NO790339L - PROCEDURES FOR TREATING CERTAIN SCRAPES - Google Patents

Description

Procedure for processing certain scrap metal

the invention generally relates to the dissolution of certain scrap metal and more specifically to a leaching method for dissolving scrap metal containing the combination of (a) a metal from the group of cobalt, nickel and mixtures thereof and (b) a refractory metal from the group of molybdenum, chromium, tungsten and mixtures thereof .

In recent years, the use of so-called "superalloys" has increased due to the need for objects with the special properties obtained by using these materials, such as high-temperature strength, resistance to oxidation at high temperature and corrosion resistance, etc. As the use of these superalloys increases, the amount of scrap metals and other waste materials containing these superalloys also increases. The usual superalloy scrap includes such physically very different objects as used and quality-unsatisfactory articles, turning shavings, grinding shavings, casting ladle shells, mold runs, spark waste and waste<p>products such as,

sludge and slag. As used herein, the term "scrap" is therefore intended, without limitation, to include scrap in any of the above forms.

Due to the fact that scrap metal of the above types contains metals of great economic importance, there is currently a clear need for suitable methods by which such scrap can be treated in order to recover at least some of the valuable metals from the scrap in commercially usable form. These scrap metals include significant amounts of the superalloys which generally contain significant amounts of nickel and/or cobalt, and various schemes have been proposed to recover these valuable metals from such scrap.

In US<p>patent no. 3649487 a method is described in which metal scrap<p>which contains the combination of at least one base metal from the group of iron, nickel, cobalt and copper and at least one metal with a high melting point from the group of chromium, molybdenum and tungsten , is treated electrolytically to extract the valuable base metals from the scrap. In the aforementioned method, the metal scrap is pretreated to convert the above-mentioned metals with a high melting point into their corresponding carbides, borides, silicides, nitrides and/or phosphides. The metal scrap pretreated in this way is then anodically dissolved at a voltage of less than 4 V in a neutral or acidic electrolyte of salts of a non-oxidizing acid. Due to the combination of the pretreatment step and the maintenance of a low voltage during the electrolysis, it is stated that the dissolution of the metal scrap takes place in such a way that the base metals are dissolved and that one or more of these are finally deposited on the cathode, while the metals with a high melting point remains essentially undissolved and is retained on the anode in the form of an anode slime or

-skeleton.

US Patent No. 3607236 describes a method in which superalloy metal scrap is dissolved by percolation of chlorinated, hot, aqueous hydrochloric acid through a layer of the scrap. After each pass, the hydrochloric acid leaching liquid is saturated again with chlorine gas. The described dissolution process is only partially selective towards tungsten and is not selective towards the other valuable metals. Essentially the entire amount of the superalloy metal scrap is thus dissolved in the leaching liquid. The enriched leachate from this dissolution process is treated using various methods such as carbon adsorption, filtration, solvent extraction, stripping, oxidation and selective precipitation

to separate and recover tungsten, molybdenum, iron, cobalt, chromium and nickel.

US Patent No. 3544309 describes a method where nickel alloy scrap is dissolved in aqueous hydrochloric acid. The chromium, iron, cobalt and nickel content of the enriched leachate is isolated and separated by means of a series of precipitation steps using magnesium oxide and by means of a solvent extraction step. Hydrochloric acid and magnesium oxide are finally reconstituted in this process, and these agents are recycled for the dissolution and precipitation stages of the process. It is important to note that the patent holder warns the users of the invention against carrying out any treatment of the metal scrap that will increase its carbon or nitrogen content, as such pre-treatment is stated to make the scrap insoluble.

Each of the above-mentioned processes is subject to certain disadvantages which tend to prevent an extensive commercial application of the processes. Thus, it is assumed that the costs of the method according to US Patent No. 3649487 are unsatisfactory because of the process's need for electrical energy and because of the high investment and operating costs that are usually associated with the operation of electrolysis cells. The method according to US patent documents no. 3607236 and no. 3544309 does not lead to any significant separation of the valuable metals during the dissolution of the metal scrap, and each of these processes thus requires a series of relatively complicated and expensive treatment steps for the enriched leaching liquid after the dissolution step to achieve separation and recovery of the valuable metals from the enriched leach liquid. Both of these processes are also based on extreme dissolution conditions in order to obtain a reasonably rapid dissolution of the normally acid-resistant alloys.

According to the invention, however, it has been shown that a highly selective and acceptable rapid dissolution of added scrap metal containing the combination of (a) at least one metal from the group of nickel, cobalt and mixtures thereof and (b) at least one refractory metal from the group of molybdenum, tungsten, chromium and mixtures thereof, can be obtained by means of acid leaching or ammoniacal leaching of specially prepared scrap.

The invention is therefore mainly aimed at providing a new leaching process for dissolving scrap metal which contains the combination of (a) at least one metal from the group of cobalt, nickel and mixtures thereof and (b) at least one refractory metal from the group of chromium, molybdenum, tungsten and mixtures thereof, where the nickel and/or cobalt content of the scrap essentially dissolves in the leaching liquid, while refractory metals mainly remain in the leaching residue.

The invention also aims to provide

a non-electrolytic acid leaching process or ammoniacal leaching process for dissolving scrap metal of the above type and where the added scrap metal comprises alloys which

are normally not soluble in aqueous acid solutions or ammonia solutions.

The invention also aims to provide a non-electrolytic acid leaching process or ammoniacal leaching process for dissolving scrap metal of the above-mentioned type, where the nickel and/or cobalt content of the scrap metal is mainly dissolved, and where the resulting leaching liquid can be treated to extract nickel- and/or the cobalt content from this.

The invention also aims to provide a new ammoniacal. leaching process for dissolving scrap metal of the above type, where the nickel and/or cobalt content in the scrap metal is essentially dissolved, the leaching process can be carried out in a non-electrolytic or electrolytic way.

According to the invention, added scrap metal containing the combination of (a) at least one metal from the group of nickel, cobalt and mixtures thereof and (b) at least one refractory metal from the group of chromium, molybdenum, tungsten and mixtures thereof is melted, an effective amount of carbon is added to the smelting, and the thus carburized melt is solidified so that the content of the refractory metal or refractory metals (b) is converted into carbides. This carburized added scrap metal is then dissolved in an ammoniacal leaching liquid or dissolved in a non-electrolytic way in an acid leaching liquid or an ammoniacal leaching liquid, whereby the nickel and/or cobalt content of the added material is mainly transferred to the leaching liquid, while the decarburized refractory metals (b) in the supplied material essentially remain as a residue. The resulting dissolution products, comprising the leach liquor and the leach residue, can be separated and each of the products can then be processed to recover the desired valuable metals therefrom.

The drawing schematically shows a flowchart for the present method together with some preferred and optional embodiments thereof.

As mentioned above, the original scrap feed in the practice of the present invention generally includes any scrap metal containing the combination of:

(a) at least one metal from the group nickel, cobalt and mixtures thereof, and (b) at least one refractory metal from the group chromium, molybdenum, tungsten and mixtures thereof.

The scrap metal inputs of interest can have a number of different compositions which are either calculated or purely random, and they can include e.g. mechanical mixtures of one or more of the following materials: nickel-based alloys, cobalt-based alloys, steel and iron alloys, magnet alloys, nickel-copper alloys and aluminum alloys, etc.

Of particular interest are the scrap metal supplies that contain significant amounts of the so-called cobalt- or nickel-based alloys, especially superalloys. Superalloys usually have a low iron content of less than approx. 40% by weight and are often resistant to attack by and dissolution in aqueous acid leaching fluids under conditions that are not extraordinary. Such cobalt- or nickel-based superalloys can e.g. consists of alloys conforming to AMS Specifications 5536G, 5532B and 5608 and to ASME Code Cases 1642-2 and 1410-4. Superalloys are also generally believed to be resistant to attack by and dissolution in ammoniacal.. leaching fluids under conditions that are not extraordinary. Of particular interest are the scrap metal supplies which, when melted, give a molten mass which, just before it solidifies, will contain at least approx. 24% by weight nickel plus cobalt and at least 8% by weight of one or more of the refractory metals from the above-mentioned group (b).

It should of course be recognized that the scrap used as starting materials may also contain: various valuable metals or metalloids (in elemental or chemically bound form, such as oxides) in addition to the special metals according to groups (a) and (b) which are necessary for the implementation of the present invention. Suitable scrap that can be used as starting materials can thus also contain e.g. one or more elements, alloys or compounds of magnesium, iron, boron, aluminium, yttrium, lanthanum, silicon, titanium, zirconium, hafnium, vanadium, niobium, manganese, copper and similar metals.

Depending on the exact nature of the original scrap metal supply, its composition and the origin of its constituents, it is also intended that the execution of the present invention can often be favorably influenced when the scrap metal supply is subjected to normal conditions prior to the treatment according to the invention cleaning processes for scrap metal, such as degreasing and particle cleaning.

Decarburization of the refractory metals of group (b) in a suitable original scrap feed may be carried out by melting the feed, with or without a slag, by any suitable process, such as by "smelting in a arc furnace or induction furnace Coal is then introduced into the resulting molten metal

bath using any suitable method, e.g. by injecting graphite or coke into the molten metal bath using an inert carrier gas, such as argon. The carbon can optionally also be added in a ladle during tapping of the molten scrap.

A further generally suitable way of adding carbon is to provide sufficient carbon in the original scrap metal supply before it is melted. This can be done e.g. by adding carbon to the original feed before melting or by suitable pre-selection and mixing of two or more different metal scrap<p>, where one or more of these scraps already contain sufficient carbon to effect carburization of the refractory metals from the group (b ) in the obtained composite original scrap supply. When using any of these methods, a molten metal bath containing dissolved carbon will therefore be obtained. The carburization of the refractory metals from group (b) naturally takes place upon solidification of the carbonaceous molten bath, the carbides forming a separate phase in the base mass of metals from group (a).

The molten bath can be solidified in connection with a tapping step, as in the casting of pig iron ingots or blocks. However, it is usually convenient and economically advantageous to bring the melt to solidification by means of ordinary shot formation or atomization methods, whereby the solidified, carburized metal supply is obtained at least in the form of relatively small particles, whereby a large contact area for the solidified , carbonized feed becomes available for the acidic leaching solution. This improves the yield in the leaching step. The particularly chosen solidification method will usually depend on the available equipment,

of financial considerations and of the calculated subsequent treatment steps. It is also intended that before the leaching, the solidified/

charred feed is crushed or ground by any known method to reduce the feed to a smaller particle size.

Important effects of the carburization of refractory metals

from group (b) in carrying out the present method is (1) to convert the content of the refractory metals chromium, molybdenum and tungsten in the original scrap metal feed into one or more separate carbide phases which are predominantly soluble in the subsequent leaching treatment, and (2) at least if the original scrap metal supplies contain one or more alloys which are normally highly resistant to attack by and dissolution in acidic or ammoniacal leaching fluids,

to fundamentally change the character of such alloys, such as e.g. to make the ground mass phase of cobalt and nickel content that is formed during the carburizing, soluble in the leaching liquid that will later be used. The smallest amount of carbon that will give these two results is therefore the smallest amount of carbon that must be present in the bath of molten scrap metal.

Although not intended to be bound by the discussion below, it is believed that the minimum effective amount of carbon that must be present in the bath of molten scrap metal for a given set of conditions will depend at least in large part on (1) the overall composition of the starting scrap metal, especially the content of refractory metals from group (b), and (2) the special leaching liquid to be used to dissolve the charred product. It is assumed e.g. that the less the corrosive effect of the leaching liquid to be used, the greater will generally be the need for the carbon content of the charred scrap. In a similar way, the greater the total content of refractory metals from group (b) in the output tap, the greater the required amount of carbon will be. Taking the above into account, it will therefore generally appear that a carbon content for the carburized solidified scrap metal feed of 0.5-6% by weight carbon is effective for carrying out the present method. Furthermore, for the execution of the present method, it should be noted and understood that the carburizing of the scrap metal used as starting material does not necessarily lead to a transformation of the content of refractory metals from group (b) simply into the simple corresponding carbides, but that it also as a rule, will lead to at least part of the refractory metals being transformed into more complex carbide forms. If e.g. chromium, molybdenum, nickel and cobalt are present in the metal scrap, at least one of different types of carbides containing two or more of these elements in combination will be able to form.

As mentioned above, the original scrap metal supply may also contain significant amounts of oxidized or oxidizable content of other valuable metals than those that must be present in the scrap metal supply, such as aluminium, silicon, magnesium, boron, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium , niobium, tantalum and manganese. The extraction of these oxidizable metal contents may be undesirable because they are of relatively low commercial value, because they are present in an insufficient amount to be of commercial interest, or because the presence of these metals in the leaching liquid or in the leaching residue products according to the invention could lead to difficulties in the subsequent treatments or extraction processes that could be used in connection with the leaching liquid or the leaching residue products. According to a preferred embodiment of the invention, it is therefore avoided that at least part of these oxidizable metals are present in the solidified, carburized scrap metal supply or that their content is reduced by using one or more optional oxidation steps between the melting step and the carburizing step. The oxidation step can be carried out by any suitable method known in the art. For example, oxygen can be injected into the molten bath of original scrap metal feed via a lance and in the presence of a calcareous slag. When using this method, the oxidizable metal compounds will migrate from the molten metal and combine in the slag. They can then be removed by slag. If desired, a new protective slag can then be provided before the charring step is started.

The decarburized and solidified scrap metal supply is dissolved by leaching the supply with essentially any leaching liquid from the group of aqueous acids or ammoniacal liquids, the liquid having to be able to attack and dissolve elemental cobalt and nickel. If aqueous acid leaching liquids are used, the leaching step is not carried out electrolytically. Suitable acid leaching liquids will generally be aqueous acids, such as hydrochloric acid, perchloric acid, nitric acid, sulfuric acid or phosphoric acid. Of these, aqueous hydrochloric acid and sulfuric acid are generally preferred as leaching fluids because they are readily available and inexpensive and because different processes for extracting the nickel and/or cobalt content from aqueous sulfate and chloride solutions of these metals are known and applied in practice.

The concentration of the aqueous acid leaching liquid or the ammoniacal leaching liquid is also generally not of decisive importance, provided of course that the concentration is sufficient for elemental cobalt and/or nickel to dissolve from the carburized metal scrap. The contact between the leaching liquid and the charred scrap can be carried out in a simple way under stirring, at elevated temperature and/or at elevated pressure to promote a rapid dissolution of the cobalt and/or nickel content. When using an ammoniacal leaching liquid, the leaching can

if desired is carried out electrolytically to promote dissolution.

Example 1

An initial scrap metal supply was prepared and melted in an electric arc furnace. The original scrap metal supply consisted of 454 kg of mixed cemented carbide, 4999 kg of shavings, 412 kg of other mixed materials, 218 kg of quicklime (CaO) and 136 kg of flux<p>ate (CaF.-,). The shavings that formed part of the original scrap metal supply were obtained from grinding operations and consisted of a mixture of metal particles and grinding wheel waste. During the preparation and melting steps, the entire amount of hard scrap metal and part of the shavings were filled into the furnace before the heating was started. The rest of the shavings, lime and fluorspar were added to the furnace during the melting of this preliminary charge.

A sample of the resulting molten metal bath was taken and, by analysis, showed 0.63% carbon by weight. Most, but not all, of the slag was then removed. A certain oxidation of strongly oxidizable elements in the scrap usually takes place during the melting down, and the oxides obtained are generally found in and thus removed together with the slag. The bath's temperature was regulated to approx. 1677°C, and oxygen was injected into the bath at a pressure of 4.9 kp/cm 2 for 5 minutes using an iron pipe lance with a diameter of 1.27 cm and covered with a refractory material. A sample of the molten metal bath was repeated and on analysis showed 0.33% carbon by weight. The rest of the slag was then removed and replaced with a new slag consisting of 91 kg of fluorspar and 36 kg of quicklime.

After that, approx. 159 kg of graphite powder injected into the scrap metal bath by means of an iron pipe lance covered with a ceramic material, and using argon as carrier fluid. A small amount of the fluorspar was then added to promote liquefaction of the slag.

A sample of the scrap metal bath was then taken and on analysis showed 2.78% by weight C. A further 34 kg of graphite was then injected into the bath in the manner described above, and a sample was again taken from the bath. The carbon content in this sample was 3.21% by weight. The bath's temperature was then regulated to approx. 1454°C and the melt was tapped into the ladle, with a further 9 kg of graphite being added to the ladle during the tapping stage.

Water jets were used to break up the tapping flow as the melt was tapped into a water-filled pit. The obtained shot-converted, charred scrap metal product was washed, dried and sieved. 3491 kg of retained hail and 1073 kg of hail smaller than the sieve size were recovered during the sifting.

Of the original scrap metal charge, losses due to spills amounted to approx. 41 kg, and the quantity of ladle shells was roughly estimated at approx. 91 kg.

Apart from the burnt lime and fluorspar, a total of 6057 kg of hard metal scrap, shavings, various other materials and graphite were added to the bath. About. 4,696 kg of scrap metal was recovered in the form of retained shot product, shot product smaller than the sieve size, waste and ladle shells. The difference between the total original charge weight and the weight of the total amount of extracted metal is due to grinding wheel waste that is introduced into the original charge, but which is found in the slag, loss of carbon due to oxidation, loss of easily oxidizable scrap metal elements, which aluminum and silicon, to the slag, metal contained in the slag and other common losses and measurement errors.

The composition of the hail product was determined for a sample taken from the tapping stream and is reproduced in Table 1-1. The low value for silicon is a good indication that the preparation and treatment of the original scrap metal feed effectively removed easily oxidizable elements, such as silicon and aluminum, which are usually present.

100 g of the sieved shot and 400 ml of 20% by weight aqueous HCl leach liquor were charged into a flask fitted with a condenser. The liquid was boiled for 24 hours, the contents of the flask filtered and distilled water added to the liquid filtrate to bring its volume up to 500 ml. This diluted leach liquid was then analysed, and the results of the analysis are reproduced in Table 1-2 below.

Based on the compositions of the shot (Table 1-1) and of the leaching liquid (Table 1-2), the percentage contents of the various elements in the original scrap metal supply that were transferred to and dissolved in the leaching liquid are calculated. These results are reproduced in the table 1-3 below.

It appears from an examination of tables 1-3 that solutions of nickel and cobalt in the leaching liquid take place to a far greater extent than the solution of chromium, molybdenum or tungsten.

Example 2

18 kg of a meltable scrap feed was charged into an air induction furnace and melted down. A chemical sample weighing 0.12 kg and hereafter designated XRS-1 was taken. Another sample of the molten bath was cast in the form of a rod and is hereafter referred to as Ri. This rod weighed 0.16 kg and had a diameter of 9.5 mm. The term "sample" is used herein to denote all metal cast at the same time and given the same number. All "R" samples are in the form of bars. In practice, therefore, a part consisted of

the samples of several separate pieces of metal.

0.45 kg of nickel oxide (NiO) and 0.45 kg of a pre-melted slag consisting of 3 parts by weight CaO, 1 part by weight CaF2 and 1 part by weight A^O^ were added, and the bath was then maintained at a temperature of slightly above 1538°C for 5 minutes. The slag was then removed,

and a chemical sample, XRS-2, and a sample of a cast rod weighing 0.20 kg, hereafter denoted R2, were both taken from the molten metal bath. Carbon was then added portionwise to the molten bath which was kept in the oven. Each such portionwise addition was carried out by stirring 0.23 kg of graphite into the molten bath. After each carbon addition and before the next, a test rod was cast, and in some cases a chemical sample was taken. Identification of the chemical samples and rod samples cross-referenced to the particular stage of the process from which they were taken, and furthermore the weight of each rod sample and the result of the carbon analysis for this are reproduced in the table 2-1 below.

The chemical samples XRS were both analyzed, and the results and analyzes are reproduced in Table 2-2.

An examination of Table 2-2 shows that aluminium

and silicon were both substantially removed during the treatment of the original scrap mixture by the present process.

Portions of 10-14 g of each rod core were placed in separate glass flasks fitted with condensers, together with 1 L each of 20% by weight aqueous HCl leach liquor.

The resulting scrap/leach fluid systems were then boiled

24 hours and cooled, and the contents were filtered and sufficient

distilled water added to the leach liquor filtrates until the volume of each filtrate was increased to its original 1 1. The weight of

each particular rod sample that was used in the various leaching systems according to the example, and the analyzes of the compositions for the various leaching liquid products obtained are reproduced in the table 2-3 below.

The percentage amounts of the elements in the rod sample that were dissolved in the leaching liquids are given in Table 2-4. The analysis for the composition of the sample XRS-2 was used as a basis for calculating the weight % for each of the elements that were present in the analyzed original bar sample and that were recovered in the leaching liquid.

The beneficial effect of the melting-carburization-solidification treatment according to the invention for facilitating the separation of nickel and cobalt from chromium, molybdenum and tungsten is clearly evident from an examination of Tables 2-4. Before this treatment was finished (the leaching liquids for the samples Ri and R2), it appears that apart from the elements cobalt and manganese, the percentage amounts of the other elements in the rod samples and which are dissolved in the leaching liquids are approximately equivalent. This suggests that each of the samples RI and R2 were relatively evenly dissolved. In contrast, the carburized and solidified bar samples (R3-R14) gave far higher percentages of dissolved nickel, cobalt and iron than of chromium, molybdenum and tungsten. The samples Ri and R2 contain 0.08% carbon, while all other samples contain over 1.3% carbon.

It should also be noted that, in general, the amounts of nickel, cobalt and iron in the original scrap metal supply and which were dissolved in the leaching liquids were considerably smaller for samples RI and R2 than for the charred samples. This is because the non-carbonated samples Ri and R2 had a significantly higher resistance to attack by the acidic leaching liquid. In stark contrast to this, the basic phase elements (Fe, Co, Ni) in the carbonized original tap feed samples were mainly dissolved. A further confirmation of this was provided by a visual examination of<p>the tails RI and R14 after they had been leached. After leaching, sample RI had a surface with a metallic appearance and which was covered with large pits. Sample R14, on the other hand, had a matte surface but appeared to retain its original overall size and shape. Cross-sectional sections of the two leached bar samples were made and examined metallographically using standard methods. The unetched sample Ri had a shiny metallic appearance. The size of the sample was reduced where a pit occurred. The unetched sample R14 had a small, central core with a shiny metallic appearance surrounded by a large matte area that extended all the way to the original boundary of the sample. This dull area indicates a carbide meshwork that was retained after the groundmass phase containing iron, nickel and cobalt had been dissolved by the acidic leaching fluid. The results of the present example therefore suggest that the melt-carburization-solidification treatment that forms part of the present invention can contribute at least two highly beneficial effects. Firstly, it promotes the separation of nickel, cobalt and, if present, iron from chromium, molybdenum and tungsten. Secondly, the aforementioned treatment can greatly reduce the resistance to acid of a normally acid-resistant scrap alloy and thus make the elements in the basic phase of the treated alloy much more susceptible to attack and dissolution by the acidic leaching liquid.

Example 3

40.8 kg of an original scrap metal supply was melted

1 an induction furnace, the temperature of the molten bath obtained was increased to 1516°C, and a chemical sample of the molten metal (sample A) was taken. Nickel oxide (NiO) was slowly added to the bath in an amount of 1.54 kg. About. 5 minutes after the addition of the nickel oxide was finished, the formed surface slag or the formed foam was removed from the bath, and a chemical sample of the molten metal (sample B) was taken. The temperature of the bath was then lowered to and maintained at between 1471 and 1366°C for the remainder of the smelting. In a similar manner as described in example 2, portionwise additions of graphite were made to the bath, each addition being followed by a sampling of the molten metal. The method used in the present example for the addition of carbon differed from the method used according to example 2 in that varying amounts of graphite were added for each portion-wise addition and that the amounts of the molten metal samples taken and some of the types of cast samples which were made from the molten metal samples, in certain cases were different. In the present example, certain of the molten metal samples all weighed approx. 2.3 kg, and they were cast into sample slabs in a graphite mold. A total of 14 cast test pieces were made and were designated with numbers from 1 to 14, cast test piece 1 being made from a sample taken before the first addition of carbon. A chemical sample (sample C) was taken from the molten bath after the last graphite addition and after the cast test piece 14 had been made.

The analyzes for the chemical samples are reproduced in table 3-1. The 14 cast<p>robes were analyzed to determine carbon,

and the results of these analyzes are reproduced in tables 3-2

and 3-3.

Pieces of each cast specimen (with the exception of specimen 1) were separately ground to 45 x D mesh (Tyler) in a steel pestle mortar. About. 40 g of each ground sample was filled into separate flasks fitted with condensers, together with 400 ml per flask of a 20% by weight aqueous HCl solution. The contents of the flasks were then boiled for approx. 4 hours, cooled and filtered, and sufficient distilled water was added to each leach filtrate to bring its volume up to 500 ml.

The leaching liquids were then quantitatively analysed, and the results of these analyzes are reproduced in table 3-2. As the metal analysis for the chemical sample B (Table 3-1) was taken as representative of the various metal contents in the cast test pieces, the weight percentages for Ni, Co, Fe, Cr, W and Mo and which had been transferred from the test pieces to the leaching liquids, calculated. The results of these calculations are reproduced in table 3-3. It is clear from an examination of table 3-3 that the addition of carbon to the molten, original scrap mixture promotes the separation of nickel and cobalt from chromium, molybdenum and tungsten after acid leaching of the solidified, charred scrap mixture.

Example 4

A portion of the cast rod sample Ri 4 (4.81 wt% C)

according to Example 2 was ground to -100 mesh (Tyler) using a steel mortar and pestle. 10 g of this ground, charred scrap metal feed sample was charged into a flask fitted with a condenser, together with 100 ml of 10 N sulfuric acid. The flask was placed on a hot plate, and the contents of leaching liquid boiled for approx. 3 hours and 45 minutes while continuously flushing the flask with air. The leach liquid was then cooled to room temperature and filtered. The leaching liquid filtrate was analyzed quantitatively to determine Cr, Mo and Ni, and it was found that the filtrate contained 2.86 g chromium per liter, 0.90 g molybdenum per liter and 34.31 g of nickel per litres. The leachate filtrate i^SO^ concentration was determined to be 6.62 N. No analysis was made to determine other elements that may have been present in the leachate filtrate.

In the same way as in example 2, the metal analysis for the chemical sample XRS-2 according to example 2 (table 2-2) was taken as representative of the metal content in the ground sample R14 described here before it was leached. The weight percentages for the elements in the original scrap and which had been transferred to the leaching liquid were thus calculated to respectively 11.8% Cr, 8.7% Mo and 59.3%

Nine. The dissolved percentage amounts of chromium and nickel according to

the present example is somewhat lower than those obtained in example 2, and it is assumed that this is due to an incomplete reaction of the ground sample R14 with the sulfuric acid leaching liquid. This assumption is supported by a comparison of the weight ratio between dissolved nickel and chromium for the sample R14 according to example 2 dissolved in a hydrochloric acid leaching liquid, with the weight ratio which

according to the present example was obtained with the sulfuric acid leach liquor. According to example 2, this ratio is 12.3, while the ratio according to the present example is 12.0. The incomplete reaction in the present example is at least partially assumed

to be due to an incomplete contact between the ground rod sample and the leaching liquid. It was noted that the acid leach liquor foamed during the leaching step for the present example, and a significant amount of the ground sample was apparently deposited on

side of the flask above the level of the boiling leach liquid.

The beneficial effect of a carburization of the original scrap metal supply in that it promotes the separation of nickel from chromium after acid leaching of the scrap metal supply, is nevertheless evident from this example, and furthermore that an aqueous sulfuric acid solution is an acceptable leaching liquid.

Example 5

A bath of an original metal supply was made by melting 23.6 kg of scrap metal with a carbon content of approx. 3.76% by weight and 3.8 kg of scrap metal with a carbon content of less than 0.15% by weight in an induction furnace. The temperature of the resulting molten metal bath was increased to 1377°C, and a chemical sample was taken from the bath. Additional bath samples were then cast in the form of bars. The chemical sample was analysed, and its composition is given in Table 5-1. The carbon content of a bar sample was determined to be 3.28% by weight. Because of this substantial original carbon content in the molten metal bath, no further additions of carbon were made to the bath before it was solidified and leached. A piece of one of the rod samples was ground to -80 mesh (Tyler) in a pestle and mortar. A 40 g portion of this ground, ground rod sample was filled with 400 ml of 10N H-^SO^ into a flask fitted with a condenser. The acid leach liquor was then boiled for 4 hours while air was bubbled through it. After cooling, a sufficient amount of distilled water was added to the flask to increase the leachate volume to 500 ml, and the diluted leachate was then filtered from the leachate residue. The leaching liquid filtrate was analysed, and the results of the analysis are reproduced in table 5-2. The weight percentages for certain of the scrap elements that were dissolved in the leach liquor were calculated based on the metal analysis for the chemical sample and are also reproduced in Table 5-2. This example shows the beneficial effect of the carburizing step in promoting the separation of nickel and cobalt from chromium and molybdenum. The use of an aqueous sulfuric acid solution as leaching liquid is also shown. Moreover, a melting practice where the carbonizing carbon is already in the original scrap metal supply when the meltdown takes place is shown.

Example 6

40.8 kg of scrap with a metal content consisting entirely of an alloy material complying with ASME Boiler Code Case No. 1410-4, was melted by adding the bulk of the scrap charge to an induction furnace, after which power was supplied to the furnace and approx. 0.8 kg of nickel oxide was added to the melt charge. After the melting of this original scrap charge was finished, the remaining part of the slope was added and melted, and a chemical sample (sample A) of the smelt was taken. Then a further 0.8 kg of nickel oxide was added to the molten scrap,

and after a short holding period of approx. 8 minutes, the oxide slag was removed from the surface of the melt. Another chemical sample (Sample B)

of the molten slag was taken, and it was determined that the bath temperature was 1493°C. The temperature of the bath was then kept within the range of 1393 and 1502°C for the rest of the smelting.

The molten scrap was charred by portionwise additions of graphite. After each addition of graphite, a sample of the metal weighing approx. 2.3 kg, taken and groaned in a graphite mold. In this way, 14 cast samples were made, and after the last core for the casting had been taken, another chemical sample (core C) of the molten bath was taken.

The composition of each of the chemical samples was determined analytically, and the results of these analyzes are reproduced in the table 6-1 below.

Of the fourteen cast samples, sample No. 13, with a carbon content determined by analysis to be 3.59 by weight, was selected for treatment by ammoniacal leaching. A rod-shaped specimen (Specimen C) was machined from this cast specimen to be used for the leaching step. A control specimen (Specimen 0) was machined from a forged piece of the same alloy used for the original scrap feed. The composition of the control sample is given in table 6-2.

The surfaces of both<p>rubber pieces were prepared by first grinding the surfaces against a 120 grit sanding belt to clean the surface. The specimens were then cleaned in distilled water and dried for 40 minutes at 93°C in an oven. Samples C and 0 weighed respectively 20.6455 g and 22.5107 g.

An ammoniacal leach was prepared by placing 148.3 g (NH 3 )SO 4 , 73.6 g NaOH and 500 ml water in a 1000 ml flask. The pH obtained was 10.3.

The flask was then fitted with a purge tube and the flask with its contents of ammoniacal leachate was heated to and maintained at approx. 66°C at the same time as a slow flushing with oxygen was carried out. After the temperature had been reached, the test pieces C and A were placed in the leaching liquid and kept immersed in it for approx. 42.5 hours. Apart from an aggregation time of

for just over 10 hours, the oxygen was continuously flushed through the solution.

After being removed from the solution, the test pieces were rinsed in distilled water, rinsed in alcohol, again rinsed in distilled water, dried for 10 minutes at 93°C in an oven and weighed.

The degree of dissolution for each of the test pieces and obtained by the above-mentioned ammoniacal leaching treatment of the test pieces was determined from the weight loss. The weight of test pieces C and 0 was determined after the leaching step to be respectively 20.6336 g and 22.5093 g. For the control sample, i.e. sample 0, the weight loss due to leaching was thus only 0.0014 g. For<p>robe sample C, the weight loss was determined to be 0.0119 g. The treatment of the original scrap supply by the method according to the invention thus led to an approximately 8.5-fold improvement in the amount of metal that was dissolved, compared to the untreated scrap. After the leaching step, the resulting leach liquid can be physically separated from the leach residue using any suitable conventional method for separating solids and liquids,

such as centrifugation, filtration, decantation etc. The leach liquid product can then usually be used as a main feed material or auxiliary feed material for various commonly known processes for refining or extracting nickel and cobalt containing ores.

It is e.g. in the article "Turbine Mixer Fundamentals and Scale-up Method at the Port Nickel Refinery", Metallurgical Transactions B, vol. 6B, March 1975, by P.B. Queneau and co-workers described methods used at a commercial refinery where nickel

.. and cobalt is extracted from nickel/copper sulphide mats. On Fig. 1 of this article is shown a block diagram of the interrelationship between unit operations at the Port Nickel refinery of America Metal Climax, Incor<p>orated. In this, according to the nickel and cobalt extraction scheme, it is stated that the acidic copper sulphate electrolyte that is formed as a by-product is used as leaching agent

by a separate and separate electroextraction process for copper. This acidic electrolyte is aerated and simultaneously brought into contact with granulated nickel/copper sulphide mat to thereby dissolve the nickel content as sulphate and to form a solid scrap of elemental copper and compounds of copper and iron. It is described that this wrecking is carried out so that when the aeration-leaching step is over, a nickel-containing leaching liquid is obtained with a content of soluble iron and copper of less than 5 parts per million. This leach liquor is then treated to separate the cobalt and nickel compounds and to recover these in the form of metals. Metal-containing sulfate leach liquors, as formed according to Examples 4 and 5 above, can therefore generally be used in the commercial nickel and cobalt recovery process described in the above-mentioned article. When e.g. the content of soluble copper or iron in the leaching liquid product formed by the present method is substantial, the liquid can be introduced into the leaching step of the extraction process which is carried out at atmospheric pressure, together with the acidic copper sulphate electrolyte. If, on the other hand, the sulphate leach liquid product formed by the present invention already has a sufficiently low iron and copper content or it is treated separately to remove iron, copper or possibly other unwanted metal compounds from it, the sulphate liquid product can then be introduced directly for nickel/ the cobalt separation and for the extraction step of the extraction process described in the article.

In another paper entitled "The Falconbridge Matte Leach Process", P.G. Thornhill, E. Wigstol and G. Van Weert, Journal of Metals, The Metallurgical Society of AIME, July 1971, pp. 3-18, a process is described in which nickel is extracted from nickel/copper sulphide mat by leaching the mat with hydrochloric acid . This leaching step is carried out with hydrochloric acid of sufficient strength to selectively dissolve the nickel component of the mat, while the copper and platinum compounds are destroyed. The leach liquor is treated with an oxidizing agent to remove dissolved hydrogen sulfide and to convert iron to the trivalent state. The iron compounds are then removed by solvent extraction of the liquid with tributyl phosphate. The resulting iron-free liquor was then treated with triisooctylamine to remove cobalt and any remaining dissolved copper from the previous solvent extraction step. Nickel chloride of good purity is then crystallized from the liquid. As it is described that the method is to be carried out commercially for the production of metallic nickel, this is then produced by high-temperature hydrolysis of the nickel chloride, whereby a product of elemental nickel and a by-product of HCl gas is obtained which is used to regenerate acid for leaching fresh mat . Other methods for producing elemental nickel from the nickel chloride have been described, such as by direct reduction or by electroextraction from electrolytes that are enriched by dissolving the nickel chloride crystals.

It thus appears that the present method as described in example 1 can lead to a leaching liquid product which is suitable for use as original input material or as auxiliary input material in the nickel extraction method described in the above-mentioned article.

In The-Winning of Nickel,Joseph R. Poldt, Jr., edited by

P. Queneau, D. Van Nostrand Company, Inc., 1967, pp. 299-315,

is described a commercial process for the extraction of nickel as carried out in practice at the refinery of Sherritt Gordon Mines, Limited in Fort Saskatchewan. This process is based on multi-stage ammoniacal leaching of the nickel-containing sulphide ore called pentlandite, whereby nickel and copper compounds are leached and passed over the ammoniacal leaching solution. The nickel and copper-containing leaching solution is boiled to recover part of the ammonia and to precipitate the copper content in the form of sulphide.

The copper-free leach solution is then treated to convert incompletely oxidized and unsaturated sulfur compounds to sulfate ions. The dissolved nickel content is then converted to a powder of elemental nickel by direct reduction with hydrogen. According to the present invention, it is aimed that the charred scrap and/or leaching liquid products obtained according to the invention should be able to be integrated into refining processes of the

type indicated above. Under the assumption that harmful materials are not present or that they are first removed, e.g. the charred scrap product obtained according to the invention is mixed with the sulphide ore to form a mixture of ore and charred scrap which is suitable as a composite feed material for the ammoniacal leaching step in the refining process. In addition, a separate ammoniacal leaching treatment of the charred original scrap supply can be carried out, and if harmful pollutants

nings. are present, these can be removed from the obtained leaching liquid which is then introduced as supplementary feed material into one or more of the refining processes carried out downstream in relation to the ore leaching step.' The ammoniacal leaching liquid product that can be obtained by the present method can thus be combined with the liquid fraction that emerges from the leaching step for the sulphide ore, the combined leaching fractions then being treated together to extract nickel and at least part of the ammonia from them.

Claims (28)

1. Procedure for processing certain metal scrap, characterized in that an original scrap metal supply is provided which contains the combination of (a) at least one metal from the group nickel, cobalt and mixtures thereof and (b) at least one refractory metal from the group chromium, molybdenum, tungsten and mixtures thereof, after which this feed material is melt-carburized sufficiently so that the metals from group (b) become at least mainly insoluble in the later leaching step with aqueous acid and so that the metals from group (a) or any acid-resistant alloys in the feed material become soluble in the acid leaching step, after which the melt-carburized feed material is leached non-electrolytically with an aqueous acid leaching liquid capable of dissolving the metals from group (a), whereby the metals from group (a) are mainly dissolved in the liquid, while the metals from group (b) remain mainly as an undissolved leach residue. 2. Method according to claim 1, characterized in that the original scrap metal supply in a molten state comprises at least 25% by weight of the metal from group (a) and at least 8% by weight of the refractory metal from group (b). 3. Method according to claim 1 or 2, characterized in that the scrap metal supply comprises a nickel-based alloy. 4. Method according to claim 3, characterized in that the nickel-based alloy is a superalloy. 5. Method according to claim 1 or 2, characterized in that the scrap metal supply comprises a cobalt-based alloy. 6. Method according to claim 5, characterized in that the cobalt-based alloy is a superalloy. 7. Method according to claims 1-6, characterized in that the melt carburization is carried out so that a melt-carburized scrap metal feed product with a carbon content of 0.5-6.0% by weight is obtained. 8.. Method according to claims 1-7, characterized in that carbon for the carburizing step is added to the scrap metal supply before it is melted. 9. Method according to claim 8, characterized in that carbon for the carburizing step is added by mixing a metal scrap that contains insufficient carbon to carburize the metal scrap, with another metal scrap that contains an excess of carbon to carburize the metal scrap, so that a composite original scrap metal supply that contains sufficient carbon to carburise it. 10. Method according to claims 1-9, characterized in that oxidizable metal compounds are removed from the original scrap metal supply by oxidation treatment of this before the carburizing step is carried out. 11. Method according to claim 10, characterized in that the oxidation treatment is carried out by melting the scrap metal supply and introducing molecular oxygen into it. 12. Method according to claim 10, characterized in that the oxidation treatment is carried out by melting the scrap metal supply and that the molten supply is mixed with an oxidizing slag. 13. Method according to claims 1-12, characterized in that hydrochloric acid is used as aqueous acid leaching liquid. 14. Method according to claims 1-12, characterized in that sulfuric acid is used as the aqueous acid leaching liquid. 15. Method according to claims 1-14, characterized in that the leaching liquid product from the leaching step is treated to extract dissolved compounds of valuable metals from it. 16. Procedure for processing certain metal scrap, characterized in that an original scrap feed is provided which contains the combination of (a) at least one metal from the group nickel, cobalt and mixtures thereof and (b) at least one refractory metal from the group chromium, molybdenum, tungsten and mix- . ings thereof, after which the feed material is melt-carburized to the extent that the metals from group (b) become at least mainly insoluble in the subsequent ammoniacal leaching step and that the metals from group (a) in the alloys present in the feed material and which are resistant to up- solution in the ammoniacal leaching liquid, become soluble in the ammoniacal leaching step, and the melt-charred feed material is leached with an ammoniacal leaching liquid which is able to dissolve the metals from group (a), whereby the metals from group (a) are mainly dissolved in the liquid, while the metals from group (b) remain mainly as an undissolved leach residue. 17. Method according to claim 16, characterized in that the original scrap supply in a molten state contains at least 25% by weight of the metal from group (a) and at least 8% by weight of the refractory metal from group (b). 18. Method according to claim 16 or 17, characterized in that the scrap supply comprises a nickel-based alloy. 19. Method according to claim 18, characterized in that the nickel-based alloy is a superalloy. 20. Method according to claim 16 or 17, characterized in that the scrap supply comprises a cobalt-based alloy. 21. Method according to claim 20, characterized in that the cobalt-based alloy is a superalloy. 22. Method according to claims 16-21, characterized in that the melt carburizing is carried out so that the obtained melt-carburized scrap supply has a carbon content of 0.5-6.0% by weight. 23. Method according to claims 16-22, characterized in that carbon for the carburizing step is added to the scrap supply before it is melted. 24. Method according to claim 23, characterized in that carbon for the charring step is supplied by mixing a scrap that contains insufficient carbon to char the scrap, with another scrap that contains an excess of carbon to char the scrap, whereby a composite original scrap supply is obtained which contains enough carbon to char it. 25. Method according to claims 16-24, characterized in that oxidizable metal compounds are removed from the original scrap feed by oxidizing treatment of this before the carburizing step. 26. Method according to claim 25, characterized in that the oxidizing treatment comprises melting the scrap feed and introducing oxygen into it. 27. Method according to claim 25, characterized in that the oxidizing treatment comprises melting the scrap feed and treating the melted scrap feed with an oxidizing slag. 28. Method according to claims 16-27, characterized in that the ammoniacal leaching liquid product from the leaching step is treated to recover dissolved compounds of valuable metals from group (a) from this.