step (iii) involves a redox process. Suitably, the subjecting step (iv) also involves a redox process. The steps (iii) and (iv) may be simultaneously or sequential. It will be appreciated that the term "redox" means an oxidation-reduction process, typically a chemical redox process. [0031] Preferably, during step (iii) the at least one inorganic heavy metal oxide in the contaminant solution is converted to the at least one insoluble inorganic sulfidised heavy metal compounds in the form of a metal sulfide or metal sulfate salt. Suitably, the conversion results from reduction of the metal oxide to the metal sulfide and/or sulfate salt in a redox process. Typically, the redox process/reaction is allow to proceed for about 5 to about 120 minutes, preferably from about 10 to about 30 minutes, most preferably for about 20 minutes. [0032] Thus, preferably, during the converting step (iii) the at least one inorganic heavy metal oxide is converted to the at least one insoluble inorganic sulfidised heavy metal compounds by a chemical reduction process. Suitable, the at least one insoluble inorganic sulfidised heavy metal compounds are selected from PbS, HgS, HgSO 4 , CdS and As 2

S

3 . The reduction reaction produces ammonium hydroxide (NH 4 0H) as a side product in the case of lead, mercury, cadmium sulfide generation, and ammonium hydroxide (NH 4 0H) and ammonium hydrosulfide ((NH4)HS) in the case of arsenic trisulfide generation. [0033] In a preferred embodiment, the insoluble inorganic sulfidised heavy metal compounds are separated from the contaminant solution, preferably by filtration or centrifugation or other suitable separation means known to the person skilled in the art. [0034] Preferably, the contaminant solution further comprises ammonium hydrogen sulfate ((NH4)HSO 3 ), or a combination of ammonium hydroxide (NH 4 0H) and sulfur dioxide which react in solution to form ammonium hydrogen sulfate ((NH4)HSO 3 ). Thus, the ammonium hydroxide (NH 4 0H) byproduct is converted to a species that is used up in a later part of the -7 process. The ammonium hydrosulfide ((NH4)HS) side product is converted to ammonium sulfate ((NH4) 2

SO

4 ) which is later crystallized out of the solution and recovered. [0035] Preferably, in the method of the invention, during the converting step (iii), elemental sulfur (S) is formed in the contaminant solution from the reduction of sulfur dioxide (SO 2 ) by ammonium hydrosulfide ((NH4)HS), a reaction which forms ammonium sulfite (NH4) 2

SO

3 as a byproduct. Suitably, this reaction takes place in the redox rector at the same time as the heavy metal reduction described above. The ammonium sulfite ((NH4) 2

SO

3 ) formed is then oxidised by air to ammonium sulfate ((NH4) 2

SO

4 ) in a further redox process. The ammonium sulfate ((NH4) 2

SO

4 ) byproduct is later recovered from solution by crystallisation. Advantageously, the elemental sulfur thus formed may reacts with zero valent mercury in the contaminant solution to form insoluble mercuric sulfide (HgS), which can easily be separated from the contaminant solution by, for example, filtration or centrifugation, or other suitable separation means. [0036] Preferably, in the method of the invention, the inorganic ammonium polysulfide (NH4) 2 Sn) is formed in the contaminant solution by the reduction of elemental sulfur (S) by ammonium sulfide ((NH4) 2 S) in the contaminant solution. The inorganic ammonium polysulfide (NH4) 2 Sn) remains in solution and thus can be easily separated from the insoluble heavy metal salts. [0037] In a preferred embodiment of the invention, the inorganic ammonium polysulfide (NH4) 2 Sn) formed in the contaminant solution is decomposed under conditions suitable to form elemental sulfur, ammonia (NH 3 ) and hydrogen sulfide (H 2 S). Typical conditions include treatment with heat at steam temperature and pressure of from about 0.1 to about 5 MPa, preferably from about 0.2 to about 4.5 MPa, more preferably from about 0.35 to about 3.4 MPa and wherein the elemental sulfur, ammonia and hydrogen sulfide are formed in a vapour. [0038] After the decomposition step, the vapour may be condensed to form a solution comprising regenerated ammonium sulfide ((NH4) 2 S), and liquid elemental sulfur (S). The elemental sulfur (S) is then preferably separated from the ammonium sulfide ((NH4)2S) solution, for example, by filtration or by centrifugation to provide an elemental sulfur (S) residue, together with a supernatant solution comprising ammonium sulfate ((NH4) 2 SO4). [0039] In a particularly preferred embodiment, the ammonium sulfate ((NH4) 2

SO

4 ) is recovered by crystallization. Before crystallization the solution volume can be reduced to about 60% if the original volume to concentrate -8 [0040] In a preferred embodiment, the method of the invention further comprises pre treating the non-ferrous metallurgic flue gas to form the dust free non-ferrous metallurgic flue gas of step (i). Suitable pre-treatment include prior art dust/ash collection methods. [0041] In a related embodiment, the present invention provides a method of removing sulfur dioxide and heavy metals from metallurgic flue gas, comprising: (i) purifying a pre-treated non-ferrous metallurgic flue gas with an ammonium sulfide absorption solution to absorb the sulfur dioxide and the heavy metals Hg, As, Cd, and Pb from the pre-treated flue gas; ii) separating sulfidised heavy metal compounds formed during the purification from the ammonium sulfide absorption solution, using a separation processing technology, thereby cleaning the non-ferrous metallurgic flue gas. [0042] In a further related embodiment, the invention provides a method of recycling sulfur dioxide and heavy metals from metallurgic flue gas, comprising: (i) preliminarily removing dust from the non-ferrous metallurgic flue gas and cooling the flue gas to a temperature of below 40 C; (ii) atomising an ammonium sulfide solution at 3 - 5% weight using the vortex nozzle located in the absorption tower, whereby the metallurgic glue gas is purified and absorbed through the mist of the ammonium sulfide solution before the purified flue gas exits, (iii) allowing the liquid rich with the absorbed sulfur dioxide and the heavy metals to enter a self-redox reactor whereby the rich liquid is reacted in the reactor for 20 - 30 minutes, before filtering the precipitate at the bottom of the reactor and returning the filtered residue returns to a raw material pool; (iv) decomposing the filtrate under the conditions of heating under 0.36 Mpa pressure and obtaining the decomposed ammonium product; (v) cooling hydrogen sulfide mixed with ammonia to produce the ammonium sulfide solution, and returning solution to the system for recycling to generate the pure sulfur which can be centrifuged to obtain sulfur having a water content of 1 - 2%; -9 (vi) heating the liquid without solid sulfur and concentrating the liquid to 60% of the original volume of the solution; and crystallised the solution at 40 OC and separating to give ammonium sulfate. [0043] The method of the invention removes sulfur dioxide and heavy metals from emission waste gas and recycles them into useable form, and thus provides a particularly useful technology that saves energy, protects the environment and provides for a more efficient use of natural resources. [0044] In one embodiment, the method of the invention involves the following steps: (i) The non-ferrous metallurgic flue gas is pre-treated to remove dust and is then cooled to a temperature below 40 OC. (ii) A 3 - 5 % by weight ammonium sulfide solution is atomised using a vortex nozzle located in an absorption tower and the metallurgic flue gas is passed through the atomised ammonium sulfide solution to absorb sulfur dioxide and heavy metal oxides contaminants from the flue gas. (iii) The contaminant rich liquid discharged from the absorption tower solution is rich in sulfur dioxide and heavy metal oxide contaminants. The discharge is sent to a self redox tank for 20 - 30 minutes allowing the formation of a su/fidised heavy metal precipitate. In the reactor, ammonium hydrogen sulfate in solution is reduced to elemental sulfur which then reacts with ammonium sulfide in solution to generate ammonium polysulfide. The elemental sulfur formed reacts with any zero valent mercury present in the solution forming a mercury sulfide precipitate. The self redox tank can be simultaneously be used as a precipitation tank to allow extraction of the precipitate from the bottom of the tank by separation by filtration or centrifugation. The precipitate can then be stored in a raw material tank to be used as a raw material as required. The filtered liquid is further processed as described below. (iv) After removal of the sulfidised heavy metal precipitate, the filtrate (ammonium polysulfide rich solution) is recycled by a decomposition process that involves heating the filtrate using steam at a pressure of 0.35 - 3.4 Mpa to produce vapour and sulfur. The vapour formed, containing ammonia and hydrogen sulfide, is then cooled to form an ammonium sulphide solution which can be returned to the system. The sulfur -10 formed contains small amounts of water that can be separated by centrifugation. After centrifugation, the sulfur is recovered in granular form with a moisture content of from 1 -2%. [0045] The desulfidised supernatant is then steamed to obtain ammonium sulphate from the solution. The supernatant is then condensed the in the steamer until the volume of the solution of (NH4)SO 4 is reduced to 60% of the original solution. The steamed solution is then cooled below 40 OC to crystallise ammonium sulfate in the crystallisation tank. The ammonium sulphate produced is separated and removed by centrifugation. [0046] In a further still embodiment, the invention provides a method of removing sulfur dioxide and heavy metals from metallurgic flue gas, comprising: pre-treating a non-ferrous metallurgic flue gas to remove dust; cooling the flue gas to a temperature of below 40 C; purifying the flue gas by passing the cooled flue gas through an atomised ammonium sulfide solution to remove sulfur dioxide and heavy metal oxides from the flue gas; removing the ammonium sulfide solution containing the sulfur dioxide and heavy metal oxides; reducing the ammonium sulfide solution such that the sulfur dioxide and heavy metal oxides are reduced to insoluble sulfidised heavy metal compounds, ammonium sulfite, and elemental sulfur; separating the insoluble sulfidised heavy metal compounds from the ammonium sulfide solution to form a substantially heavy metal free ammonium sulfide solution, optionally removing the sulfidised heavy metal precipitate; decomposing the heavy metal free ammonium sulfide solution under conditions suitable to form sulfur and regenerate ammonium sulfite. [0047] To atomise the ammonium sulfide solution at 3 - 5% weight using the vortex nozzle located in the absorption tower. The metallurgic glue gas is purified and absorbed through the mist of the ammonium sulfide solution. The purified flue gas exits. The rich liquid with the absorbed sulfur dioxide and the heavy metals enter the self-redox reactor and the rich liquid is reacted in the reactor for 20 - 30 minutes, to filter the precipitate at the bottom of the reactor and the filtered residue returns to the raw material pool. The filtrate decomposed under the conditions of heating under 0.36 Mpa pressure and to obtain the decomposed ammonium product. Hydrogen sulfide is cooled to mix with ammonia to produce the ammonium sulfide solution, and the solution is returned to the system for recycling and the pure sulfur is generated which can be centrifuged to obtain sulfur having a water content of 1 - 2%.

-11 [0048] The liquid without solid sulfur is heated and concentrated to 60% of the original volume of the solution and then is crystallised at 40 OC and separated to give ammonium sulfate. [0049] The principle of simultaneously removing SO 2 and heavy metal pollutants from the metallurgic flue gas is as follows: [0050] Absorption: (NH4) 2 S solution absorbs S02, while simultaneously (NH4) 2 S efficiently remove heavy metal oxides of Hg, As, Cd, Pb in the flue gas by absorption. The absorption reactions are as follows and generate the indicated inorganic sulfur compounds: S02 + H 2 0 = H 2 SO3

H

2 SO3+ (NH4) 2 S - (NH4)HSO 3 + H 2 S

H

2 S + (NH4) 2 S -* 2(NH4)HS 2(NH4)HS + 2SO2 = (NH4) 2

SO

3 + S + H 2 0 (NH4)2SO 3 + 0.502 = (NH4)2SO4 (NH4)2S + PbO + H 2 0 - PbSj+ 2(NH4)OH (NH4)2S + HgO + H 2 0 - HgSj + 2(NH4)OH

SO

2 + H 2 0 + HgO -* HgSO41 (NH4)2S + CdO + H 2 0-- CdSj + 2(NH4)OH 4(NH4)2S + As 2

O

3 + 3H 2 0 -* As 2 S31 + 3(NH4)OH + (NH4)HS (NH4)2S + PbO + H 2 0 - PbSj +2(NH4)OH

NH

4 0H + SO 2 = NH 4

HSO

3 [0051] Self-redox reaction: ammonium hydrogen sulfate generated in the absorption tower is reduced in the redox reactor to elemental sulfur. The reaction is as follows: - 12 2(NH4)HSO 3 Io (NH4) 2

SO

4 +S + + H 2 0. [0052] Meanwhile, the elemental sulfur produced can react with any liquid mercury present in the solution to generate a mercury sulfide salt precipitate, thereby removing any zero valent mercury present: S +Hg =HgSI. [0053] The elemental sulfur then reacts with ammonium sulfide in the solution to generate ammonium polysulfide in the solution: (NH4) 2 S + (n-1)S -* (NH4) 2 Sn. [0054] The sulfuridised heavy metal compounds Hg, As, Cd, Pb all precipitate and can be removed by filtering, the precipitate and the raw material can be mixed up for recycling. [0055] Recycling the ammonium sulfide solution: The ammonium polysulfide solution arising as described above is decomposed by heating to allow separation of sulfur from the ammonium sulfide solution. The reaction is as follows: (NH4) 2 Sn = H 2 S + 2NH 3 + (n-1)S [0056] Ammonium sulfide is then regenerated by cooling the vapour comprising the hydrogen sulfide and ammonia and then can be reused in the process. [0057] The said metallurgic flue gas in the present invention refers to exhaust waste gas of a non-ferrous metallurgy process. Advantages of the present invention [0058] The SO 2 and heavy metal oxide compounds in metallurgic flue gas are simultaneously removed in a highly efficient manner. The treated flue gas exhaust meets the required emissions limits for these components. The sulfur removal efficiency is 95% and the

SO

2 content in the treated exhaust gas is less than s 400mg/m 3 ; the efficiency of the removal of the four heavy metals, mercury, arsenic, cadmium and lead is consistently higher than 90%. The content of Hg s 0.012mg/m 3 , As s 0.5 mg/m 3 , Cd s 0.052 mg/m 3 , and Pb s 0.07 mg/m 3

,

-13 and the side product sulfur is produced with a purity 98% and the total recovery rate of mercury is greater than 80%, the total recovery rate for lead, arsenic, cadmium is 65%. [0059] A method of SO 2 removal from metallurgic flue gas that generates useful side products such as sulfur and ammonium sulfate, etc., has a number of advantages, for example, provide low cost, yet highly efficient sulfur removal from flue gas. Furthermore, the sulfur generated by the method of the invention can be used in a broader range of applications than other sulfur products. In addition, sulfur is easily stored and transported. The present invention is suitable for industry scale up and is a particularly useful metallurgic flue gas cleaning technology as the method generates recyclable byproducts. [0060] The present invention removes heavy metals from metallurgic flue gas by generating recoverable sulfidised heavy metal compound precipitates that can be reused, while simultaneously avoiding release of unacceptable levels of heavy metal emissions in metallurgic flue gas exhaust, as well as avoiding undesirable secondary pollution of wastewater. DETAILED DESCRIPTION OF THE INVENTION [0061] After pre-treatment to remove dust from the metallurgic flue gas, the flue gas is cooled to a flue gas temperature of below 400C. [0062] A 3 to 5 % by weight ammonium sulfide solution is prepared. The ammonium sulfide solution is then atomised in a flue gas absorption tower using a specially designed vortex nozzle. The metallurgic flue gas is then purified by passing the flue gas up through the atomised mist of the ammonium sulfide solution in the absorption tower whereby sulfur dioxide and heavy metal pollutants absorbed by the atomised solution. The absorbed sulfur dioxide and heavy metal oxides of Hg, As, Cd and Pb are then scrubbed out of the atomised solution and collected for discharge from the absorption tower. DESCRIPTION OF THE DRAWING [0063] A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which shows the process flowchart for the present invention (Figure 1) in which: 1 represents the metallurgic flue gas to be treated. 2 represents a cooling tower. 3 represents an absorption tower. 4 represents the purified flue - 14 gas. 5 represents self-redox reactor. 6 represents a filter device. 7 represents a storage tank. 8 represents a heater. 9 represents a cooler. 10 represents a centrifuge. 11 represents an evaporator. 12 represents a crystallization tank. 13 represent a further centrifuge. 14 represents a circular pump. 15 represents a sulfidised heavy metal compound. 16 represents solid sulfur. 17 represents ammonium sulfate. PREFERRED EMBODIMENTS OF THE INVENTION [0064] The present invention will be explained below using the following embodiments, but the scope of protection is not limited to the specifically provided example. Example 1 - A method of recycling of sulfur dioxide from zinc metallurgic flue gas [0065] In this embodiment, the flue gas to be treated is a zinc metallurgic flue gas. The volume of the gas is 10,000 m 3 /h. The flue gas at the furnace exit contains sulfur dioxide: 4 6%, Hg: 0.2 mg/m 3 , As: 1 - 5 mg/m 3 , Pb: 10 - 15 mg/m 3 , and Cd: 2 - 7 mg/m 3 . [0066] After dust has been removed from the zinc metallurgic flue gas 1, the zinc metallurgic flue gas 1 enters the cooler 2 and the temperature of the zinc metallurgic flue gas is cooled to below 40 OC. [0067] A 3% by weight ammonium sulfide solution is atomised in the absorption tower 3 using a vortex nozzle. The cooled flue gas then enters the absorption tower 3 where it passed up through the mist to ensure thorough contact with the atomised ammonium sulfide solution to ensure the sulfur dioxide and the oxidised metals Hg, As, Cd and Pb are absorbed from the flue gas in a highly efficient manner. The purified flue gas 4 is then exhausted from the top of tower 3. [0068] The solution, which is rich in sulfur dioxide and heavy metals, then enters the self redox reactor 5 where a redox reaction is carried out for 20 minutes. In this process, metal sulfide compounds of mercury, lead, arsenic and cadmium are formed and precipitate from the solution. The clear liquid remaining in the reactor can then sent back to the absorption tower 3 for atomisation through the circulation pump 13. The precipitate collected at the bottom of the redox reactor 5 is pumped to filter 6 for filtering. The filtered residue contains the heavy metal sulfidised compounds 15, which are then collected in a raw material pool. The filtrate then flows to storage tank 7 for accumulation, and thereafter is sent to the solution heater 8 to be -15 heated by steam under a pressure of about 0.35 MPa until boiling. This decomposes ammonium polysulfide formed during the redox reaction into sulfur, H 2 S and NH 3 . The H 2 S and

NH

3 enters the cooler 9 to reform the ammonium sulfide which to return to the reactor to mix with the clear liquid remaining that is to be reused. The sulfur generated from the ammonium sulfur decomposition is separated while it is still hot using the centrifuge 10 to obtain the solid sulfur product 16 having a moisture content of 2%. [0069] (4) After sulfur removal, the supernatant is sent to evaporator 11 until condensed to 60% of the original volume, and is returned to the crystallization tank 12 to be cooled down to 40 OC to facilitate crystallization. The ammonium sulfate crystals formed are separated by centrifugation 13 to obtain the ammonium sulfate product 17 (see Figure 1). [0070] Using the above method, the efficiency of sulfur removal from the flue gas is 95% and the concentration of SO 2 in the flue gas at the furnace exit is $400mg/m 3 . The efficiency of heavy metal removal for mercury, arsenic, cadmium and lead is consistently larger than 90% and the concentration of Hg in the flue gas and furnace exit is less than Hg 50.012 mg/m 3 , As 50.5 mg/m 3 , Cd 50.5 mg/m 3 and Pb s 0.7 mg/m 3 . The purity of the sulfur side product 99%. The lead recycling rate is 65% and the total recovery rate of mercury is 75%. Example 2 - A method of recycling of sulfur dioxide from lead metallurgic flue gas [0071] In this embodiment, the flue gas to be treated is lead metallurgic flue gas. The volume of the gas is 50,000 m 3 /h, and then the flue gas at the furnace exit contains sulfur dioxide 8 - 15%, Hg: 0.4 mg/m 3 , As: 1 - 5 mg/m 3 , Pb: 35 - 45 mg/m 3 , and Cd: 1 - 3 mg/m 3 . [0072] After treatment to remove dust, the flue gas is cooled to below 40 OC. [0073] A 5% by weight ammonium sulfide solution is atomised in an absorption tower using a vortex nozzle. The metallurgic flue gas is purified by passing it through the mist of the ammonium sulfide solution whereby the pollutants therein are absorbed by the mist. The purified flue gas can then be exhausted from the absorption tower. The liquid rich in absorbed sulfur dioxide and heavy metals then enters the redox reactor where the concentrated liquid is reacted for 30 minutes. During this process, sulfidised metal compounds of mercury, lead, arsenic and cadmium precipitate from the solution.

- 16 [0074] The precipitate at the bottom of the reactor is filtered and the filtered residue is sent to a raw material pool. [0075] Ammonium sulfur in the filtrate is then decomposed under conditions of heat and pressure under 0.36 Mpa to obtain decomposed ammonium products. The hydrogen sulfide and ammonia generated in the decomposition step are cooled in the cooler 9 to produce regenerate the ammonium sulfide solution, which is returned to the system for use in the recycling process. The pure sulfur generated during the decomposition can be centrifuged to recover sulfur with a moisture content of 1%. [0076] The supernant, from which the solid sulfur has been removed, is then heated and concentrated to 60% of the original volume of the solution and allowed to crystallise at 40 OC. The crystallized ammonium sulfate can then be separated and removed from the system. [0077] In the above method, the efficiency of sulfur removal from the flue gas is 96% and the concentration of S02 in the flue gas at the furnace exit is $350mg/m 3 . The efficiency of heavy metal removal for mercury, arsenic, cadmium and lead is consistently larger than 90% and the concentration of Hg in the flue gas and furnace exit is less than Hg s 0.01 mg/m 3 , As s 0.1 mg/m 3 , Cd s 0.1 mg/m 3 and Pb s 0.6 mg/m 3 . The purity of the sulfur side product formed > 99%. The lead recycling ratio is 70% and the total recovery ratio of mercury is 90%. Example 3 - A method of recycling of sulfur dioxide from nickel metallurgic flue gas [0078] In this embodiment, the flue gas to be treated is nickel metallurgic flue gas. The volume of gas generated by the furnace is 36,000 m 3 /h, contains SO 2 : 0.8 - 1.1%, Hg: 0.2 mg/m 3 , As: 3-5 mg/m 3 , Pb: 10 - 25 mg/m 3 , and Cd: 1 - 3 mg/m 3 . [0079] After treatment to remove dust, the flue gas is cooled to below 40 OC. [0080] A 4% by weight ammonium sulfide solution is atomised using the vortex nozzle located in the absorption tower. The metallurgic flue gas is purified by passing it through the mist of the ammonium sulfide solution whereby the pollutants therein are absorbed by the mist. The purified flue gas can then be exhausted from the absorption tower. The liquid rich in absorbed sulfur dioxide and heavy metals then enters the redox reactor where the concentrated liquid is reacted for 25 minutes. During this process, sulfidised metal compounds of mercury, lead, arsenic and cadmium precipitate from the solution.

-17 [0081] The sulfidised metal compound precipitate at the bottom of the reactor is then filtered and the filtered residue is sent to the raw material pool. The filtrate is then decomposed by heating under 0.4 Mpa pressure to obtain the decomposed ammonium products as described above. The hydrogen sulfide and ammonia mixture are cooled to produce a regenerated ammonium sulfide solution, which is then returned to the system for recycling and the pure sulfur generated can be centrifuged to obtain sulfur having a water content of 2%. [0082] The liquid without solid sulfur is then heated and concentrated to 60% of the original volume of the solution and then is crystallised at 40 OC and separated to give an ammonium sulfate product. [0083] From the above method, the efficiency of sulfur removal from the flue gas is 95% and the concentration of SO 2 in the flue gas at the furnace exit is $400mg/m 3 . The efficiency of heavy metal removal for mercury, arsenic, cadmium and lead is consistently larger than 90% and the concentration of Hg in the flue gas and furnace exit is less than Hg s 0.010 mg/m 3 , As s 0.1 mg/m 3 , Cd s 0.1 mg/m 3 and Pb s 0.6 mg/m 3 . The purity of the sulfur side product is> 99%. The lead recycling ratio is 70% and the total recovery ratio of mercury is 90%. DEFINITIONS [0084] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains. [0085] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term 'about'. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, '%' will mean 'weight %', 'ratio' will mean 'weight ratio' and 'parts' will mean 'weight parts'. [0086] The terms 'preferred' and 'preferably' refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one -18 or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention. [0087] The terms 'a', 'an' and 'the' mean 'one or more', unless expressly specified otherwise. The terms 'an embodiment', 'embodiment', 'embodiments', 'the embodiment', 'the embodiments', 'an embodiment', 'some embodiments', 'an example embodiment', 'at least one embodiment', 'one or more embodiments' and 'one embodiment' mean 'one or more (but not necessarily all) embodiments of the present invention(s)' unless expressly specified otherwise. [0088] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. A method of purifying a non-ferrous metallurgic gas, comprising the steps of: (i) cooling a dust free non-ferrous metallurgic flue gas to a temperature of below 95 OC; (ii) contacting the flue gas with an atomised ammonium sulfide solution to form a contaminant solution comprising one or more inorganic sulfur compounds and at least one inorganic heavy metal oxide; (iii) converting the at least one inorganic heavy metal oxide in the contaminant solution into at least one insoluble inorganic sulfidised heavy metal compound; (iv) subjecting the inorganic sulfur compounds in the contaminant solution to conditions that form elemental sulfur and regenerate a solution of ammonium sulfide.

2. The method of claim 1 wherein during converting step (iii) the one or more inorganic sulfur compounds are converted to inorganic ammonium polysulfide in the contaminant solution.

3. The method of claim 1 or claim 2 wherein the inorganic sulfur compounds in the contaminant solution are selected from one or more of sulfur dioxide (SO 2 ), ammonium sulfide ((NH4) 2 S), sulfuric acid (H 2 SO4), ammonium hydrogen sulfate ((NH4)HSO 3 ), hydrogen sulfide (H 2 S), and ammonium hydrosulfide ((NH4)HS).

4. The method of any one of the preceding claims further comprising the step of recovering the elemental sulfur formed in step (iv).

5. The method of any one of the preceding claims further comprising the step of reusing/recycling the ammonium sulfide solution generated in step (iv) in step (ii).

6. The method of any one of the preceding claims wherein the concentration of the ammonium sulfide solution used in step (i) is from 3 to 5% by weight.

7. The method of any one of the preceding claims wherein prior to step (iii), the contaminant solution is separated from the flue gas. - 20

8. The method of any one of the preceding claims wherein the converting step (iii) involves a redox process.

9. The method of any one of the preceding claims wherein during step (iii) the at least one inorganic heavy metal oxide in the contaminant solution is converted to the at least one insoluble inorganic sulfidised heavy metal compounds in the form of a metal sulfide or metal sulfate salt.

10. The method of any one of the preceding claims wherein the at least one insoluble inorganic sulfidised heavy metal compounds are selected from PbS, HgS, HgSO 4 , CdS or As 2 S 3 .

11. The method of any one of the preceding claims wherein during the converting step (iii) the at least one inorganic heavy metal oxide is converted to the at least one insoluble inorganic sulfidised heavy metal compounds by a chemical reduction process.

12. The method of any one of the preceding claims wherein the insoluble inorganic sulfidised heavy metal compounds are separated from the contaminant solution.

13. The method of any one of the preceding claims wherein insoluble inorganic sulfidised heavy metal compounds are separated from the contaminant solution by filtration or centrifugation.

14. The method of any one of the preceding claims wherein the contaminant solution comprises ammonium hydrogen sulfate or a combination of ammonium hydroxide and sulfur dioxide which react in solution to form ammonium hydrogen sulfate.

15. The method of any one of claims 2 to 14 wherein during the converting step (iii) elemental sulfur is formed in the contaminant solution from the reduction of sulfur dioxide by ammonium hydrosulfide.

16. The method of any one of claim 15 wherein the elemental sulfur formed reacts with zero valent mercury in the contaminant solution to form insoluble mercuric sulfide.

17. The method of claim 15 wherein the insoluble mercuric sulfide formed is separated from the contaminant solution. -21

18. The method of any one of claims 2 to 17 wherein the inorganic ammonium polysulfide is formed in the contaminant solution by the reduction of elemental sulfur generated in any one of claims 15 to 17 by ammonium sulfide in the contaminant solution.

19. The method of any one of the preceding claims wherein ammonium hydrosulfide is generated from the reaction of ammonium sulfide with hydrogen sulfide in the contacting step (i).

20. The method of any one of the preceding claims wherein hydrogen sulfide and ammonium hydrogen sulfate are formed from the reaction of sulfuric acid and ammonium sulfide in the contacting step (i) and/or the reaction of sulfur dioxide and ammonium hydroxide, wherein the ammonium hydroxide is formed as a byproduct of converting step (iii).

21. The method of any one of the preceding claims wherein sulfuric acid is formed from the reaction of sulfur dioxide and water in the contacting step (i).

22. The method of any one of claims 2 to 21 wherein the inorganic ammonium polysulfide in the contaminant solution is decomposed under conditions that form elemental sulfur, ammonia and hydrogen sulfide.

23. The method of claim 22 wherein the decomposition conditions are heat at steam temperature and pressure of from about 0.1 to about 5 Mpa and wherein the elemental sulfur, ammonia and hydrogen sulfide are formed in a vapour.

24. The method of claim 23, wherein the vapour is condensed to form a solution comprising regenerated ammonium sulfide, and liquid elemental sulfur.

25. The method of any one of claims 22 to 24 wherein the elemental sulfur is separated from the ammonium sulfide solution by centrifugation to provide a elemental sulfur residue and a supernatant solution comprising ammonium sulfate.

26. The method of claim 25 wherein ammonium sulfate is recovered from the supernatant by crystallization. - 22

27. The method of any one of the preceding claims wherein the flue gas is contacted with the atomised ammonium sulfide solution by passing the flue gas through the atomised solution.

28. The method of any one of the preceding claims wherein the contaminant solution is separated from the flue gas by washing out and/or rising out the contaminant solution from the flue gas.

29. The method of any one of the preceding claims further comprising pre-treating the non-ferrous metallurgic flue gas to form the dust free non-ferrous metallurgic flue gas.

30. Product obtained by the method according to any one of claims 1 to 29.

31. Product according to claim 30 selected from an insoluble inorganic sulfidised heavy metal compound, ammonium sulfate, ammonium sulfite or elemental sulfur