TECHNICAL FIELD

The present invention concerns a machine, and the relating method, for the continuous production of metal ingots.

The complete process comprises the loading of the requested amount of metal in special moulds and. the subsequent melting of the metal contained within said moulds. The metal content in each mould is then solidified and extracted from the mould. All these operations occur continuously, even though the forward movement of the ingots is intermittent, so that the precious metal has enough time to melt, solidify and cool down.

BACKGROUND ART

It is well known that the production of ingots, be they of gold, silver, precious alloys or other metals or alloys, is normally carried out according to two different processes. In order to produce small ingots having a limited weight for objects such as coins, medals etc., a molding process using presses ("coining", if it concerns coins), starting from semi-finished products such as billets or blanks, is preferred. Generally, the weight of the finished products is comprised between 5g and 50g.

However, when ingots of considerable weight must be produced,. the material is preferably melted and then solidified in suitable moulds.

The precious metal to be treated may be directly loaded into the mould in the form or homogeneous or inhomogeneous powder, grit or cut-outs of various sizes. Subsequently, the loaded moulds are transported in a melting zone.

Alternatively, the material can be formed only by substantially pure powdery material previously subjected to a refining process.

The liquid metal is then solidified, though remaining in the mould. The thus obtained solid ingot is then taken out of the mould.

Actually, besides having an almost absolute purity if made with pure metal, or an exact percentage of pure metal if made with an alloy (the so-called "count") , the ingots which are available on the market must nave an extremely precise size and weight and a pleasant external appearance, flawless surfaces, no depressions ks and a uniform colour; moreover, they must have a perfect internal metallograpnic structure, no blowholes or micro-porosity and, above all, no structural tension.

Therefore, to avoid defective ingots, the whole production cycle must be carried out with great care and precision, particularly during ^' the steps of melting, solidification and cooling of the metal in the mould. In fact, since these products must also be aesthetically pleasant for a buyer, an ingot fundamental requirement is a f1awless surface fini sh .

Ingots having a more than acceptable surface finish are obtained thanks to the use of the present continuously operating machine.

Furthermore, the present machine has been designed to automate the various steps of dosing, transporting, melting and solidifying of the metal-- filled moulds, so that, at the end of a cycle, said metal will have taken the desired shape thanks to a repeatable process, with a consequent increase in the overall efficiency in terms of timing, cost and ease of management.

Another relevant aspect of the technical problem solved, by the present invention is that of sealing, with respect to a controlled atmosphere containing inert gas (e.g. nitrogen, argon, or a mixture of nitrogen/hydrogen with a maximum of 4.5% hydrogen), the machine parts hosting the melting, solidifying and cooling of the molten material contained in the ingot.

In fact, as already known, the moulds are typically made of graphite, and therefore they can be consumed by combustion in an oxygen-containing atmosphere at temperatures above 100°C.

In actual use, this leads to a rapid deterioration of the moulds after a few working cycles, with the consequent need to replace the damaged moulds with new ones.

However, the use of suitable inert atmospheres in certain sensitive areas of the machine allows a dramatic increase of the lifetime of the moulds, with an obvious benefit in terms of overall costs.

Moreover, the operation under a boosted controlled atmosphere allows, by suitably modulating the solidifying step, to obtain an extremely high-quality surface finish, the so-called "mirror finish".

However, as already known, the present configuration of the machines for the continuous production of ingots makes difficult to separate the inner area from the surrounding environment, because the moulds continuously move in the inlet and outlet areas into/from, the processing tunnel.

As a matter of fact, this implies the presence of two areas {inlet and outlet area of the moulds) wherein the inert gas injected into the tunnel tends to disperse into the external environment ,

In the absence of a valid sealing system, the only way to prevent the inlet of oxygen in the tunnel is to inject continuously the inert gas, with the imaginable related consumption and operating costs.

In time, various systems to minimize the inlet of oxygen (and therefore to reduce the inlet of a protective gas in the plant) have been tested,, but these systems have not proved to be conclusive.

The most used ones range from, systems consuming the entering oxygen (by means of free flames) to systems reducing the passage section (by means of curtains made of materials suited to processing temperatures) of the ingots from/to the processing tunnel.

Further commercially available systems provide for the opening and the closing of the inlet and outlet areas of the processing tunnel by means of sliding doors.

Finally, some applications prevent outside air from entering in the processing tunnel by means of air knives. However, open flames systems do not solve the problem of a leakage of protective gas, their only aim being the consumption of the oxygen contained in the air. This, however, has obvious negative side effects on graphite moulds .

Curtained systems try to seal the incoming and outgoing moulds by exploiting the elasticity of the curtains. As a. matter of fact, the passage of gas between the inside and the outside is only reduced, but never entirely avoided. The use of sliding doors eliminates contamination when they are closed, but not during the insertion and the extraction of the moulds. Since the closing time of the doors lasts as long as the opening time, the consumption of protective gas remains consistent.

The use of air knives in the inlet and in the outlet areas prevents the inside and the outside atmosphere from mixing, but, by its very nature, is a perpetual source of oxygen which will tend to mix with the protective gas, even if the air knife is perfectly ducted to the outside.

In contrast, the system of the present invention faces the problem of a contamination between the inside and the outside by exploiting the elasticity of a suitable membrane to create a seal around the mould.

The inlet of compressed air in one or more chambers closed by the membrane on the sides of the mould causes the adherence of the membrane on the outside of the mould, thus making it a sealing gasket,

This occurs in the step wherein the moulds are stationary inside the machine processing tunnel.

When the outer handling system pushes forward the whole row of moulds, a slight reduction of pressure decreases the adhesion of the membrane on the moving elements; as soon as the moulds stop, the repressurization of the chambers separates again the inside from the outside.

DISCLOSURE OF INVENTION

Therefore, the main object of the present invention is to provide a continuously operating machine providing for an excellent surface finish of the ingots by means of a careful optimization of the operating parameters of said machine, in particular the electric power of the induction furnace and those parameters controlling the cooling fluid in the solidifying and cooling stations.

A further object of the present invention is to provide a pneumatic sealing valve which can effectively isolate from the external environment the portion of the machine where the melting, the solidification and the cooling of the ingots occur,

Moreover, the present invention concerns a machine characterized by the fact that the electronic control unit can continuously and/or intermittently synchronize the advancing speed of the moulds, the operation and the power of heating means arranged in the second station for the metal direct melting, and the opening/closing of at least a pair of sealing valves.

Moreover, the present invention concerns a method for the continuous production of metal ingots characterized in that it comprises a first step for setting, by means of a continuous and precise control of the temperatures in a metal melting station and of the temperatures in at least one solidifying and/or cooling station, the properties (inlet temperature, flow rate etc.) of a cooling liquid entering at least one solidification and/or cooling down station. Moreover, the present invention concerns a method characterized in that it comprises a second step to synchronize the opening/closing of at least one pneumatic sealing valve depending on whether mould pusher means are operated on not, and depending on the temperatures set in the first step.

Therefore, the present invention discloses a machine for the continuous production of metal ingots, as claimed in claim 1, or in any of the claims directly or indirectly depending on claim. 1,

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention some preferred embodiments will now be described, purely by way of non-limiting examples and with a. reference to the accompanying drawings, wherein:

- figure 1 snows a lay-out of a complete machine for the continuous production of metal ingots according to the teaching of the present invention;

- figure 2 shows an enlargement of a longitudinal section of a pneumatic sealing valve used in the machine shown in figure 1;

- figure 3 shows a first configuration of a cross section of the pneumatic sealing valve of figure 2; and

- figure 4 shows a second configuration of a cross section of the pneumatic sealing valve of figure 2, BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1, 100 indicates as a whole a iria.chi.ne for the continuous production of metal ingots according to the teaching of the present invention.

The machine 100 includes the following stations:

1) a first station 10 for loading precious metal (in the form, of homogeneous and inhomogeneous powders, grit or cut ^¬ outs of different, sizes) in moulds (ST) , and for replacing the air within the machine and/or the moulds with a pressurized inert gas through a gas exchange device 10A;

2) a second station 20 for melting the metal present in the mould (ST) ;

3) a third station 30 for solidifying the molten metal in the moulds (ST) ;

4) a fourth station 40 for cooling of moulds (ST) and the metal contained in them;

5) a fifth station 50 for unloading the moulds (ST) ; and

6) a sixth station 60 for emptylng the moulds (ST) by means, for example, of a conveyor belt and/or an equivalent handling system (NT) ,

The moulds (ST) , already loaded with the exact amount of metal (in the form of homogeneous and inhomogeneous powder, grit or cut-outs of various size) , are accumulated, close to the first station 10 and automatically (by means of belts, pistons, suitable mechanical elements) or manually conveyed before the machine inlet 100,

By way of example, a supply pusher 11 of the moulds (ST) (also called "brackets" or "ingot moulds") is present at the beginning of a cycle.

In order to improve the finish of the ingots, the moulds (ST) , equipped with a lid made of a refractory and/or a suitable material (not shown), can be alternated with other elements called "spacers" (not shown) , which allow to move the moulds (ST) at the same speed.

As soon as the moulds (ST) are positioned before the machine inlet, the pusher 11 intermittently, so as to be synchronized with the following operations (see below), inserts the moulds (ST) within the second melting station 20 wherein an induction melting furnace is located. The sliding of the moulds (ST) can be eased by a plurality of rotating rolls and/or guides made of cooled ceramic material and/or metal (not shown) , arranged on the support surface of the moulds (ST) , transversely to the path of the moulds (ST) .

In the second station 20, an ordinary electric oven or a furnace equipped with gas burners can be used instead of an induction furnace.

However, in this kind of machines the best results have been obtained thanks to induction furnaces allowing an accurate repetition of the treatments on different stocks of ingots .

Advantageously, in the induction furnaces used in this application, the management of the melting temperatures is optimized in a pyrometric closed.-- loop control of the mould. The translation of the moulds (ST) occurs by means of the pusher 11, which intermittently thrusts the whole mould block (ST), already present in the machine 100, and also intermittently expels a similar number of moulds (ST) leaving ^' the emptylng station 60.

In the second melting station 20, the moulds (ST) only spend the necessary time to melt the noble metal contained in them.

The metal in the moulds (ST) is melted according to the temperature and time parameters set by the operator.

As already stated, in order to reduce the wear of the moulds (ST) occurring at the metal melting temperatures, the process is carried out under an inert atmosphere obtained by insufflating a. suitable inert gas (for example argon, nitrogen, or a mixture of nitrogen/hydrogen with a maximum of 4,5% of hydrogen) ,

The exchange of gas inside the machine 100 is carried out using the aforementioned gas exchange device 10A.

In the embodiment, shown in figure 1, the second melting station 20, the third station 30 for solidifying the molten metal and the fourth station 40 tor cooling the moulds (ST) are contained in a single tunnel 70 made of a single block 80,

Usually, in the solidifying station 30, the moulds (ST) have temperatures around 600 °C, with cooling plates having a temperature comprised between 100 °C and 150 °C.

These parameters may dramatically vary depending on the type of ingot to be melted (material and weight) and depending on the production rate (pieces/hour) . Moreover, machines for the production of larger ingots can possibly melt smaller pieces with suitable moulds (for example, in the oven for 1 kg ingots, 100 g ingots can be cast by means of four-cavity moulds), thus considerably complicating the schemat izat ion of parameters,

Each of the ends 7 OA and 70B of the tunnel is selectively closed by a respective innovative closing valve 200, illustrated in greater detail in figures 2, 3, 4 (see below) ,

As hereinafter better explained, by using the valves 200 the tunnel 70 is selectively sealed, so that the inert gas (for example argon, nitrogen, or a mixture of nitrogen/hydrogen) injected into the tunnel 70 is not dispersed outside the block 80.

The machine 100 comprises an electronic control unit (CC) processing a specific algorithm and receiving a series of signals. This electronic control unit (CC) also controls the operation of the various actuators, as better seen by describing the operation of the machine 100.

In particular, a signal detected by one or more optical pyrometers 21 will be sent to the electronic control unit (CC) by means of a line (LI) . These signals refer to the temperature of the moulds within the melting station 20 and/or to the external temperature of the moulds (ST) leaving the second melting station 20.

The third station 30 for solidifying the molten metal also contains one or more thermocouples 31 which detect the temperature of a solidifying plate 32 and of the cooling fluid, and sends a corresponding signal to the electronic control unit (CC) through a line (L2) .

The metal plate 32 is cooled by a liquid, and ensures the solidification of the metal; the flow rate of the cooling liquid and its temperature are calibrated and set by an algorithm according to formulas programmable by an operator to optimize the solidification of the metal.

To improve the process, the metal plate 32 is insulated with suitable insulation panels and/or heating systems (electric resistors, induction coils, gas burners, infrared lamps, laser, etc.) (not shown) in order to modulate the operating conditions of this station. In particular, the lid 32A of the metal plate 32 may be provided with a heating device 32B, advantageously, but not necessarily, with electrical resistances.

Also in the fourth station 40 a respective plate 41 is cooled by a liquid; it brings the moulds (ST) and the relative ingots to room temperature, so that they can be safely handled as soon as they come out of the hot part of the machine 100.

In known manner, the electronic control unit (CC) controls the feedback operation of said pusher 11 by means of a line (L4), and the feedback operation of the induction furnace in the second melting station 20 through a line (L5) .

Moreover, the two valves 200 are feedback driven and controlled, always by the electronic control unit (CC) , by means of a line (L6) .

The conveyor belt (NT) or an equivalent system is instead feedback controlled and monitored by the electronic control

(CC) by means of a line (L7) .

As previously stated, the entire tunnel 70 is filled with a protective atmosphere created by inserting an inert gas in the tunnel 70.

The protective environment not only avoids the deterioration of the moulds (ST) , but also a possible oxidation of the molten metal by preventing any contact of the metal with the oxygen present in the environment surrounding the machine 100, and allowing at the same time to obtain a surface finish called "mirror finish".

In the present case, at least two sealing valves 200, namely a tunnel inlet valve and a tunnel outlet valve, are used to ensure the sealing of the tunnel 70.

Such sealing valves 200 allow the inlet and the outlet of the moulds (ST) in and from the tunnel 70, but prevent an outflow of the protective gas by means of an elastic membrane 201 (figures 2, 3, 4) .

As shown in more detail in figures 3 and 4, the elastic membrane 201 comprises an upper portion 201A, a lower portion 20 IB and two side portions 201C.

The elastic membrane 201, in turn, is enclosed in a. housing 202, which is also tubular and has a square, rectangular or circular cross section .

Therefore, an air chamber 203 is defined between the housing 202 and the elastic membrane 201, said chamber being feci with compressed air and/or inert gas through a supp1y conduit 204 (figure 2 ) .

As stated above, the present invention uses the elasticity of the membrane 201 to create a seal around the mould (ST) . The inlet of compressed, air in the air chamber 203 closed by the membrane 201 facing the sides of the mould (ST) let said membrane 201 adhere to the outside of the mould, thus acting as a sealing ^' gasket.

In particular, as shown in figure 4, the upper portion 201A adheres to the lid of the mould, the lower portion adheres to the outer surface 9OA of a plate 90 on which the mould (ST) slides, whereas the side portions 201C rest on the sides of the moving mould (ST) . This occurs in the step wherein the moulds (ST) are stationary within, the tunnel 70.

Actually, figures 3 and 4 illustrate two possible embodiments of the elastic membrane 201.

In the embodiment of figure 3, the elastic membrane 201 is formed by a single piece having a tubular shape and a square (or rectangular or circular) cross section; while figure 4 shows another possible embodiment provided, with four rectangular separated elements.

When the external handling system pushes forward the whole row of moulds (ST) by means of the pusher 11 (figure 1), a slight reduction of pressure in the air chamber 203 is responsible for a decrease in the adhesion of the membrane 201 on the moving moulds.

As soon as the moulds (ST) stop, the repressuri zat. ion of the air chamber 203 separates again the inside of the tunnel 70 from the external environment.

The main advantages of the machine for the continuous production of metal ingots object of the present invention are the following:

- flawless upper surfaces of the ingots; - significant reduction of protective gas leakage f rom the treatment tunnel, and only during the sliding step of the moulds (namely when the compression pressure is smaller) , otherwise totally blocking its passage without any thermal stress; and

- significant reduction in the consumption of the moulds.