WO 2011/060505 PCT/AU2010/001565 Method of Processing Metallurgical Waste Powder Field of the invention 5 The present invention relates to a process for processing waste powder produced in a metallurgical process. More specifically, the present invention relates to washing of granules of the powder to recover target species and/or to reduce the content of components that would otherwise inhibit further processing and/or use of the granules. The present invention also relates to apparatus suitable for implementation of the process 10 of the invention. Background to invention Steel production using electric arc furnace (EAF) technology has been increasing over 15 recent decades at the expense of other steel-making methodologies. When steel is produced in an electric arc furnace between 10-20kg of dust is produced per tonne of steel output. The dust forms as a result of volatile metals passing into the vapour phase at the prevailing furnace temperature and being oxidised. These metals are generally present in the dust as free oxides or in the form of composite oxides with iron oxide. The dust may 20 also contain other metals such as zinc, lead, copper, manganese and chromium due to the scrap iron raw material that is used or due to the use of additives in the steel-making process. For example, when galvanised steel scrap is used as raw material, most of the zinc vapourises.and ends up in the dust as zinc oxide and zinc ferrite. The dust also contains significant amounts of chlorides. 25 EAF dust is regarded as toxic waste due to its content of heavy metals and it is treated, recycled or land-filled. As world-wide production of EAF dust is several million tonnes per year, processing or disposal of the dust is a significant issue. Disposal of the dust as land-fill is an environmental concern and from that perspective processing of the dust is a 30 preferred option. Processing of the dust is also desirable since the dust contains many constituents that it would be desirable to reclaim.
WO 2011/060505 PCT/AU2010/001565 -2 Numerous techniques exist in the art for processing EAF dust. However, there remains the need for alternative processing methodologies that can provide practical advantages. The present invention seeks to provide a new process that enables EAF dust to be processed. 5 Summary of the invention Accordingly, the present invention provides a process of processing finely divided material produced in a metallurgical process, which process comprises: 10 continuous washing of a first portion of granules formed from the finely divided material by movement of the granules through a wash solution, the suspended solids content of which increases as a result of washing of the granules; wherein washing of the granules takes place in a vessel comprising a second portion of 15 granules formed from the finely divided material that are provided in the vessel to remove solids from the wash solution thereby reducing the suspended solids content of the wash solution; and wherein the first and second portions of the granules are continuously removed from 20 the vessel and replenished with fresh portions of granules for processing. Central to the present invention is the continuous washing of granules (formed from a finely divided material) in a vessel that also contains granules the function of which is to remove from the wash solution solids that are present in the wash solution as a result of 25 granule washing. Advantageously, the process of the present invention may be operated continuously by suitable removal of granules from the vessel and replenishment of "fresh" granules. In practice, at least some of the first and second portions of granules are in contact with 30 each other, with the first portion of granules being provided above (on top) of the second portion of granules. In other words, the invention involves movement through a wash solution of upper and lower portions of granules, with the granules in the lower portion WO 2011/060505 PCT/AU2010/001565 -3 serving to absorb suspended solids that are present in the wash solution as a result of washing of the upper portion of granules as that portion is moved through the wash solution. Some intermingling of the granules may take place, although this is preferably not extensive and the relative speeds at which the first and second portions of granules are 5 being moved may be adjusted accordingly. Detailed discussion of the invention The present invention will now be described with emphasis on processing of EAF dust, 10 and this is believed to be a particularly useful application of the invention. However, it will be appreciated that the principles underlying the present invention may be applied to the processing of waste products produced in metallurgical processes other than electric arc furnaces. Herein such waste products are referred to generically using the term "finely divided material". One skilled in the art would understand how the present invention may 15 be applied to other forms of waste products other than EAF dust. It is important in accordance with the present invention that the EAF dust to be processed is provided in the form of granules that retain integrity during processing. If the granules disintegrate during processing a sludge would form. The use of granules also facilitates 20 ease of handling (increased bulk density) and processing, and subsequent use. The granules may be formed as an initial step of the process of the invention. Alternatively, the invention may be implemented using previously prepared and stored granules. Generally, the EAF dust will have an average particle size of less than about 100 pm. The 25 granules will usually have an average particle size of from 0.5-15 mm. The granules may be formed by an agglomeration process in which EAF dust is subjected to high intensity mixing in the presence of a binder. Formation of granules in this way is known and one skilled in the art would familiar with the materials, equipment and methodology to be employed. Interestingly, once formed and stockpiled for drying the granules do not tend to 30 agglomerate into large clumps. If the granules do aggregate however they can easily be broken up.
WO 2011/060505 PCT/AU2010/001565 The granules should be -sufficiently robust to retain integrity during processing in accordance with the present invention. In this regard the granules may be evaluated using dry and wet tumbling tests as follows. 5 In the dry test the sample is prepared by proportioning selected size fractions that are based on the sample size distribution. The sample is then introduced into an "I" tube 100 mm in diameter and 800 mm in length. The ends of the tube are plugged to prevent sample loss. The tube is then tumbled end over end at 20 rpm for 10 minutes. The proportion of sample retained at 2 mm or more average size is determined. This test is intended to provide some 10 relative measure of granule strength in relation to agglomeration conditions, dust blending, binder additions, curing etc. Typically, after testing at least 80 wt% of granules, preferably at least 85 wt% of granules, exhibit an average particle size of 2 mm or more. In a wet test the sample is prepared similarly to the dry test sample. The sample is sealed 15 in a I litre jar containing 0.6 litres of water and tumbled end over end at 30 rpm for 10 minutes. The wet test provides measure of the physical resistance of the granules to degradation in terms of washing and weathering capacity. Typically, after testing at least 80 wt% (dry basis) of granules, preferably at least 85 wt% (dry basis) of granules, exhibit an average particle size of 2 mm or more. 20 The present invention advantageously involves washing of granules rather than powder/dust. Washing of powders/dusts can present problems with formation of sludge and produces a final filter cake that may not be readily amenable to further processing. In the present invention the granules remain intact through the washing process and show 25 minimal degradation. After washing the granules can be stockpiled, for example to allow for drainage of water. The washed granules can also be handled and transported with ease. The binder used to form the granules believed to modify the matrix of the granules to give required properties. The binder used may actually have an influence on the components 30 that are retained in the granules after washing. Binders such as ground slag, lime, cement and sodium silicate could be suitable. A cementitious binder has been found to be effective.
WO 2011/060505 PCT/AU2010/001565 -5 After formation the granules are allowed to dry/cure at ambient temperature before being processed in accordance with the present invention. This may take a number of days. 5 In the process of the present invention a (first) portion of granules are subjected to washing by movement of the -granules through a wash solution (water). It is believed to be important-that the granules are moved through the water'rather than flushing of a static bed of granules with a flow of water. In this regard washing of the granules may be regarded as a continuous process. During this washing the granules remain submerged in the wash 10 -solution. The granules may be moved through the wash solution by any suitable means that facilitates movement of the granules and intimate contact of the granules with the wash solution. By way of example, an axial screw may be used to move granules through a 15 wash solution. Typically, the granules will have a residence time of from I to 4 hours in the wash solution so as to ensure removal of target species, such as soluble metal chlorides. The residence time can vary depending upon the prevailing characteristics (content) of the wash solution 20 and its effectiveness in removing target species from granules. After having been washed as described this portion of granules may be discharged from the vessel in which washing is performed, rinsed as necessary with clean water to remove contaminated wash water and allowed to dry. Samples of washed granules may be 25 analysed for quality control purposes. Washing of the granules causes fines to be transferred into the wash solution thereby increasing its turbidity. In accordance with the -present invention this is mitigated by the use of another (second) portion of granules that are provided below the first portion of 30 granules and in contact with the water solution through which the first portion of granules are being moved. The effect of the second portion of granules is to capture solids suspended in the wash solution thereby reducing its suspended solids content. In other WO 2011/060505 PCT/AU2010/001565 -6 words the second portion of granules filter the wash solution so that turbidity problems may be reduced or avoided. Movement of the second portion of granules through the wash solution will also lead to a reduction in the solubles content of those granules, depending of course on the characteristics of the prevailing wash solution with respect to uptake of 5 such species. To perform this filtering function (second portion of) the granules are generally required to be essentially static. However, in accordance with the present invention to allow continuous operation it is necessary to continuously remove these granules from the vessel. 10 In practice it has been found that the effectiveness of the granules in terms of reducing turbidity can be retained if the granules are moved through the wash solution relatively slowly, but certainly no faster than the first portion granules that are being actively washed. 15 The second portion of granules is provided in the vessel used for washing below the first portion of granules that are completely submerged by wash solution. The residence time of the second portion of granules in the wash solution will not exceed that of the first portion of granules, although as noted the content of the wash solution is likely to influence this. 20 The second portion of granules is also removed from the vessel after passing through the wash solution. These granules will then be rinsed as necessary with clean water to remove contaminated wash water. 25 In practice the same granules may be used for the first and second portions of granules. The granules are porous in nature and are able to absorb fines suspended in the wash solution as a result of washing of other granules (making up the first portion). The granules making up the second portion of granules are formed in the same manner as 30 the granules of the first portion, i.e. agglomeration of particulate material in the presence of a binder. It is also possible to use as granules for the second portion, granules that have already been washed in accordance with the present invention. Such washed granules are WO 2011/060505 PCT/AU2010/001565 -7 likely to exhibit the porosity required for absorbing suspended solids during the process of the invention. To operate the process of the invention in continuous manner requires continuous removal 5 of granules from the vessel and replenishment as necessary. Granules making up the second portion of granules should be replenished as required in order to maintain the efficacy of those granules with respect to filtering suspended solids from the wash solution. 10 Desirably. the present invention enables a significant reduction in soluble chlorides of EAF dust. Thus, granules washed using the invention may report a reduction in solubles chloride content of 80% or more. Further rinsing with clean water may further reduce the solubles chloride content. 15 The content of the wash solution will change as the process of the invention proceeds and it too may need to be replenished to allow uptake of target species to continue to the desired extent. The wash solution may be sampled and analysed from time to time with this in mind. 20 The present invention also provides an apparatus for implementing the process of the present invention as described. Components of the apparatus and its operation should be apparent from the discussion of the present invention. In one embodiment, the present invention provides an apparatus for performing the process of the invention, the apparatus comprising: 25 a chamber for a wash solution; means for feeding granules into the chamber; means for moving granules through an upper part of the chamber in contact with the wash solution; and means for removing granules that have been washed by movement through the wash 30 solution in the upper part of the chamber; and means for moving granules through a lower part of the chamber in contact with the wash solution; and WO 2011/060505 PCT/AU2010/001565 -8 means for removing granules from the lower part of the chamber. In practice, the chamber may be configured such that the volume of the first portion of granules is larger than the volume of the second portion of granules. In this regard the 5 chamber may be made up of an upper and lower chambers that are in communication with each other. The means for moving the granules in each chamber is typically an axial screw with rotation of the screw moving the granules. It will be appreciated that the granules in the upper part of the chamber correspond to the 10 first portion of granules and that the granules in the lower part of the chamber correspond to the second portion of granules. Brie f discussion of drawings 15 An embodiment of the invention is illustrated in the non-limiting accompanying figures. Figures Ia and I b are schematics showing apparatus suitable for implementation of 'the process of the present invention. The apparatus Figure la is a side view and Figure lb a top view. 20 The apparatus comprises a hopper (1) that in use feeds granules to be processed into an upper chamber (2) including a wash solution. A vibrating in-feed conveyor (Ia) may be used to deliver granules from the hopper. Within vessel (2) the granules will be moved through the wash solution (from right to left in the embodiment shown) using an axial 25 screw (3) driven by a motor (4). At the end of the screw (3) remote from the hopper (1) is a rotating wheel (5) that is adapted to receive granules that have been moved through the length of the upper chamber (2) under the action of the screw (3) and the lower chamber (8) under the action of the screw (7). The wheel (5) in turn feeds granules onto a conveyor belt (6) which removes the granules for stockpiling. The granules may be rinsed on wheel 30 (5). The apparatus also includes a second screw (7) provided in a lower chamber (8). The screw (7) is driven by a motor (9). In use the screw (7) will move granules through wash solution provided in the lower chamber (8). The upper and lower chambers (2, 8) communicate with each other with respect to wash solution but granules in chamber (8) are WO 2011/060505 PCT/AU2010/001565 -9 replenished from chamber (2). The upper chamber (2) and screw (3) are somewhat bigger than the lower chamber (8) and screw (7). Granules that have been moved through the lower chamber (8) by the action of the screw (7) are also discharged on to the rotating wheel (5). 5 In use the granules are washed continuously in the upper chamber (2) by movement of the granules through wash solution by the action of screw (3). During this washing step the granules remain submerged in wash solution. As the granules are washed the turbidity of the wash solution will increase. This is mitigated by use of the lower chamber (8) that is in 10 communication with the upper chamber (2). In the lower chamber (8) the screw (7) moves granules relatively slowly compared to the screw (3) and the granules in that chamber find fines that have been discharged into the wash solution by washing of granules in the upper chamber (2). Thus, the granules in the lower chamber (8) filter the wash solution thereby reducing the suspended solids thereby reducing the suspended solids content of the wash 15 solution. Granules are continuously fed into the apparatus at one end for processing and continuously removed at the other end for rinsing or stockpiling. The wash solution may be replenished from time to time by a process of continuous removal and topping up. This 20 will be necessary in order to maintain the efficacy of the wash solution with respect to uptake of species introduced into the wash solution during the washing process. Figures 2-5 show different views/perspectives of the apparatus illustrated in Figures Ia and I b, or componentry of the apparatus. Reference numerals apply as per Figure Ia and I b. 25 In the embodiment shown approximate dimensions are as follows: overall length: 9m diameter screw (3): 2m 30 diameter screw (7): 0.5m diameter wheel (5): 3m WO 2011/060505 PCT/AU2010/001565 - 10 It is envisaged that the screw (3) will be rotated at 0.4 RPM and that the output of this screw onto the wheel (5) will be 2500 kg/hr. The screw (7) is likely to be rotated at a speed of 0.2 RPM and the output of this screw onto the wheel (5) is likely to be 600 kg/hr. 5 The granules produced in accordance with the present invention may be subjected to subsequent processing as is conventional in the art. The granules may be processing to recover retained target species, for example. Alternatively, if target species have been removed from the granules using the present invention, they may -be put to practical use, for example as feed to produce strong sinter and briquettes. The used wash solution may 10 also be processed subsequently to recover target species using conventional methodologies. The following non-limiting example illustrates an embodiment of the present invention. 15 Example Granules having a distribution of particles sizes are produced by mixing an EAF dust (approximately 85 wt%), cementitious binder (approximately 2 wt%) and water (approximately 13 wt%) in a high intensity mixer. The granule size depends upon the 20 operating conditions of the high intensity mixer. The size characteristics of the granules pre- and post-washing are included in the following table. The granules are processed using apparatus having the characteristics- set out above with the screws (3,7) being rotated at 0.4 RPM and 0.2 RPM, respectively. The output of the 25 screw (3) is 2500 kg/hr and the output of the screw (7) 600 kg/hr. The apparatus is replenished with (first and second portions of) granules at the same rates. The granules used for absorbing suspended fines were the same type as the granules being Washed. The wash water is continuously replenished to maintain set soluble chloride extractor rates. 30 Before washing the granules contained 11000 mg/kg of total soluble chloride. The as washed granules had a total soluble chloride content of 2100 mg/kg. This result showed a soluble chloride reduction of 80.9%. When the as washed granules were rinsed with clean WO 2011/060505 PCT/AU2010/001565 water at a water:solid ratio of 3:1, the soluble chloride reduction increased to 83.6%. This last result shows the importance of rinsing the washed granules with clean water to remove contaminated wash water. 5 Typical Size Distribution FEED MATERIAL POST WASHED PRODUCT Size (mm) g %Retained %Cumulative %Pass g %Retained %Cumulative %Pass "+16.0 2.0 0.2 0.2 99.8 18.0 2.0 2.0 98.0 "+11.2 17.1 1.9 2.1 97.9 46.0 5.1 7.1 92.9 "+8.0 56.5 6.3 8.5 91.5 59.0 6.6 13.7 86.3 "+4.0 210.7 23.6 32.1 67.9 227.0 25.3 39.0 61.0 "+2.0 281.6 31.6 63.7 36.3 280.3 31.2 70.2 29.8 "+1.0 180.6 20.3 84.0 16.0 169.7 18.9 89.2 10.8 "-+0.5 79.8 9.0 92.9 7.1 78.0 8.7 97.8 2.2 Residual 63.2 7.1 100.0 19.3 2.2 100.0 Total 891.5 .100.0 897.3 100.0 The influence of the accumulated soluble chloride content in the wash solution and the rate of removal of soluble chloride from the granules was surprising. Management of the 10 soluble chloride in the wash water was found to be a major factor in the effectiveness of the washing operation. Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention 15