ALUMINIUM ALLOYS SCRAP

The subject of this Invention is a method to reprocess aluminium and aluminium alloy scrap. The method according to the Invention comprises a process of consolidating and melting shreds of aluminium and aluminium alloys, as well as other aluminium waste materials and aluminium granules. In particular, the method according to the Invention may be utilized for recycling waste materials consisting of beverage cans made of aluminium alloys.

According to one of the known solutions, the aluminium or aluminium alloy scrap is shredded, i.e. fragmented into very small pieces, using diverse shredding machines. Those small pieces called 'shreds' are cleaned in order to remove unwanted chemical substances such as printer's inks, leftover products contained in aluminium containers, or other substances.

Various cleaning methods are used to clean the shredded scrap; among other things, a process of pyrolysis or flushing the shreds with chemical solutions showing a specific, predetermined chemical composition. In practice, the effectiveness of a pyrolysis process used to clean aluminium scraps depends on the shredding degree of aluminium shreds; the shreds must be adequately shredded to prevent the formation of closed spaces, which may not be completely cleaned because the access to them is difficult. Next, the properly cleaned aluminium shreds are melted; the final product is a solid alloy in the form of various sections. The commonly used solutions comprise the shredding of aluminium scraps, the cleaning of the shredded product in a pyrolysis process, and the melting of the shredded and cleaned material; however, their essential drawback is the oxidation of some part of the cleaned aluminium shreds in the course of melting the shreds. This drawback causes raw material losses.

There are also other solutions known, where the aluminium shreds are consolidated in the course of pressurized consolidation processes. For example, one solution is known from the German Invention as disclosed under the Patent No. DE4213441; according to this solution, the appropriately cleaned aluminium shreds, i.e. each and every impurity such as lubricants or coolants are absolutely removed, are briquetted under pressure .

Another solution is known from the Invention as disclosed under the British Patent No. GB540105. According to the method under this Patent, shreds made of light metals are consolidated at a high temperature, which is, at least, equal to or higher than the oxidation temperature of those light metals, preferably at a temperature between 250° and 500°C. In the case of aluminium shreds, the consolidation temperature is ca . 400°C and the consolidation pressure is between 0.5 and 2.5 tonnes per 1 cm ² . In one of the pressurized consolidation options, an air hammer is used to consolidate the shreds. Another Invention as disclosed under a Patent No. KR20100024534 refers to the recycling of discarded aluminium cans. In the solution according to latter Patent, in the course of melting the scrap shreds, the recycled raw material losses as mentioned above are avoided owing to the following method applied: at the beginning, the aluminium cans are sorted out and cleaned in order to remove all other wastes. Next, the cans are heated to a temperature at which the organic substances and the printer's inks on the cans are charred. The cleaned aluminium cans are pulverized and aluminium granules constitute the final product manufactured. Next, those aluminium granules are consolidated under an appropriate pressure.

The Patent No. RU2379357 discloses another Invention. This Invention comprises a method where the metallic waste materials including aluminium scrap are cleaned through dipping them in a mixture of chemically neutral agents and, next, the shreds are further cleaned under a high pressure and at a temperature increasing successively from 0.22 to 0.70 of the melting point of the metal that the dipped shreds are made of. The shreds cleaned with the use of those two methods are placed in a disc extruder where they are briquetted under a pressure ranging between 500 to 1000 kg/cm ² .

According to the present Invention, the essence of the method for reprocessing aluminium and aluminium alloys scrap, and, specifically, the discarded beverage cans made of aluminium alloys, consists in that the pre- shredded and cleaned, using a pyrolysis process, aluminium scrap is consolidated in a roll press, in its closed profiles of a trapezoidal cross-section, at temperatures ranging from 150°C to 390°C; the consolidation process continues until the density value of the aluminium scrap is close to the theoretical density of aluminium or aluminium alloy, i.e. not less than 90% thereof; next, the consolidated material is melted.

One preferable embodiment of the solution according to the present Invention provides that the aluminium or aluminium alloy scrap is shredded to obtain a fraction size of less than 50 mm; at the same time, the fraction size distribution is highly differentiated to make it possible to increase the charging consolidation of the shreds whereas the 'charging consolidation' means the consolidation and density of the scrap material being introduced into a charging device in the roll press.

Another preferable embodiment of the solution according to the present Invention consists in that, prior to being rolled in the closed profiles of a trapezoidal cross-section, the shredded aluminium or aluminium alloy scrap of a charging consolidation degree ranging between 40% and 54% of solid aluminium is pre- consolidated under a certain pressure in order to obtain its charging density of 70% to 80% of solid aluminium or solid aluminium alloy. Next, this pre-consolidated material is rolled in the closed profiles until its density reaches a value close to the value of theoretical density of solid aluminium or aluminium alloy.

There is one more preferable embodiment of the solution according to the present Invention; it consists in that the rolling process in the roll press, in its closed profiles of a trapezoidal cross-section, continues under a pressure of 70 to 180 MPa.

A particularly preferable embodiment of the solution according to the present Invention is to use a continuous process for the reprocessing of aluminium or aluminium alloy scrap.

The method according to the present Invention has the following advantage : the losses of raw material that is melted are minimized and the energy consumption is reduced. According to the already known solutions, for the purpose of producing aluminium or aluminium alloy 'pigs' that are ready for casting into a simple shape, the cleaned scrap is either melted or consolidated in the course of a pressurized consolidation process. According to what was said above, during the melting process, raw material losses occur. As regards the pressurized consolidation process ongoing under an increased pressure, the manufactured 'pigs' are characterized by non-uniform (varying) physical-chemical parameters. The reason thereof is that different types of aluminium scrap are shredded; usually, they are made of various aluminium alloys. Practically, even with the highly shredded scrap material, during the pressurized consolidation process, it is impossible to manufacture ^pigs' showing a uniform, reproducible physical-chemical structure similar to that produced when the scrap shreds are melted.

The method according to the present Invention provides for the melting of a consolidated material, not scrap shreds .

The density of the consolidated material is close to the density of a solid metal; therefore, it is possible to avoid processed material losses arising owing to the oxidation of the small sized shreds during the process of melting. In the recycling industry, material losses occurring in the course of melting process essentially affect the profitability of aluminium scrap collection and recycling processes. Provided the sequence of the above described actions is followed, unexpected (surprising) benefits are gained. First of all, the energy consumption during the entire process is essentially reduced. The plasticity of the material to be rolled increases because, during the pyrolysis process, the highly shredded aluminium shreds are heated to a temperature ranging between 150° and 390°C, thus, they turn out to be softened. At the same time, the rolled material heats up the rolls in the roll press, therefore, they do not absorb heat from the shreds being rolled. Consequently, and compared to the melting of cooled scrap material, considerably less energy is necessary to melt the aluminium pigs manufactured during the rolling process. Important is also the shape of rolls in the roll press. A closed profile of a trapezoidal cross-section appeared to be the optimal shape of the press. Owing to this profile, the workspace of the rolls impacts, in all planes, the rolled and consolidated mass of shreds; this fact is very important in the case the metallic fragments being rolled have different shapes and sizes. The applied trapezoidal cross-section of the press results in a higher consolidation efficiency.

E x a m p l e I

The method for reprocessing aluminium and aluminium alloys scrap as exemplified by the embodiment described below consists in that aluminium beverage cans are shredded in a shredder in the course of a continuous shredding process in order to obtain a differentiated fraction size not exceeding 50 mm, and to obtain a charging density of ca. 55% of solid aluminium. Next, the shredded aluminium scrap is cleaned in a pyrolysis process at a temperature of 250°C. Thereafter, the shreds, heated during the pyrolysis process, are introduced directly into a charging hopper situated in the roll press whereas the roll press has closed profiles of a trapezoidal cross-section. In the roll press, under a pressure of 70 Pa and at a temperature of ca. 200°C, the shreds are consolidated, so, that their density value reaches 92% of the density of solid aluminium. The final products manufactured are aluminium pigs of a trapezoidal cross-section; using a feeding device, those pigs are introduced directly into a melting furnace.

E x a m p l e I I

Another example of the embodiment of the method being the subject of the present Patent Claims shows that the discarded aluminium cans are shredded in order to obtain a fraction size of ca. 5 mm, and, next, they are cleaned in the course of a pyrolysis process at a temperature of 400°C. The cleaned shreds' charging density is ca. 45% to 50% of the density of solid metal, their temperature after the completed pyrolysis process is ca. 390°C. Prior to the rolling in a roll press with closed trapezoidal profiles, the material is pre-consolidated in an open roll press; consequently, the charging density of the consolidated shreds increases to a value of ca . 75% of the density of solid aluminium. Only when the material has been pre-consolidated as described above, it may be rolled in a roll press with closed profiles of a trapezoidal cross-section; the pressure applied during the rolling process is 120 MPa, and the temperature is ca. 320°C. The final product after the completed rolling process is an aluminium bar that has a trapezoidal cross- section, its density value is ca. 98% of the theoretical density of aluminium, and its temperature is ca . 300°C; next, the aluminium bar showing the above indicated parameters is melted.