Processing methods and processing apparatus

Abstract

A method and apparatus for processing a material are provided, the material being the upper layer from a metal melting process, the material containing one or more salts, the material containing one or more metals, the salts and/or metals being recycled as a result of the method/apparatus. The method includes feeding the material to a leaching step; obtaining a leachate from the leaching step; feeding the leachate to a drying step or spray drying step; obtaining a solid from the drying step or spray drying step. Off gases from the leaching step are used to provide heat to the drying step. The drying step provides a product well suited to being turned into pellets for reuse.

Claims

1. A method of processing a material, the material being the upper layer from a metal melting process, the material containing one or more salts, the material containing one or more metals, the method including: a) feeding the material to a slag press and compressing the material; b) feeding the compressed material to a size reduction; wherein steps a) and b) are provided before the temperature of the compressed material has reduced below 350 C.; the method further including: c) reducing one or more dimensions of the compressed material in the size reduction step, the size reduction step including a crushing step; d) feeding the crushed material to a leaching step; e) obtaining a leachate from the leaching step; f) feeding the leachate to a spray drying step; and g) obtaining a solid from the spray drying step.

2. A method according to claim 1 in which the leaching step further provides one or more other outlet streams, including an off gas stream, the off gas stream being further processed by combustion, with heat and/or power generated by the combustion being used in the spray drying step.

3. A method according to claim 1 further comprising a step for feeding hot gas to the spray drying step, the hot gases being from an off gas stream processing and/or possessing energy generated by an off gas stream processing.

4. A method according to claim 1 including the further step of compressing and/or moulding the solid into pellets.

5. A method according to claim 1 wherein the leaching step comprises a multiple pass leaching process where multiple passes through one leaching tank are provided and/or a multiple stage leaching process where passage through a number of leaching tanks is provided.

6. A method according to claim 1 including a step for obtaining a solid from the spray drying step, by separating the solid and a gas in the spray drying step.

7. A method according to claim 6 including a step for compressing and/or moulding the solid into pellets in one or more further steps.

8. A method according to claim 7 wherein the step for compressing and/or moulding is provided without adding any further components to the solid.

9. A method according to claim 7 wherein the compressed and/or moulded solids are be fed to a furnace to provide a material on the surface of a molten metal in a furnace.

10. A method according to claim 7 wherein the compressed and/or moulded solids are produced with a controlled ratio of sodium chloride to potassium chloride.

11. A method according to claim 7 wherein the step for compressing and/or moulding solids is provided without adding any binders or moisture.

12. A method according to claim 1, wherein the one or more salts are selected from sodium chloride, sodium chlorate, potassium chloride, or potassium chlorate.

13. A method according to claim 1 further comprising a step for the recycling of one or more metals produced.

14. A method according to claim 1, wherein steps a) and b) are provided before the temperature of the compressed material has reduced below 525 C.

15. A method according to claim 1 further comprising a step for recycling one or more salts produced.

16. A method according to claim 1 wherein the one or more metals are selected from aluminium or aluminium oxide.

17. A method according to claim 1 wherein steps a) and b) are provided before the temperature of the compressed material has reduced below 760 C.

Description

(1) Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 is a flow diagram for a process for the recovery of salt from an aluminium recycling process, including improvements according to the invention;

(3) FIG. 2 is a flow diagram for a process according to an embodiment of the invention;

(4) FIG. 3 is a schematic illustration of a spray dryer for use in the invention.

(5) There are various problems with the large scale centralised process. These include:

(6) The transport costs in taking the compressed slag 9 to a suitable centralised plant for processing;

(7) The significant regulatory issues (particularly if country borders are crossed) with such transportation;

(8) The safety risk from accidents and the like involved in such transportation;

(9) Having to use a centralised process plant, outside of the salt producer's control;

(10) The increased energy requirements and difficulty in achieving size reduction of the cold compressed slag;

(11) The variable nature of the feed slag, due to its containing different salt levels, different slat proportions, different contaminants and the like as it arrives from various different aluminium recyclers, giving process control issues;

(12) The variable nature of feed slag impacting upon the dry salt produced (in terms of its chemical make up, moisture content and size distribution) meaning the product is not well controlled or defined and so impairs it as a good salt feed for the furnace.

(13) In FIG. 1, a rotary furnace 1 is used to recycle aluminium, such as scrap which is contaminated with paint, lacquers and the like. The furnace 1 generates an aluminium product stream (not shown) together with an off gas stream 3 and a slag stream 5.

(14) The off gas stream 3 is subjected to suitable processing to recover/use or render safe the gases it contains.

(15) To prevent oxidisation of the aluminium and to provide other benefits, a salt is added to the top of the melt in the furnace 1 to form a barrier. The salt, together with some aluminium, forms the slag and hence the slag stream 5. The salt is mainly sodium chloride, but with potassium chloride too and other materials. A typical salt slag (also known as aluminium salt cake) produced by the secondary aluminium industry may contain 15-30% aluminium oxide, 30-55% sodium chloride, 15-30% potassium chloride, 5-7% other materials (such as aluminium and impurities including carbides, nitrides, sulphides and phosphides etc.

(16) After processing, the aluminium is drained from the furnace 1 and the slag is the main material remaining. The slag is formed mainly of the salt, together with the removed contaminants and other components picked up in the melting process.

(17) When the slag stream 5 is removed from the furnace 1 it is general emptied into bins (not shown) and then fed to a slag press 7 to allow for mechanically assisted agglomeration of the aluminium from the slag. The aluminium is generally returned to the furnace 1 after separation. The compressed material from the slag press 7 is classified as compressed slag 9.

(18) The compressed slag 9 is allowed to cool. The compressed slag 9 still contains a material amount of aluminium and a large amount of salt. The compressed slag is classified as a hazardous material, with respect to environmental and other laws, in many countries and so requires further consideration.

(19) One option is to transport the compressed slag 9 to a suitable plant for the recovery of the aluminium and treatment to recover the salt. This involves road transportation of the compressed slag 9 from the site of the furnace 1 where the compressed slag 9 is generated, to the distant processing plant. This involves significant transport costs, significant regulatory issues (particularly if country borders are crossed) and a safety risk from accidents and the like.

(20) The typical process plants are large installations which provide centralised treatment of the compressed slag 9 arising from various aluminium melting plants.

(21) The compressed slag 9 is cold by the time processing starts and is fed to a salt crusher 11 to reduce the compressed slag 9 in size. The cold nature of the compressed slag 9 presents difficulties and increased energy requirements for the size reduction steps. For instance, pneumatic breakers may be needed to initially break up the compressed slag 9 into smaller lumps. The size reduction may include the illustrated salt crusher 11, together with size based separation steps, such as sieves.

(22) The size reduced salt stream 13 is then fed to an eddy current separator 15. This generates a ferrous stream (not shown) which is recycled, an aluminium stream 17 which may be collected and sold and/or may be returned to the furnace 1 and a cleaned salt stream 19.

(23) The cleaned salt stream 19 is fed to a leaching tank 21, together with a water stream. The leaching process dissolves the salt content of the cleaned salt stream 19, ionises the sodium chlorate and so provides a dissolved salt stream 23. The remaining solids, generally aluminium oxides and non-metallic particles form suspended solid stream 25. The leaching off gas stream 27 is fed to a combined heat and power unit 29 where it is burnt to provide power and heat to the overall process, in particular to the steam dryer 31. Typical gases include hydrogen, methane, and ammonia. Phosphine and hydrogen sulphide may also be present.

(24) The suspended solid stream 25 is fed to a filter 33 to separate the solids from the liquid. The solids are a useful by product of the process which can be sold. The liquids are returned (not shown) to the leaching tank 21.

(25) The dissolved salt stream 23 is passed to an evaporator 35 to give rise to a steam stream 37, a gas hydrocarbon stream 39 (which also goes to the combined heat and power unit 29) and a wet salt stream 41.

(26) The steam stream 37 is fed to a condenser 43 and the resulting condensed water stream is fed back to the leaching tank 21.

(27) The wet salt stream 41 is taken to a super heated steam dryer 31. This produces a steam/water vapour stream (not shown) which is returned to the leaching tank 21 and the products stream, the dry salt stream 43. The dry salt stream 43 can provide salt feed 45 for the furnace 1.

(28) The present invention attempts to address these issues and/or provide improvements in them as a result of the process approach and changes made.

(29) Referring to FIG. 2, the overall process of an embodiment of the invention is provided. Once again, the rotary furnace 51 is used to recycle aluminium and the furnace 51 generates an aluminium product stream (not shown) together with an off gas stream 53 and a slag stream 55.

(30) Features in common with the approach of FIG. 1 are not repeated, but are still applicable to the FIG. 2 embodiment. For instance, the off gas stream 53 is subjected to suitable processing to recover/use or render safe the gases it contains.

(31) The slag stream 55 is removed from the furnace 51 it is then fed to a slag press 57. The compressed material from the slag press 57 is again the compressed slag 59.

(32) In this embodiment of the invention, the compressed slag 59 is processed at the same site as it is generated at. This is possible because of the lower capital cost and lower unit sizes viable for the overall process selected in this embodiment when compared with the prior approach. A typical localised plant might be configured to handle 1 tonne or less per hour of salt, whereas centralised plants may handle 5 or more tonnes per hour.

(33) A first benefit arising from this is that the compressed slag 59 is only allowed to cool to the desired extent. That cooling still aids handling and reduces the risk of the hot aluminium reacting further, but the compressed slag 59 is not allowed to cool to an extent that would cause negative impacts in the subsequent size reduction stage.

(34) The compressed slag 59 is fed to a slag crusher 61 to reduce the compressed slag 59 in size. However, as the compressed slag 59 is not cold and hard, the size reduction is easier to achieve and the size distribution resulting is more readily controlled. This gives a better size distribution and/or reduced amount of fine material, for instance sub 1 mm material. This is significant in improving the saleability of the solids recovered from the suspended solids stream 75.

(35) After size reduction, the size reduced slag stream 63 is then fed to an eddy current separator 65. This generates a ferrous stream (not shown) which is recycled, an aluminium stream 67 which is returned to the furnace 1 and a cleaned salt stream 69.

(36) The cleaned salt stream 69 is fed to a leaching tank 71, together with a water stream 73. The same general leaching process is provided.

(37) The known nature of the slag 59, because the aluminium recycling plant operator and the slag processing plant operator is the same, mean that the feed material to the leaching tank 71 is better known and more consistent.

(38) The dissolved salt stream 73, the suspended solid stream 75 and the leaching off gas stream 77 arise as before. The leaching off gas stream 77, however, is now made use of to increase the energy efficiency of the overall system. The leaching off gas stream 77 is fed to a combined heat and power unit 79 where it is burnt to provide power and heat to the overall process. In particular, the power can be made of throughout the process to drive one or more processes and the heat can be made use of in the spray dryer 97. Typical gases present include hydrogen, methane, and ammonia.

(39) The suspended solid stream 75 is fed to a filter 83 to separate the solids from the liquid. The solids are a useful by product of the process which can be sold and with improved properties and hence value, as mentioned above.

(40) The dissolved salt stream 73 is then subjected to different processing steps than before.

(41) The dissolved salt stream 73 passed to the spray dryer 97, described in more detail below. The spray dryer benefits from heat recycled from the furnace 51 and/or from the combined heat and power unit 59. The salt stream 91 arising is much drier than from the evaporator in the FIG. 1 form.

(42) In an embodiment not shown, the dissolved salt stream 73 may be passed to an evaporator to give rise to a steam stream, a gas hydrocarbon stream (which also goes to the combined heat and power unit 79) and a wet salt stream 73 which then passes to the spray dryer 97 described above. An issue with the salt stream 91 is that the particle size is relatively small. However, the applicant has identified that this issue can be address by feeding the salt stream 91 to a pelletiser 99, described in more detail below. Furthermore, the applicant has identified that the salt stream properties and size are ideally suited to pelletisation without the need for further treatment or additives, such as binders.

(43) As the product stream, the dry salt stream 93 is now pelletized, this allows for the optimisation of the physical properties, such as the size of the pellets, for the subsequent reuse of the dry salt stream 93 as a salt feed 95 for the furnace 51.

(44) In addition, the pellets are known to be of a standard content and so offer more controlled feeds to the furnace 51, for instance in terms of the ratio of sodium chloride to potassium chloride present (which is used to control the sodium chloride melting temperature). The blending of different pellets is also possible to provide other variation in the feed to the furnace 51.

(45) Finally, the pellets are also more suited to storage, without moisture take up, for instance, than the dry salt product of the FIG. 1 form.

(46) As illustrated in FIG. 3, the spray dryer 97 receives the dissolved salt stream 73 via a balance tank 200, with the flow into the tank controlled by a valve 202 to maintain the solution level. Low to open and high to close level probes are used to operate the valve 202. A water inlet 204 is also provided to the balance tank 200. The outlet 206 supplies the dissolved salt stream 73 to a peristaltic pump which pumps the dissolved salt stream 73 to the atomiser 210.

(47) The atomiser 210 also receives filtered air.

(48) The atomiser 210 makes use of a two fluid nozzle atomiser offering co-current mode, but rotary atomisers and two fluid atomisers in fountain mode are possible too. The high velocity air encounters the low velocity fluid to disintegrate that into droplets.

(49) The atomiser 210 is positioned in the upper part of the drying chamber 214, with the drying chamber 214 being formed of a cylindrical upper part 216 and conical lower part 218. The bottom of the conical part 218 provides the powder outlet 220 and moist air side outlet 222.

(50) The atomised fluid directly encounters the hot air which has passed through the heating stage 212. The heating stage 212 includes the electrical heat source and baffled air distributors to control the flow pattern and heat transfer in the air.

(51) Upon entering the drying chamber 214 through the atomiser 210, the dissolved salt stream 73 comes into intimate contact with the hot air. This provides for the drying in the hot air stream. Away from the atomiser 210 the drying chamber 214 allows the separation of the dry powder from the hot air, with both passing to their respective outlets.

(52) The dry powder is conveyed away by a fluidised ejector. Cooling and packing of the powder can then be provided for.

(53) The separated hot air enters an air stream going to atmosphere which is provided with bag filters to remove any remaining powder, with cyclones offering an alternative form of filtering.

(54) The following table defines some typical operating characteristics of a suitable spray dryer.

(55) TABLE-US-00001 TABLE 1 Characteristic Value Units Processed material Chloride based salts Average residence time in dryer 11 s Material feed rate 100 Kg/hr Feed solids content 30 % Rate of powder production 30 Kg/hr Evaporation rate of dryer 70 Kg/hr Residual moisture in product 1 % Drying air inlet temperature 190 C. Drying air outlet temperature 97 C. Powder temperature 85 C. Mass flow rate of drying air 2454 Kg/hr Heat input to dryer 103905 Kcal/hr Heat input to dryer 120 KW Operational running load 20 HP Electrical supply 440-3-50 VV-ph-Hz Instrumentation 24 V

(56) The spray drying product has closely controlled particle size distribution, residual moisture content, bulk density and morphology

(57) In terms of the results achieved, the variation in moisture content achieved with varying conditions is detailed in Table 2.

(58) TABLE-US-00002 TABLE 2 Feed Inlet Outlet Rate Salt Conc Moisture Trial Temp C. Temp C. L/hr wt % NaCl:KCl wt % 1 150 90 1.2 24.2 66:34 0.27 2 195 102 2 24.2 66:34 1.05 3 150 90 1.2 24.6 50:50 0.31 4 195 100 2 24.6 50:50 1.64 5 150 90 1.2 16.7 66:34 0.32 6 196 102 2 16.7 66:34 7 150 90 1.2 17.0 49:51 1.18 8 197 99 2 17.0 49:51 1.40

(59) The particles generated, when measured using SEM imaging, are generally around 5 to 30 microns in diameter and are blocky crystals in appearance. Optical microscope imaging also confirmed that the particles tend to form agglomerates of 120 to 2500 microns in diameter.

(60) The pelletiser 99 is important in increasing the size of the product to a size at which it is not prone to airborne transportation out of the furnace.

(61) Roll based compactors were used to examine the suitability of the powder to pelletizing. The powder was fed using gravity through two counter rotating rollers. Friction between the material and the roller surfaces brought the powder into the narrow space between the rolls and the high loads present gave good compaction of the powder. The powder held together in the compacted form without the need for any binders, water or other additives to be present.

(62) Roll based pelletisers, where the rolls bear opposing halves of the recessed spaces where the pellets form where then used to form pellets successfully.

(63) The technique readily achieves moisture contents below 0.2 wt % and allows less than 2% of the material to be below 149 microns in diameter (so minimising the amount of fines present).