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The present invention relates to a lubrication method in a rolling mill. More particularly it relates to a lubrication method using used oil, i.e. degraded and/or contaminated oil.
Cold-rolling mills consume a large amount of oil for lubricating the contact between the rolls and the product being rolled. The lubrication is carried out by means of a mixture of water and oil called an emulsion, typically comprising between 0.5 and 4% of oil in water.
The emulsion generally flows in a closed circuit, as for example illustrated in documents JP 56-041011, JP 6-246330 or in FIG. 1. Within the circuit, the emulsion is collected, treated by means of filters and oil skimmers and is then sent back to the rolling mill. During use, fresh oil is mixed with the recycled emulsion in order to maintain the oil concentration constant.
Over time, the emulsion degrades and/or is contaminated by iron fines, by the presence of micro-organisms, by the acid coming from the preliminary pickling lines, by oils used in other portions of the rolling mill, etc. Accordingly, a portion or the totality of the emulsion must be emptied and stored in a waste tank before it is subsequently treated in a specialized center. This generates a significant amount of wastes. The wastes from the lubrication method not only include the emptied emulsions but also the oily sludges that are deposited in the filters, on the equipment, as well as the oil from the filtration of skimmed oils.
Presently, used emulsions are generally de-oiled in an emulsion-separation plant and/or dissociated into oil and into water (see FIG. 1). During the storing of the wastes, the used emulsions from the rolling may also be mixed with other used emulsions used in any other step occurring in the manufacturing of steel sheets. The dissociated oils are then sent to a purification plant where they are evaporated. These dissociated oils may be mixed, before purification, with other used oils, used during the manufacturing of steel sheets. These oils may then be burnt, for example in blast the furnaces of a steel-working plant.
The handling of these wastes and their treatment generate costs that are highly significant and that could be reduced by using again the wastes within the rolling process, whether this is hot or cold rolling.
The present invention aims to provide a solution that allows the reuse of the used emulsions, oily sludges and used oils within the rolling process and, in particular within the hot-rolling process.
It thus aims to reduce the amount of wastes produced and the costs associated with their handling and treatment.
The present invention relates to a lubrication method in a rolling mill by means of a lubricant comprising used oil.
According to particular embodiments of the invention, the method comprises at least one or a suitable combination of the following characteristics:
said used oil stems from a waste or a mixture of wastes from one or several of the steps occurring in the manufacturing of a metal;
said used oil at least partly stems from a waste coming from a cold-rolling mill;
said used oil is reused as a lubricant in a hot-rolling mill;
the waste comprises a used emulsion, an oily sludge or a used oil;
the used emulsion and the oily sludge are treated by means of a separation technique to extract said used oil;
said used oil is taken from a supernatant layer above the aqueous phase of the used emulsion;
the used oil taken from the supernatant layer above the aqueous phase of the used emulsion is reused as a lubricant in a hot-rolling mill without being subjected to any purification treatment;
the used emulsion is mixed with other wastes beforehand;
the lubricant comprises a mixture of used oil and water;
the mixture comprises an emulsifier;
the lubricant comprises the used oil that is not mixed with water;
the waste stems from a lubrication process in a cold-rolling mill or from a pickling process before the cold-rolling mill;
The result is a force reduction of the order of 30% during the hot rolling of a steel sheet.
FIG. 1 schematically illustrates the complete cycle of the emulsion, according to the state of the art, in a cold-rolling mill.
FIG. 2 illustrates the force reduction resulting from the lubrication of a hot-rolling cage by means of various lubricants including one reference and those according to the invention.
The present invention relates to a method for lubricating a rolling mill by means of a lubricant comprising used oil, which will also be called waste oil. Preferentially, the used oil according to the invention is reused in a hot-rolling mill requiring a lower oil quality than that of a cold-rolling mill. However, the present invention does not exclude the reuse of waste oil in a cold-rolling mill.
The expression <<used oil>> more particularly designates an oil that is degraded and/or contaminated and that, in this condition, no longer has the properties required for optimum lubrication of a cold-rolling mill even after filtration and provision of fresh oil. It thus differs from a recycled emulsion/oil within a closed loop such as described in documents JP 56-041011 and JP 6-246330 of the state of the art.
The expression <<used oil>> also includes degraded and/or contaminated oil as defined above having further been subjected to one or several treatments before it is reused.
The degraded and/or contaminated nature of an oil depends on the application field and is not easy to define in a universal way. In the case of a cold-rolling mill, the emulsion is thoroughly monitored. It may be decided to empty the emulsion on the basis of several criteria which inter alia are a modification of the pH of the emulsion, of the stability of the emulsion, of the size of the particles of the emulsion, of the viscosity of the oil, of the rolling forces, etc. Each rolling mill has its own operating window. Therefore, it is delicate to quantify the wear of an oil without unduly limiting the scope of the invention.
The used oil may stem from oily sludges, a used emulsion, and/or, more simply, from a waste oil, all three of which will be commonly called wastes. These wastes may stem from a lubrication process of a cold-rolling mill and/or from any other step occurring in the manufacturing of a metal, and more particularly in the manufacturing of steel sheets (pickling, winding, etc.). The present invention also extends to the use of a used oil stemming from other applications (French fry shops, change of oil from engine, etc.). The used oil may thus be obtained from a mixture of oils stemming from several sources of wastes.
The used oils may be extracted from sludges or used emulsions by means of a separation techniques selected according to the composition of the wastes and to the amount of oil required for the application. More simply, the separation may be carried out naturally within the tank by destabilization of the emulsion at rest. Thus, in the waste tank, the used oil will form a supernatant layer above the aqueous phase and may be directly withdrawn.
As a non-limiting example, the lubrication method according to the invention is illustrated below for a hot-rolling mill but, as already mentioned, the method according to the invention may also be implemented on a cold-rolling mill.
Two used oils stemming from lubrication in an industrial cold-rolling mill for steel sheets, and mixed with used preservation oils used on pickling lines, have been tested in a pilot hot-rolling mill and compared with a fresh reference oil. The tests related to the rolling of a ferrous metal, namely steel. Both used oils were taken from the upper layer which formed in the waste tank by destabilization of the water/oil emulsion. They comprise less than 0.5% of water and have not been subjected to any purification treatment before reuse. Both used oils stem from the same tank and have color variations (very dark brown and dark brown versus very pale brown for fresh oil) which express variations in compositions and/or in concentrations of emulsifier within the layer in suspension.
First of all, the emulsions were prepared with a 1% concentration of oil by means of a mixer. The properties of the oil and water mixture without or with emulsifier were evaluated. The emulsifier concentration in the mixtures comprising the latter, amounts to 2%. The tests have shown that both used oils have different capabilities of mixing into water. Thus, for a first used oil, called <<waste 1>>, the mixing is carried out with difficulty in the absence of an emulsifier, while in the presence of an emulsifier, the mixing is carried out properly. In the case of the second used oil, called <<waste 2>>, good mixing properties are observed whether with or without an emulsifier. In parallel, an emulsion with 1% of fresh oil was prepared as a reference.
The prepared emulsions were sprayed on rolling rolls with pressures of 3 and 7 bars giving an emulsion flow rate of 3 L/min and 4.75 L/min, respectively.
FIG. 2 illustrates the average percentage of force reduction for the various emulsions tested relative to a rolling test carried out without lubrication, moreover everything else being equal. One observes a force reduction that may attain 30% with the used oil, called <<waste 1>>, and may be greater than the one obtained with the reference oil. It is further observed that the addition of emulsifier tends to reduce the lubrication performances.
Additional tests were carried out with pure used oil, i.e. not dispersed in water. The pure oil was sprayed with flow rates of 10 and 20 L/min on rolling rolls, according to the method described in document WO 2009/046505 for example. The results of these tests are also shown in FIG. 2. A force reduction greater or equal to 30% is observed.
The present invention was illustrated for the rolling of ferrous metals. The lubrication method according to the invention using used oil may be extended to lubrication within the scope of rolling of non-ferrous metals.