PELLETING OR BRIQUETTING OF DISPERSE METALLURGICAL WASTES WITH THE AID OF PLASTIC BINDER DERIVED FROM TREATED PLASTICS WASTE
The present invention relates to the utilization (re-use) by cold pelleting of disperse wastes or environment- contaminating powders and sludges, respectively, formed in the metallurgy.
An economical and environment protection problem in the field of metallurgy is that several kinds of powders or powder-like materials, i.e. sludges are formed both in the manufacturing of iron and steel as well as in the preparation and fishing of iron alloys (master alloys) used in the steel manufacturing. Although the ingredients of the powders and sludges are very advantageous, however, they cannot be recycled into the main technological processes and cannot be kept in deposition without processing because they have a high dispersity grade.
One of the material wastes contain mainly iron or iron compounds, respectively. Such materials are on the one hand so called oily scales or fine chips qualified as dangerous wastes; on the other hand, dry scales considered to be harmless. Powder-like alloying substances (ferrosilicon,
ferrotitanium, ferrochromium, ferrovanadium, ferro- phosphorus) in form of powders are less environment- polluting below a certain particle size, however, they are also environment-damaging through inhalation or due to their dissolution into the ground waters. According to the prior art, a small part of powders qualified as dangerous are rendered harmless while a major part thereof are put in deposits. While being rendered the wastes harmless, contaminations are removed but the re-use of the product has not been solved. The re-feeding of powders into the furnaces or steel-alloying ovens causes serious problems and their re-use for the ore dressing is similarly cumbersome.
A known method of re-use consists in the pelleting of powders and the metallurgical utilization of this lumpy powder material. ■ The pelleting of the powders cannot be accomplished without a binder. The binder should have such properties that the pellets formed after compression possess an appropriate compressive strengh as well as a significant abrasive resistance for the technological-metallurgical use and further, for the handling and transfer, etc. of the material. Moreover they should be suited as a feed to the technical requirements of the furnace or steel-producing apparatus.
An other important requirement for not dry powders and sludges consists in that the contaminants (mainly oil) should not impede the binding via pelleting. A purification or pre-treatment of the waste material before use has to be eliminated since this would increase the expenditures. In addition to the so-called "green strength", the pellets has to maintain a significant proportion of their initial strength after having been fed into the furnace, i.e. at a higher temperature, otherwise its behaviour will not be identical or similar to that of the usual ore sinters. Another requirement against the binder is that the usual presses should be suitable to process the mixture without any essetial transformation, i.e. the technological operation should be carried out within the usual pressure and temperature ranges.
Various processes have been developed for the lumping or pelleting of fine-granular waste powders possessing a valuable metal content, since the powders mentioned can usually be pelleted only by means of auxiliary binder(s).
According to DE-P3139 375 10% of cement and/or caustic lime are used as binders. At least 15% of water are added thereto and after standing for 3 to 14 days the pellets
reaches the desired strength; till then, the pellets should be stored under roof. It is obvious that the high amount of ballast material and the long time of drying significantly impair the re-use of the waste, processed according to the cited process.
Another type of binders the organic polymers such as polyacrylamide, carboxymethylcellulose, polyacrylic acid salts, etc. are used for pelleting as well. According to US 5,102,455 iron ore powders are mixed in a wet state with an aqueous solution of anionic polymers and then pelleted. According to US 5,100,467 in addition to the same polymer solutions, bentonite is also added to the pelleting processes carried out similarly as disclosed in the references EP 0,413,603 and EP 0,225,171. EP 0,296,068 discloses the binding with the aqueous solution of the cited polymers and specifies the quality of the organic polymeric solutions and bentonite material as well as the method of mixing these components to the ore concentrates.
A common disadvantage of thes known processes is that polymeric binders are expensive; a long and energy- consuming drying is required to achieve the apropriate "green strength" of the pellets. A low water content is also
expedient for the further use since a high water content would cause foaming of the metallurgic inset material and this if would diminish the strength thereof at a high temperature. It appears also clearly from the referrences that an appropriate green strength cannot be accomplished without addition of a clay mineral increasing the amount of silica in the pellets which is disadvantageous in the metallurgic re-use.
Numerous processes are known, which suggest byproducts of sugar manufacture, mainly molasses as binders. Considering that the by-products in themselves do not provide a required pellet strength, lime or other similar inorganic additives should be used.
According to EP 0,630,980 the metallurgical waste is firstly coated with a fine hematite emulsion advantageously influencing the binding capacity of the mixture of caustic lime with molasses used as a binder.
According to DE 294 2899 the caustic lime is prepared by slaking in situ with water, calcium oxide being present in the metallurgic wastes, while additioning also molasses lime mixtures, soda and sulfite wastes of paper industry as other binders.
According to US-P 5,100,464 gypsum-free cement and molasses are employed as binders. A variant of this is published in the US-P 4,725,358, where iron powder, Si02 and carbon powder are pelleted and utilized thus replacing ferrosilicon.
A process is disclosed in DE-P3,642,411, according to which on-containing powders are bound to various carbohydrates occurring as by-products of sugar industry and simultaneously, the reductive power of carbon generated by pyrolysis of the binder is utilized. As mentioned above, these processes are associated with the drawback that the water has to be removed. Moreover, it is rather difficult to adjust the mixture ratios to form a product with a constant strength and composition suited to the requirements of the subsequent metallurgic process.
The use of known organic binding materials in themselves for pelleting is disclosed e.g. in US-P 4,865,642 and EP 0,329,281 where the use of by-products of the hydrocarbon industry, such as waxes, tar, bitumen, distillation residues and similar combustible materials as pelleting binders are suggested. The principle of this process comprises mixing binding materials - which have a low
melting point but are solid at room temperature - at a temperature above their melting point homogenously as liquids with the powder-like material and the cooling it in a mould after compaction. Considering that the product very easily disaggregates and is unsuitable for re-use, it is used for the environment-protection and rendering wastes harmless. In order to counterbalance the disadvantageous properties of asphalt and other similar easily softening organic binders, the addition (additioning) of lignosulfonates, carbohydrates and silicates as semi-"hardening" additives are suggested in the US-P 4,659,374; however, the difficulties related to the kinds of bitumen materials possesing a low softening temperature are not solved because of the sulfur content of the bitumens. On the other hand, to reproduce an even and smooth pelleting is rather difficult.
Object of the invention is the re-use of disperse wastes formed in the metallurgy by means of a cold-pelleting process using a binder, which can be employed with better result than the known binders. According to the invention this object has been solved by using as a binder the product prepared according to the PCT patent specification WO 99/29481.
The product disclosed in PCT WO 99/29481 is a result of a processing of crushed mixed plastic waste containing at least 20% by weight of low density polyethylene with a softening point lower than 150°C and of 5 to 40% by weight of a mineral oil or a derivative thereof calculated on the weight of the plastic waste by means of intense mechanical kneading and moulding at a temperature between 150°C and 250°C, said kneading continued until the formation of a quasi-homogenous melt phase mass whereafter introducing the mass into an extruder to obtain a granular product.
Herein, the term "binder" or "binding material" means the product prepared by the process according to the PCT specification referred to above.
Actually, the advantageous behaviour of this pelleting binder in furnaces is connected with the fact that unexpectedly several technological properties of the binder suggested are much more preferred as compared to the known binders. The specific feature of the binder is its softening occurs in a broad temperature interval from 150°C up to 250°C. The particles of the powder may easily be incorporated into the binder under compression at a low temperature range, however, they will not flow out from the
powder mixture because of its higher melting point. In comparison to the carbohydrates and other non- thermoplastic materials, it is actually endowed of the advantage that the binder used is thermoplastic, its softening and binding function occur without any considerable gas evolution whereas gases - at first water vapours - arise e.g. from molasses at pyrolysis forming the binding, a phenomenon being disadvantageous from the technological point of wiev.
In comparison to other organic binders, a further advantage of the binder employed consists therein that it can be ground to a fine powder and therefore, it can excellently and homogeneously be mixed with the waste powder mixture requiring the binding and agglomeration. The properties of the binder are the same as that of cement and other powder-like inorganic binding materials, however with the significant difference and benefit that the metall- contaming pellets to be processed is not diluted by eventually highly unpreferable fireproof substances getting in to the metallurgic process. According to the invention, when the particle size of the binder are below 0.5 to 10 mm, they are preferred due to their morphological characteristic the particles being not spheroidal but rather branchy. In
practice, they get adhered on the particles to be bound during the mixing of the ingredients and, in spite of the significant difference in the specific weight, no separation occurs during mixing the powdery substances.
The binder suggested is preferred in comparison to the fireproof cements also therein that is has no water content and results in a product having a suitable quality at a much lower ratios of binder calculated on the material to be pelleted.
The binder suggested maintains its adhesive-binding characteristics in a broad interval its softening temperature is high as well. Therefore, the pellets prepared with such a binder maintains a significant part of his physical properties after compression. A surprising result appears also therein that the binder used is equally useful for the pelleting of all powders (wastes) occurring in the industrial practice. The preferable behaviour of pellets in the furnace may arise from the fact that the binder is polar and therefore, the cohesion of particles is better than that of the known organic binders. The products to be pelleted can be compressed under a low or moderated pressure (30-300 bar).
For the pelleting the powder mixture contaming the binder can be processed by several kinds of compression techniques usually employed in the industry.
One of alternative used in the industrial practice is a pelleting press roll consisting of a couple of oppositely running rolls possessing a surface pattern, where the patterns as half negatives together form the pressed object. The diameter of the rolling presses is 0.8 to 1.4 m with a width of 30 to 60 cm, their peripheral velocity varies between 0.3 and 0.8 m/sec. Thus, e.g. a press of 84 cm in diameter, 50 cm in width with a peripheral velocity of 0.3 m/sec results in a performance of 10 to 15 t/h of product.
Piston pumps for pelleting are also used in the industry. The crosswork of the press patterns is in accordance with that of the pellets. In general twin-action compression is used: one press piston from both sided is pressed into the press pattern. It is important that the plastic properties of the material to be pressed be capable to adapt to the pressure values of the compression. The piston pumps (so-called "brick-moulding" machines) can be used in a broad pressure and temperature range and the duration of compression may be adjusted freely, however, they are sensitive to the consistency of the material to be compressed.
The mixture to be pelleted formed according to the invention is suitable to be processed by piston pumps. The minimum duration of pellet formation is 0.3 to 0.4 sec.
A continuous material flow can be accomplished by using the so-called "rotary" (rolling) and "ring" presses (Exter's or Apfelbeck's type) and optionally, a continous pellet manufactiiring series can be accomplised.
Powders of ores, metals, metal compounds and alloys show very various behaviour from the viewpoint of processing technology. This depends on the one hand on their hardness, compression strength, rigidity and morphology and on the other hand, on their particle size distribution. The properties of the surface also play a considerable role. If the surface is covered with foreign materials (e.g. spoil, etc.) and the surface tension is variable these factors influence the binding strength. The binder used fits well to various powders. In opposition to the cements or bitumens having the known properties, which can be modified only to a moderate extent, in the instant case a binder can be selected in each case, which can appropriately be adapted to the requirements of both the powder-like material to be pelleted as well as to those of the compression technology.
The invention is illustrated by the following Examples:
Example 1
Ferrosilicon powder containing 70% of Si(silicon) was stirred at room temperature with 3.5% by weight of a product ground to fine particles prepared according to the process described in the PCT patent specification WO 99/29481 up to an uniform distribution. The binder had a softening point of 143°C. The particle size of the binder was 0 to 2.5 mm.
The material mixture was compressed at a temperature of 160°C by means of a brick press under a pressure of 210 bar for 14 sec. The cold compression strength of the pellets was 240 kg, which decreased to 165 kg after heat-treatment. This pellet was utilized as a ferrosilicon pellet of full value in the furnace process.
Example 2
A homogenous mixture was prepared from oily iron scale having an iron content of 72.5% by weight which was considered as an industrial waste and deposited; 2.8% by weight of the powder-like binder used in Example 1 was added calculated on the weight of the industrial waste
hereafter the mixture was processed on a brick press preheated to 125°C in such a way that the mixture to be pelleted was pre-heated to 140°C. A pressed product having cushion- -like form was produced by the press operated (working) under a compression pressure of 210 bar; this product could be fed instead of iron ore pellet into a blast furnace where it could be processed similarly to the usual inset material.