Abstract:
A protective furnace gas for protecting a bath of magnesium in furnace equipment includes a mixture of inert gas such as nitrogen and 0.3 to 1.5% by volume of SO 2 , the mixture being provided in a temperature-controlled room wherein the temperature is at least 22° C. and preferably 25° to 35° C.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for operating furnace equipment for magnesium alloys in which a bath surface is covered with a protective furnace gas containing a sulfur compound. 
     THE PRIOR ART 
     In installations where liquid magnesium is present it is necessary to take special precautions to reliably prevent the access of atmospheric oxygen because magnesium is extremely combustible in the liquid state. SF 6  is used as such a protective furnace gas, for example, which reacts with the magnesium of the melt and forms a protective layer which protects the melt from the access of atmospheric oxygen. Occasionally, mixtures of SF 6  and nitrogen are used for this purpose. Since SF 6  concerns a greenhouse gas, its use is problematic for reasons of environmental protection. Moreover, the costs for this gas are relatively high. 
     It has already been considered as an alternative to use SO 2  as a protective furnace gas. SO 2  is available at relatively low cost and, like SF 6 , forms a protective layer on a magnesium melt. The application of SO 2  has been prevented up until now because it concerns an extremely unpleasant smelling gas which in medium to high concentrations is detrimental to health and promotes corrosion. During the use of SO 2  in the conventional manner it is not possible to prevent the escape of gas from the furnaces or the like which leads to an impermissible burden on the ambient environment. In particular, maximum workplace concentrations will be exceeded by far. 
     It is further known to use gas mixtures of SO 2  and dried air as a protective furnace gas for magnesium furnaces. It has not yet been managed, on the one hand, to control the toxic and corrosive effect of SO 2  and, on the other hand, to realise a secure operation of the furnace. 
     It is the object of the present invention to provide a method which, on the one hand, allows a secure treatment of magnesium melts and, on the other hand, causes the lowest possible burden on the environment. 
     SUMMARY OF THE INVENTION 
     This object is achieved in accordance with the invention in such a way that a mixture of SO 2  and an inert gas is used as a protective furnace gas, with the volume share of SO 2  being set precisely to a value which is in a range of between 0.3% and 1.5%, preferably between 0.5% and 1.0%. It was surprisingly noticed that in the case of SO 2 , there is a small range in which there is a sufficient protective effect on the one hand and a burden on the environment can be substantially prevented on the other hand. The relevant aspect of the present invention is the setting of a precise value of the SO 2  concentration in the protective furnace gas. Since very low SO 2  concentrations are used, precise process control and the avoidance of fluctuations in the gas composition is very important to prevent fires. 
     Nitrogen is used particularly preferably as the inert gas. Nitrogen is available at low cost and is not critical to the environment. In a particularly preferable embodiment of the method in accordance with the invention, it is provided that the mixture of SO 2  and the inert gas is performed in an air-conditioned room whose temperature is kept above 22° C., and preferably in a range of between 25° C. and 35° C. It has been noticed that a satisfactory mixture of SO 2  and nitrogen is only possible from a certain minimum temperature at a performance pressure &gt;2 bars. Moreover, temperature-induced fluctuations in volume and pressure of the involved gases can be securely prevented by the mixture in an air-conditioned room. A temperature of approx. 30° C. is particularly preferable. 
     Moreover, the present invention relates to an apparatus for producing a protective furnace gas for furnace equipment for magnesium alloys with a storage tank for a sulfur compound, a storage tank for an inert gas and a mixing device. The apparatus in accordance with the invention is characterized in that the mixing device is arranged to provide a precise flow rate control of SO 2  and nitrogen and is arranged in an air-conditioned room. Safety gas cells are provided in an air-conditioned room which receive the SO 2  cylinders. Nitrogen is taken from a conventional tank. The mixture of SO 2  and nitrogen is performed through electronic mass flow meters which are arranged in the air-conditioned room. The gas mixture thus produced is conveyed to the respective furnace equipment via pipelines and adjusted to consumption by way of local control systems. The furnace equipment can concern smelting furnaces, holding furnaces, dosing furnaces and pig casting belts for magnesium alloys. 
     SO 2  sensors are provided in the safety gas cells which already respond to low SO 2  concentrations. Once an SO 2  cylinder has been emptied, the same is scavenged with nitrogen together with the pipelines in order to exclude any health hazards during the exchange of the cylinders. In this manner it can be prevented reliably that any SO 2  escapes into the environment. 
     As a result of the low SO 2  concentration of the protective furnace gas, any pollution of the environment and any annoyance caused by bad smell, particularly in the furnace area, can be securely prevented. Secure operations can be ensured by the precisely set mixture ratio. 
    
    
     BRIEF DESCRIPTION OF THE FIGURE 
     The FIGURE shows a schematic diagram of an apparatus in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Components are described with the broken lines  1  which are arranged in two safety cabinets. Gas cylinders  2  are used as storage vessels for sulphur dioxide which is supplied to a collecting line  4  by way of stop valves  3 . A control line  5   a  is supplied via a manometer  5   a  which triggers a solenoid valve  6 . A supply line  8  for SO 2  is connected via a further stop valve  7 . 
     A storage vessel  9  for nitrogen is connected with the distributor line  4  in each of the two safety cabinets  1  via a line  10 , a stop valve  11  and a return valve  12  in order to provide nitrogen for scavenging the lines during the exchange of the cylinders  2 . A manometer  13  shows the nitrogen pressure in a nitrogen main line  16 . The individual cylinders  2  are in connection with a waste gas collecting line  19  via stop valves  17  and waste gas lines  18 . The distributor line  4  is vented into the waste gas collecting line  19  via separate stop valves  20 . The circuit as described above allows scavenging the respective line sections with nitrogen prior to the exchange of one of the cylinders  2 , so that any escape of SO 2  can be reliably prevented. 
     The right-hand section of the FIGURE shows the mixing device for supplying the individual consumers. Since the individual mixers  21  are principally designed in the same way, only one of them is designated with a reference numeral and is described in the description. 
     The individual mixers  21  are supplied via a first distributor panel  22  with nitrogen and via a second distributor panel  23  with sulphur dioxide. A third distributor panel  24  is used for connecting the mixer  21  with the waste gas collecting line  19 . Stop valves  25  and  26  are provided in the individual mixers  21 , which valves are connected with the distributor panel  22  or  23 . Manometers  27  and  28  indicate the respective nitrogen or sulphur dioxide pressure after the stop valves  25  and  26 . A stop valve  29  for scavenging is provided between the nitrogen line and the sulphur dioxide line, with a return valve  30  being provided downstream of the same. Highly precise flow rate meters  31  for nitrogen and  32  for sulphur dioxide are used for setting the precise quantity ratio for the two gases. Filters  33  and  34  are provided upstream of the flow rate meters  31  and  32 . The gases are joined in a supply line  38  via further stop valves  35  and  36  and a return valve  37 , which supply line is provided with a manometer  39  and a stop valve  40 . A scavenging line  41  with a stop valve  42  is used for preventing any pollution of the environment during maintenance work and the like. 
     The entire arrangement as exhibited in the FIGURE is arranged in accordance with the invention in an air-conditioned room which is kept at a temperature of approx. 30° C. In this manner the mixture ratio of the gases can be kept at a precisely predetermined value, and a favourable thorough mixture can be ensured. 
     Flow governors (not shown) can be provided in the individual consumers, which governors only influence the supplied quantity of the gas mixture, but not the composition of the gas. 
     The present invention thus not only allows savings in costs, but also a substantial reduction of the burden placed on the environment and a particularly safe operation in dealing with magnesium melts.