Background Art In the technical practice, solutions of hydrides in molten alkali hydroxides have been used for several tens of years for descaling and for cleaning surfaces of metals. Initially, sodium hydride was synthesized from sodium and hydrogen directly in the molten bath. This method of preparation was not only dangerous, but it was also uneconomical, because it required higher concentrations of hydride. The unsafety of the preparation was to be removed by means of other methods of preparation of the hydride melt, during which sodium hydride was added into the bath either in a solid form or surface protected, or in a mixture with hydroxide.

Said methods, though, have not been brought to industrial practice.

Stanek and Mostecky (CS patent 135780) found out that for a good course of descaling, the presence of sodium oxide is desirable. The hydride and oxide were put into the bath as a solution or suspension in an alkali hydroxide. In this way, active substances were passivated and the dosing was safe.

A disadvantage of all hitherto used methods of preparation of descaling baths resides in the fact that the content and ratio of the mentioned components was not controlled sufficiently.

Disadvantages of hitherto used methods are removed by the present invention, which solves the method of preparation of the bath and its control.

Summary of Invention The substance of the invention resides in a reducing melt for descaling metals, for reducing oxides on surfaces of metal products and oxides of surfaces of those metals which do not react in a chemical way with molten sodium or potassium hydroxide, said reducing melt comprising sodium or potassium oxide or their mixture in an amount of 0.01 to 2 wt% related to the total weight of the melt, sodium or potassium hydroxide or their mixture, sodium or potassium hydride or their mixture, metallic sodium or potassium or their mixture, and hydrogen, in such an amount that by reaction of one weight part of the melt with water there are liberated 0.1 to 26 volume part of gaseous hydrogen. The reducing melt includes advantageously also at least one of metals selected from the group of Fe, Cr, Cu, Ni, Mn, Ti, Mo and W, as such or as anions.

The invention also includes several methods of preparation of the reducing melt. The method of preparation of the reducing melt issues from a new finding that elementary alkali metal and hydrogen are also important components of a reducing hydride descaling bath. Direct active components are the alkali metal hydride and oxide, the elementary alkali metal serving as a reserve of said components. The ratio between the oxide and hydride is determined by the hydrogen pressure. For a good function of the bath, it is desirable that active components and alkali metal may be included in a suitable ratio and concentration.

One of the methods of preparation of a reducing melt for descaling resides in the fact that metallic sodium or potassium or their mixture, sodium or potassium oxide or their mixture and hydrogen are added to the melt of sodium or potassium hydroxide or of their mixture at the melt

temperature of 360 to 460°C, in such an amount that by the reaction of one weight part of the melt with water there are liberated 0.1 to 26 volume parts of gaseous hydrogen.

Another method of preparation of a reducing melt for descaling metals according to the invention resides in the fact that metallic sodium or potassium or their mixture, sodium or potassium oxide or their mixture, and at least one of metal oxides selected from the group including Fe, Cr, Cu, Ni, Mn, Ti, Mo and W, are added to the melt of sodium or potassium hydroxide or of their mixture at the melt temperature of 360 to 460°C in such a quantity that by the reaction of one weight part of the melt with water there are liberated 0.1 to 15 volume parts of gaseous hydrogen.

Another method of preparation of a reducing melt for descaling according to the invention resides in the fact that metallic sodium or potassium or their mixture, sodium or potassium oxide or their mixture, sodium or potassium hydride or their mixture, are added to the melt of sodium hydroxide or potassium hydroxide or of their mixture, at the melt temperature of 360 up to 460°C, in such an amount that by the reaction of one weight part of the melt with water there are liberated 0.1 to 26 volume parts of gaseous hydrogen.

Still another method of preparation of a reducing melt for descaling according to the invention resides in the fact that metallic sodium or potassium or their mixture, sodium or potassium oxide or their mixture, sodium or potassium hydride or their mixture, at least one of metal oxides selected from the group including Fe, Cr, Cu, Ni, Mn, Ti, Mo and W and hydrogen, at the melt temperature of 360 up to 460°C, are added to the melt of sodium or potassium hydroxide or of their mixture, in such an amount that by the reaction of one weight part of the melt with water there are liberated 0.1 to 26 volume parts of gaseous hydrogen.

Finally, still another method of preparation of a reducing melt for descaling according to the invention resides in the fact that metallic sodium or potassium or their mixture, sodium or potassium oxide or their mixture are added to the melt of sodium or potassium hydroxide or of their mixture, at the melt temperature of 360 to 460°C, to the resulting melt there is added such an amount of water in the form of superheated steam that by the reaction of one weight part of the melt with water there are liberated 0.1 to 15 volume parts of gaseous hydrogen.

The bath is prepared from a melt of an alkali hydroxide, preferably sodium hydroxide, viz. by adding an alkali metal, alkali metal hydride and oxide and hydrogen. Advantageously, sodium and its compounds may be used. When preparing the bath, use is made of mutual reactions between the alkali metal hydroxide, hydride and oxide and hydrogen in such a way that only some of the mentioned components are added to the molten hydroxide and the remaining ones are formed by mutual reactions. The components may be added to the melt either as pure substances or advantageously as solutions or suspensions in the hydroxide, in such an amount that by the reaction of one gram of the melt with water there is formed, under normal conditions, 0.1 to 26 ml of gaseous hydrogen and that the content of the alkaline metal oxide is within the range of 0.01 to 2 wt%.

The reactions useful for forming active components are exemplified on sodium and its compounds. Sodium hydride together with sodium oxide is formed by the reaction of elementary sodium with sodium hydroxide: 2 Na + NaOH = Na20 + NaH This reaction is an equilibrium one, and thus, in case of excess of the hydride and oxide, elementary sodium and the hydroxide are formed on the

contrary. The hydride can be formed also by the reaction between sodium and hydrogen: 2 Na + H2 = 2 NaH Hydrogen needed for the mentioned reactions may be supplied as pure hydrogen or in a mixture with nitrogen or an inert gas. According to the invention, some of the components in the bath may be formed by reactions with water or with substances liberating water in the hydroxide, or by action of gaseous oxygen-containing mixtures. Useful reactions are exemplified on sodium and its compounds: hydrogen may be formed directly in the melt by means of reaction of water with sodium.

2 Na+2 H20=2 NaOH + H2 wherein by action of water also the concentration of sodium oxide may be modified: Na2 O + H20 = 2 NaOH Superheated steam may serve as a source of water, or water may be formed directly in the melt by decomposing hydrate crystals or by neutralizing sodium hydroxide with silicon dioxide, boron oxide or carbon dioxide, or with acidic oxides of transition metals. By means of water, either directly introduced on the surface of the melt or as generated in the melt, also functional properties of the bath during operation may be modified.

Sodium oxide may be formed by oxidizing elementary sodium with oxygen (also with atmospheric oxygen): 4 Na+02 = 2Na2O

In some cases, on the contrary, there may occur an accumulation of the alkali metal oxide in the melt, which results in loss of descaling capability. Said disadvantage may be removed by converting the oxide into the hydroxide by means of water or of substances liberating water in the hydroxide.

The optimum content of sodium oxide in the bath may be advantagenously stabilized according to the invention by means of action of iron oxides to the bath. At an increased content, the alkali metal oxide reacts with the present iron oxides forming a ferrate (II) and a ferrite, which causes decrease of its content.

At a low content of the alkali metal oxide, the iron anions dissociate thus forming iron and the oxide ion. The latter then increases the alkali metal oxide activity in the bath. It is advantageous to use lower oxides of iron, because higher oxides are partially reduced to the detriment of the hydride in the bath.

Practical experience has shown that the presence of iron oxides in the bath affects favourably the descaling kinetics.

The stabilization capacity of sodium oxide is limited, and if it is exceeded, a rapid increase of the concentration of iron dissolved in the bath occurs. In such a case it is necessary to apply another technology of modification of the bath composition. Specific examples of performing the invention are given below, without limiting the scope of the invention in any sense.

Of course, it is possible, and also advantageous with respect to the operational and safety points of view, to have pre-prepared a mixture of several components, e. g. a stabilized mixture of sodium hydride, sodium oxide and sodium hydroxide, or of other combination of the above

mentioned substances. In a tank for descaling steels, other components are added, step by step, to the mixture, pre-prepared in such a way, in dependence upon different steel compositions.

When descaling steel, the work is carried out at ordinary pressure and at the melt temperature of 360°C to 460°C. With respect to stability keeping of the reducing melt, it is advantageous to use heating through the tank wall. As to the operation safety, it is desirable that no contact of the melt with water takes place.

The process of steel descaling and dosage of individual components in the melt, using the above mentioned reactions, may be controlled by determining the amount of the alkali metal hydride by means of measuring of the volume of liberated hydrogen in the following way: Sodium hydride is determined with gasometric analysis.

Testing equipment: a device consisting of Erlenmeyer flask (serving as a decomposition vessel) of capacity of 100 ml with a ground joint that is used at the same time as a weighing bottle ; when weighing, the flask is closed with a light ground-in bottle screw; - at the head of the decompositions bottle, there are two outlets on an extended ground joint inner member; furthermore, a miniature dropping funnel is connected with throughput; the end of a stalk is several millimetres under the ground joint; - a gas measuring burette is provided with a three-way cock with the volume of 400 ml (the last 100 mi are divided by 0.1 ml), and with a tempering roller.

Test procedure. A sample of the melt, prepared in advance, is put into a preliminarily weighed decomposition bottle. Weighing of the sample for various kinds is as follows :

Weight of the sample : 2.00 0.1 g; the flask is connected to the decomposition device, and, after tempering in the bath, the charge is decomposed carefully just with the necessary quantity of water; as soon as the temperature equalizing reaches a constant volume of released hydrogen, this volume is read and then the temperature of water jacket of the burette, of the bath of the decomposition device and barometric pressure are read.

Calculation. The content of sodium hydride (x) in percentage is calculated according to the formula : (b-PH20) Vt x =--------. 0. 028877 (273.1 + t) m wherein b is barometric pressure, in mbar; P H20 tension of water vapour (at temperature t) in mbar; Vt volume of released hydrogen (at temperature t) in ml ; m mass of the sample, in g; t temperature of water jacket, in °C Examples : Example 1: 12 kg of sodium and 3 kg of Na20 were dosed into 950 kg of molten NaOH at the temperature of 405 °C. During dissolving the components, a nitrogen-hydrogen mixture (75 % of N2 and 25% of H2) was introduced to the bottom of the reactor with the melt at a flow rate 360 lSmin~1 for the period of 90 minutes. The melt, prepared in such a way, reduced oxides on the surface of wires made of low-alloy and high-alloy steels.

Example 2:

2 kg of sodium and a casting of a solid solution of 2.3 kg NaH in 50 kg of NaOH were added into 950 kg of molten NaOH which contained 0. 38 g of melted iron in 1 kg of the melt, at the temperature of 400 °C.

The melt, after melting the casting, contained 0.017 g of melted iron in 1 kg of the melt.

Example 3: Into 1000 kg of molten NaOH at the temperature 400 °C there was added a casting of a solid solution of 3.9 kg of NaH and 1. 8 kg of Na20 in 50 kg of NaOH. The melt, after melting the casting, contained 0.24 % of NaH and it reduced oxides on the surface of hot drawn wires made of austenitic, ferritic and martensitic steels, of strips made of high speed steel and strips made of titanium and copper.

Example 4: Into 1060 kg of molten NaOH at the temperature of 400 °C there were added 12.4 kg of scales made of carbon steels, constituted of oxides predominantly of iron. After adding 13 kg of sodium and melting thereof a melt was formed comprising 4.2 g of meted iron in 1 kg of the melt. By treatment with superheated steam, introduced at the surface of the melt for 20 minutes, the content of melted iron was decreased to 0.38 in 1 kg of the melt, and the melt reduced oxides on the surface of cast-iron castings. The content of NaH after terminating the supply of steam was 0.14 wt%.

Example 5: Into 950 kg of a mixture of molten NaOH and KOH (720 kg of NaOH and 230 kg of KOH) at the temperature of 390 °C, a casting of a solid solution of 3.9 kg of NaH and 1.8 kg of Na20 in 50 kg of NaOH was added. The

melt, after melting the casting, contained 0.20 % of NaH and KH and it reduced oxides on the surface of hot drawn wires made of austenitic, ferritic and martensitic steels, strips made of high-speed steels and strips made of titanium.

Example 6: Into the mixture of 990 of molten NaOH and KOH comprising 730 kg of NaOH and 260 kg of KOH, at the temperature of 390 °c, there were added 13,7 kg of scales from ferritic steels constituted by oxides of iron. After adding 13 kg of sodium and melting thereof a melt formed comprising 7.37 g of melted iron in 1 kg of the melt. By supplying superheated steam onto the melt surface for 30 minutes, the content of melted iron was decreased to 0.27 g in 1 kg of the melt, and the melt reduced oxides on the surface of strips made of carbon steels. The content of NaH and KH after terminating the steam supply was 0.09 wt%.