US3069334A - Process for the production of tetraethyl lead - Google Patents

Description

Dec. 18, 1962 Filed June 9, 1958 K. ZIEGLER ET AL PROCESS FOR THE PRODUCTION OF TETRAETHYL LEAD 2 Sheets-Sheet l STORAGE STORAGE I /'DIAPHRAGM "SETTLING I rowan /s 1 I (a? me I STORAGE STORAGE I I 23 /8 :r/7

23 24 INVENTORS 101R]. Z/EGLER HERBERT LE HMKUHL ATTORN S Dec. 18, 1962 K. ZIEGLER ET AL 3,069,334

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INVENTORS KARL Z/EGL ER. HERBERT LE HMKUHL ATTORN S atent 3,069,334 Patented Dec. 18, 1962 Free 3,069,334 PROCESS FOR THE PRUDUCTION OF TETRAETHY L LEAD Karl Ziegler and Herbert Lehmkuhl, Mulheim (Ruhr), Germany; said Lehmkuhl assignor to said Karl Ziegler, Mulheim (Ruhr), Germany Filed June 9, 1958, Ser. No. 740,623 Claims priority, application Germany June 12, 1957 22 Claims. (Cl. 204-59) This invention relates to a process and a device for the production of tetraethyl lead.

In our co-pending patent application Serial No. 548,862, filed November 25, 1955, now Patent No. 2,985,568, a process is described for the production of lead tetraalkyls. This process allows an overall result characterized by the reaction equation:

Since a direct reaction between metallic lead, hydrogen and olefins is not possible, the above result is realized by a combination of two process steps. Aluminum is re acted in the first step with an olefin and hydrogen to form aluminum trialkyl. In the second step the aluminum trialkyl is made electrically conductive by the addition of a suitable compound, such as an alkali metal halide, and subsequently electrolized between an anode containing lead and a cathode of any suitable material. In this step, aluminum is deposited at the cathode while lead tetraalkyl is formed at the anode and is separated from the electrolyte. The aluminum deposited at the cathode may afterwards be reacted again with an olefin and hydrogen to form aluminum alkyl which is recycled into the process. The reaction of the aluminum with an olefin and hydrogen in the first step may be effected pursuant to our co pending applications Serial No. 484,576, filed January 27, 1955, and Serial No. 573,470, filed March 23, 1956, and Patent No. 2,835,689, according to the following reaction equation:

(2) 2AlR +Al+1 /2H =3AlHR and may, if desired, be effected in aluminum halide catalyst.

In order to make the aluminum alkyl compounds electr1cally conductive the above mentioned applications suggest the use of additives such as an alkali halide, especially sodium fluoride. According to application Serial No. 548,862 a particularly suitable electrolyte consists of the liquid complex compound of the composition NaF.2AlR which is described in application Serial No, 379,294, filed September 9, 1953, of the applicant, now Patent No. 2,844,615.

The direct production of lead tetraalkyl as proposed in application Serial No. 548,862 may be represented by the following reaction equation:

During the electrolysis, aluminum trialkyl is thus constantly used to the same extent as the lead tetraalkyl and metallic aluminum are formed. When NaEZAlR is used as the electrolyte, this has the effect of constantly impoverishing the electrolyte in aluminum so that the electrolyte composition finally reaches a ratio of NaF:AlR of about 1:1, with the formation of a definite complex compound NaF.AlR (1:1 compound). This compound may have a high melting point and a substantially lower conductivity than the compound of the type NaF.2AlR (1:2 compound) and is therefore less suited for the electrolysis, e.g. for economic reasons. It is apparent that stationary conditions can be maintained in the electrolyte if aluminum the presence of a dialkyl trialkyl is constantly added at the same rate that it is being consumed, with the formation of lead tetra-alkyl at the anode and aluminum at the cathode.

In the further development of the process it became apparent that unless special precautionary measures were taken, the aluminum deposited at the cathode would react with the lead tetraalkyl forming lead which contaminated the rest of the aluminum. This effect increased proportionately with an increase in the current density used for the electrolysis. When using higher current densities the aluminum is not deposited at the cathode in form of a compact metal but as a porous powder which reacts with the lead tetraalkyl much more rapidly. These higher current densities are, however, desirable for economic reasons. The contaminated aluminum thus obtained is not pure aluminum suitable for industrial purposes, and if used for the conversion to aluminum alkyl compounds in the production of lead tetraalkyl, complicates the process.

In said application Serial No. 548,862 it was proposed to separate the electrolytic cell by means of a diaphragm into an anode and a cathode space, in order to prevent the contamination of the catholyte with the lead tetraethyl containing anolyte and thus to obtain a refined aluminum. The separation of the anode from the cathode space has, however, far reaching consequences for the electrolysis. The chemical reactions occurring during the electrolysis change the composition of the now separate catholyte and anolyte. The diaphragm prevents a comingling of these two liquids and thus an adjustment of the differences in the composition of the electrolyte resulting during the electrolysis. Consequently during the electrolysis aluminum trialkyl free from complex bonding with alkali halide is formed in the anode space. This free aluminum trialkyl is removed together with the lead tetraalkyl and can be separated from it only with difficulty. In this manner, a part of the aluminum is lost during the process in the form of aluminum trialkyl which contaminates the lead tetraalkyl obtained.

The applicants have proposed overcoming this difliculty by adding a certain amount of sodium aluminum tetraalkyl to the electrolyte at the beginning of the electrolysis.

It was found, however, that this modification of the process was not sufiicient to avoid with certainty the above mentioned complications resulting from the separation of the electrolysis device into anode and cathode space.

One object of this invention is to avoid all the above mentioned difiiculties and allow the deposition of aluminum at the cathode which is suitable to be used as refined aluminum outside the process, and the production of lead tetraethyl which does not contain any aluminum triethyl at the anode. This and still further objects will become apparent from the following description read in conjunction with the drawings in which:

FIG. 1 is a diagrammatic perspective view of an embodiment of an arrangement of electrolysis cells for effecting the process in accordance with the invention;

FIG. 2 diagrammatically shows'an arrangement in the form of a flow sheet for effecting the process in accordance with the invention; and

FIG. 3 is a fiow sheet showing a further embodiment of the process in accordance with the invention. a

The invention relates to an improvement in the above described process for the electrolytic production of tetraethyl lead by passing an electrolysis current between a cathode and lead-containing anode through an aluminum triethyl-containing electrolyte. In accordance with the invention a diaphragm is maintained between the cathode and anode to thereby form separate cathode and anode spaces which are separated from free flow communication of the electrolyte with each other by means of the diaphragm. At least initially in anolyte is maintained in The electrolysis may be continued until one of the NaAl(C H and NaF.Al(C H has disappeared The advantages obtained by the process according to the invention are apparent from a consideration of the chemical reactions in the anolyte during the electrolysis.

For the purpose of illustrating the process of the invention it will be supposed that the complex compound NaF.2Al(C H is present in the anolyte mixture. As will be seen later, this is not absolutely necessary. The 1:2 compound decomposes into positive sodium ions and the negative complex-radical These ions are discharges at the anode where, in the presence of lead going into solution, the following reaction occurs In the brackets at the right side of the equation there appears thus at first a double molecule This decomposes into its two components The sodium ions which, so to speak, remain behind in the anode space partly transfer to the cathode space and partly find their electrochemical equivalent in the fluorine ions transferring through the diaphragm into the anode space.

For each C H group bound to lead going into solution there thus results one free Al(C H compound. This can only be combined as 1:1 compound to the extent to which sodium ions did not move out of the anode space and fluorine ions did transfer into the anode space from the cathode space.

Moreover, the Al(C H F compound formed according to Equation 4 has the greater tendency towards forming complexes than the Al(C H compound. As a result, a reaction with the excess electrolyte according to takes place.

For each C H group bound to the lead, a substantial amount of aluminum triethyl is thus liberated and separated in admixture with the tetraethyl lead.

According to the process of the. invention the electrolysis is started with an anolyte which contains one compound each of the formula NaAl(C H and of the formula NaF.Al(C H (1:1 compound). The effect of these additions is illustrated by the following reaction equation;

The combination of the Equations 4 and 5 results in the new reaction equation:

(transferring to the cathode).

The NaAl(C I-I compound, which in accordance with the invention is additionally present, enters the reaction in the following manner:

Of the three Al(C H compounds formed according to Equation 7 only two can react to a 1:2 compound so that one Al(C H compound is left which could contaminate the lead tetraethyl.

Further according to the invention, the anolyte contains additional free 1:1 compound. This 1:1 compound combines with the remaining aluminum triethyl under formation of a 1:2 compound so that the anolyte now is free of Al(C H According to Equation 4, for each one fourth Pb(C H one Al(C H compound is liberated and one Al(C H F compound formed which requires for its neutralisation exactly one NaAl(C H It is, therefore, suitable if the 1:1 compound and the NaAl(C l-I compound are added to the anolyte mixture in equivalent amounts, although this equivalence is not critical. The reaction results are not affected if there is no such equivalence; only in that case the electrolysis is suitably discontinued at the very point at which the component of which there is a deficiency has just disappeared or preferably even a little earlier. It is preferred to keep the molar proportion of NaAl (C H 3 in the range of 3:1 to 1:3 with the commercial optimum of one mole NaAl(C H,-,) to one mole 1:1 compound.

As is apparent from the above equations, the 1:2 compound is formed during the electrolysis from the anolyte components, the 1:1 compound and the NaAl(C- H compound. As a result, an anolyte mixture is conceivable which contains originally only NaF.Al(C H (1:1) compound and NaAl(C H compound and no additional 2:1 compound. In practice, however, an anolyte mixture of this type has certain disadvantages. The melting point is high so that high reaction temperatures are required. The conductivity is low since the 1:1 compound has a considerably lower conductivity, amounting to about only one fiftieth of the conductivity of the 1:2 electrolyte.

In practice, a particularly suitable anolyte contains per mole NaAl(C I-l one mole of the 1:1 compound and 0.16 to 1.70 moles and preferably 0.67 to 1.70 moles of the 1:2 compound.

If the electrolysis is continued until these electrolytes in the anode space have exactly the composition then they contain 10 to 20%, preferably 10 to 15% lead tetraethyl partly in solution, partly separated in liquid form. Per kg. electrolyte a current of 33 /3 to 66% ampere hr., preferably 33 /3 to 50 ampere hr. is required.

With these electrolyte mixtures it is of course not absolutely essential that the amount of current is exactly as indicated. The stated amounts represent the limit up to which the formation of free aluminum triethyl is avoided. tetraethyl continues to be formed; however, according to the above equations and in the absence of at least one of the electrolyte components used in accordance with the invention, free aluminum triethyl will be formed which comingles with the lead tetraethyl.

Suitably, the composition of the anolyte and the amount of the current to be applied will be so selected that upon If the electrolysis is further continued, lead removal of the used anolyte the 1:1 compound has been reacted as far as possible. On the one hand an incomplete reaction prevents the substantially optimum utilisation of the electrolyte for the production of lead tetraethyl which is desirable for economic reasons. On the other hand, the 1:1 compound is more readily soluble than the 2:1 compound in the subsequent extraction of the electrolyte for the recovery of the lead tetraethyl, thereby interfering with the recovery of the lead tetraethyl. It is best to remain a few percent below the amount of the required ampere/hours since this gives the guarantee that on the one hand the reaction has progressed as far as possible whereas on the other hand the critical limit at which one component part of the electrolyte disappears has not yet been exceeded.

Also for the development in the cathode space, the separation of the electrolysis device by a diaphragm preventing the admixing of anolyte and catholyte has far reaching consequences. As a result of the arrangement of the device in accordance with the invention the composition of the catholyte is thus without any importance for the formation of lead tetraethyl in the anode space. Thus, any cathode liquid may be used provided it fulfills the condition of not becoming admixed with the anolyte through the interposed diaphragm and not substantially interfering with the current transport. For an economic operation of the process under regeneration of the used up aluminum alkyl compound, it is however suitable to use also in the cathode space an electrolyte which contains an aluminum ethyl compound.

If the original catholyte contains the complex compound NaF.2Al(C H then sodium is primarily separated during the electrolysis. This sodium reacts with /sAl(C H to /3Al+NaC H which upon addition to Al(C H forms NaAl(C H (sodium aluminum tetraethyl). At the same time, fluorine ions pass through the diaphragm into the anode space and vice versa sodium ions from the anode space into the cathode space. In the course of the electrolysis, more and more NaAl(C H is formed at the cathode. At the same time the aluminum triethyl disappears and as a result the 1:1 compound is developed from the 2:1 compound.

Suitably the 1:2 compound is thus used at the beginning of the electrolysis as catholyte. As a result of the primary separation of sodium at the cathode, the immediate reaction with /sAl(C I-I to sodium ethyl and /3A1 there disappears for each separated equivalent aluminum /aAl(C H The simultaneously obtained sodium ethyl reacts with a further Al(C H compound I to NaAl(C H For each Na(C H there thus disappear ,Al(C I-I in other words, 1:1 compound are being formed from the originally present 1:2 compound. As was just pointed out above, in the anode space on the other hand just one NaAl(C H and one NaF.Al(C I-I (1:1 compound) react to the 1:2 compound.

Upon suitable selection in the composition of the anode and cathode liquids at the beginning of the electrolysis there thus exists according to the invention the possibility to regenerate the used up catholyte by addition of an amount of aluminum triethyl corresponding to approximately the amount of aluminum deposited at the cathode which catholyte is subsequently used as anolyte. Vice versa, there exists the possibility to free the used up lead tetraethyl containing anolyte of this lead tetraethyl and to subsequently use it as catholyte.

According to the invention, the mutual exchange. of the anolyte and catholyte may be effected intermittently by electrolysing one charge until anolyte and catholyte have the optimum composition for the exchange. Now the entire catholyte is removed, an amount of fresh aluminum triethyl approximately equivalent to the amount of separated aluminum added and subsequently introduced as one charge into the anode space from which the entire anolyte has been previously removed as one charge. This 6 anolyte is freed of lead tetraethyl and subsequently introduced into the emptied cathode space.

According to the invention the process may, however, also be carried out continuously by continuously withdrawing a certain proportion of both liquids, freeing the Withdrawn anolyte from the lead tetraethyl and adding fresh aluminum trialkyl to the catholyte and subsequently returning both portions into the other electrolyte space. In place of the intermittent exchange of the electrolyte liquid With each other it is thus possible by a proper combination of the rate of electrolysis carried out under use of 1:1 compound and NaAl(C H in the anolyte and the corresponding continuous replenishment of the catholyte containing these compounds to arrive at such a point that the condition in the anode space required in accordance with the invention, i.e. the presence of the 1:1 compound and sodium aluminum tetraethyl in the anolyte will be invariably maintained during an electrolytical process of any given length of time. In this manner, the critical limit is never exceeded beyond which free aluminum triethyl is formed in the anolyte.

In accordance with a preferred embodiment of the process of the invention part of the catholyte is thus continuously withdrawn from the electrolysis device, mixed with aluminum triethyl in an amount approximately equivalent to the amount of aluminum separated from the removed part of the catholyte and the combined liquid added to the anode space from which a corresponding amount of anolyte is continuously withdrawn, freed of the lead tetraethyl and subsequently introduced into the cathode space.

This removal can be carried out at any given electrolye composition during the electrolysis provided the catholyte already contains the 1:1 compound and so dium aluminum tetraethyl. The higher the concentration of 1:1 compound and sodium aluminum tetraethyl in the catholyte is, the smaller may be the volumes to be removed. For economic reasons it is suitable to keep the Withdrawal rates of the liquids to be exchanged as low as possible. The separation of lead tetraethyl outsidde the electrolysis device can thus, for example, be effected more easily from smaller amounts of anolyte than if the amounts of anolyte to be Worked up are large and obtained at high removal rates. Moreover, if the concentration of 1:1 compound and sodium aluminum tetraethyl in the catholyte is high, a higher yield of lead tetraethyl per volume anolyte can be obtained than if these compounds are present in smaller concentrations in the catholyte part to be introduced into the anode space.

In the case of a continuous exchange of the electrolytes the electrolysis is thus suitably started with the same electrolyte composition than when working intermittently. The electrolysis is now continued until the sodium aluminum tetraethyl, preferably however the 1:1 compound has approximately disappeared under formation of the 1:2 compound in the anolyte, without exceeding this limit. From this point on small proportions of catholyte are continuously withdrawn from the cathode space, mixed with an amount of aluminum trialkyl approximately equivalent to the amount of aluminum separated from the thus removed liquid amount and passed into the anode space from which a corresponding amount of anolyte is continuously withdrawn, freed of the lead tetraethyl and subsequently introduced into the cathode space. The continuous process, as compared with the intermittent process is thus characterized by the fact that while both processes start with the same composition of catholyte and anolyte, a stationary state is obtained in the continuous process in which the electrolyte composition in the cathode and anode space are approximately the reverse of the composition at the beginning of the electrolysis. Contrary thereto, no such stationary composition of the electrolyte is obtained in an intermittent exchange process, where the composition of the speassa electrolyte varies intermittently between the two limits at the beginning and the end of one charge.

Already in the said prior application Serial No. 548,862, it was pointed out that the aluminum trialkyl used for the production of the electrolyte and also the aluminum trialkyl which is constantly added must be free from any hydride. This requirement applies also to the process of this invention. It for example even a small amount of A1R2H is contained in the electrolyte, the latter becomes immediately dark in colour as a result of the reaction with the formed lead tetraalkyl and the formation of lead. The removal of any hydride present may be effected in the manner described in application Serial No. 548,862 by treating the aluminum trialk-yl or the electrolyte with olefins at temperatures between 60 C and 70 C.

For reasons of economy it is desirable to carry out the electrolysis at the highest possible temperatures since the conductivity of the electrolyte baths used increases greatly with increasing temperatures. On the other hand, the upper range of the electrolysis temperatures to .be used is limited by the fact that in the case of excessively high temperatures, in particular during a prolonged electrolysis, the electrolyte will again become dark colored. For the process of the invention, temperatures between the solidification point of the electrolyte mixture (in most cases in the range of room temperature and about 40 C.) and about 110 C. are suitable, temperatures between 60 C. and 80 C. are preferred. A temperature of the electrolyte of about 70 C. is particularly suitable. This temperature permits the safe working for any length of time without the appearance of any interfering side reactions in the electrolyte composition. For short periods of time, somewhat higher temperatures may also be applied.

According to the invention, the liquids used in the anode and cathode space are cooled during the electrolysis in order to obtain a temperature of about 70 C. This measure is necessary since the specific heat of the electrolyte is relatively low so that the amount of energy added during the electrolysis would excessively increase the electrolyte temperature within a short time. According to the invention this cooling is effected with particular advantage by working not with stationary but with flowing electrolytes.

Substantially larger volumes of anolyte and catholyte liquids than are required to fill the electrode spaces may be separately cycled through these spaces and cooled outside the electrolysis device. In this manner, it is possible to regulate the exit temperature of the electrolyte liquid by an appropriate selection of flow rate and entering temperature of the electrolyte into the electrolysis device. In accordance with the invention, the temperature diflference of the electrolyte at the entrance and exit point amounts preferably to 5 C. in particular to 2-4 C. When working in this manner, there are obtained a number of further advantages in addition to the easy and reliable adjustment of the temperature of the electrolyte. For example it is not necessary to provide a complicated cooling system in the electrolysis cell which permits the construction of the cell in a very compact and economical form.

Furthermore there are obtained other considerable advantages for the unimpeded execution of the process of the invention. it has already been pointed out that as a result of the higher current densities which are desirable for economic reasons, the aluminum is deposited at the cathode as a loose finely divided crystalline powder. According to the invention, this metal powder can easily be scraped from the cathode and subsequently discharged from the electrolysis device together with the flowing catholyte. The operation in the anode space is also considerably simplified as a result of the cyclical anolyte flow according to the invention. The 2:1 electrolyte possesses a certain solubility for the formed lead tetra- When working with an electrolyte temperature of 70 C. in the cell, the anolyte would for example only then separate liquid lead tetraethyl as second phase, it the content of the latter would be above 11%. As a result of the cyclically flowing anolyte according to the invention it is now easily possible to cool the anolyte outside the electrolysis cell to temperatures wihch are sufiiciently low so as not to exceed a predetermined maximum lead tetraethyl content in the anolyte liquid. The lower limit of this low temperature is restricted by the solidification point of the anolyte which is higher at the beginning of operation when there is still a great amount of 1:1 compound and NaAl (C H present than towards the end when the composition approaches the 1:2 compound. On the other hand the solubility of the lead tetraethyl in the electrolyte is somewhat smaller at the beginning than at the end. In case of an intermittent execution of the process, it is of course equally possible to maintain the temperature of the total supply of anolyte at about 70 C. without any separation of lead tetraethyl during the entire electrolysis. In this case the total lead tetraethyl formed at the end of the process is still present in dissolved form. It is then possible to cool the anolyte to such a degree, for example 5 C. that the larger proportion of the formed lead tetraethyl is separated out at once. With an anolyte which contains at the end of the operation approximately 10% lead tetraethyl itis thus possible to separate about 7080% lead tetraethyl in pure form.

Subsequent to the separation of the main lead tetraethyl content the electrolytically exhausted anolyte must in any case be completely freed of any lead before it can be used as catholyte.

According to the invention, a particularly suited process for this separation is the liquid-liquid counter current extraction. The extraction may be carried out in particular with hydrocarbons, if desired under pressure. An excellent extracting agent is for example isooctane. The distribution coetficient, for instance, of lead tetraethyl in isooctane and 1:2 compound amounts to 16:1. In an extraction with three theoretical steps it is thus very easy to arrive at a very small lead content of for example 1 mg./ cm. The volume of isooctane required for this extraction corresponds approximately to that of the electrolyte, the isooctane dissolving some of the electrolyte. Upon stripping off the isooctane, the residue separates, however, into two layers; one consisting of a small amount of electrolyte which is returned to the extraction, the other of lead tetraethyl. This type of extraction, however, presents the difficulty that as result of the lead tetraethyl content the density of the extraction agent approximates the density of the electrolyte to such a degree, that during the extraction with the first portion of extracting agent the layers separate only slowly, whereas this difliculty is not encountered with the subsequent portions used in the extraction.

Thus, according to the invention an extraction agent is suitably added to the electrolyte in the beginning. This indifferent agent may be present in amounts of from 3 to 10% vol. of the electrolyte, the electrolyte being preferably saturated therewith. The electrolyte absorbs for example 5 cm. isooctane per 100 cm. electrolyte. By a carefully measured addition of a minute amount of additional extraction agent subsequent to the electrolysis and by cooling to, for example 0 C. it is possible to more easily extract larger amounts of lead tetraethyl. imultaneously the volume of the lead tetraethyl to be separated is thus increased. This constitutes an additional advantage of the process according to the invention since experience has shown that the extraction of a small amount of a second layer from a large amount of a first layer presents certain difficulties. Since only negligible amounts of extraction agents are used up during the first step of the improved extraction of lead tetraethyl from the anolyte, it is thus possible to carry out a same number of further extractions, for instance, five with the same total amount of extraction agent.

When proceeding in this manner, and using for example isooctane as extraction agent for the separation of lead tetraethyl, the final concentration of lead tetraethyl in the electrolyte amounts to about only one-fifth of the amount obtained if the extraction agent is not admixed with the electrolyte in the beginning.

The remaining traces of lead in the anolyte can be removed by an after-treatment of the anolyte, for instance with a small amount of finely divided aluminum, the finely divided aluminum obtained from the process itself being particularly suitable since it shows a high activity for this reaction.

As has already been pointed out, the carrying out of the process according to the invention requires a reliable separation of catholyte from anolyte without, however, substantially interfering with the current transport. According to the invention, this is accomplished by separating the cathode from the anode space by the use of at least one diaphragm. In principle any material may be used for this purpose which under the operating conditions prevailing in the electrolysis device will preserve its stability against the relatively corrosive electrolyte over a longer period of time and the selective effect of which suflices to retain the larger ions and the aluminum and lead ethyl compounds and complexes in their respective electrode spaces without any substantial interference with the current transport by the smaller sodium and fluorine ions.

It has been surprisingly found that cellulosic products are a suitable material for these diaphragms, and are resistant against the chemical effect of these electrolytes even at temperatures of about 100 C. All types of porous cellulosic products with sufficiently small pores may thus be used. It is preferred to use filter paper as diaphragm material. Suitable are for example materials of which a round filter with a cross section of cm. shows a filter rate for 100 cm. distilled water at C. and a height of the column of water of 6 cm. in the range of to 200 see. A particularly suitable material is filter paper which has been compacted or chemically impregnated in order to reduce the size of the pores. In this manner, it is possible to use diaphragms with a thickness of only a few tenths of a mm. In order to increase the mechanical resistance, these diaphragms may be reinforced by a filter consisting of cloth or by inserting the diaphragm between a fabric such as cotton. Other suitable diaphragm materials are for instance glass fiber webs or fabrics.

Any slow transfer of the anolyte into the cathode chamber through the diaphragm which may possibly still occur can easily be eliminated by maintaining a definite difference in the electrolyte levels. This difference in level is adjusted in such a manner that during the entire process a slow flow from the cathode into the anode space through the diaphragm is maintained or that this difference in level prevents at least a transfer of the anolyte into the cathode space. The diflference in level to be maintained depends upon the pore size of the diaphragm so that with a diaphragm with very fine pores a lower difference in level can be chosen than in the case of a diaphragm with coarse pores. Inasmuch as the total electrical resistance of the cell on the other hand is largely dependent upon the pore size of the diaphragm, the best technical effect consists in a compromise between these opposing influences which can very easily be established by a few experiments.

Alternatively, the reliable separation of anode and cathode space may be obtained by an auxiliary measure in accordance with the invention. With the help of at least one additional diaphragm, a center space for the reception of an electrolyte can be formed between the anode and cathode spaces. An electrolyte, which preferably does not contain any lead, can be conducted through the center space in a slow stream. It absorbs the lead tetraethyl portions transferring over from the anode space and carries them immediately out of the electrolysis device. In this manner, the catholyte can also be reliably protected against the diffusion of any lead compounds. After leaving the electrolytic cell, the electrolyte flowing through the center space may, if necessary, be purified from any lead by addition of traces of finely divided aluminum, suitably from the process itself, and subsequently recycled.

The operation of the anode space during the electrolysis is relatively simple. It is merely necessary to replace the used up proportions of lead by fresh lead. This may, for instance, be readily accomplished by feeding lead rods through an opening provided for this pur pose into the anode space in the same amount in which they are being used up during the electrolysis.

At the cathode, aluminum is deposited during the electrolysis. Without any special precautionary measures, this aluminum would be deposited at the cathode in the form of crystals and might conceivably thus increase the volume of the cathode until it reaches the diaphragm which it may even pierce. It is therefore necessary to prevent this accumulation of large amounts of aluminum crystals at the cathode. Good results are obtained, for example, by the provision of mechanical stripping means such as rods, grids, frames, etc. with cathode and stripping means are moved relatively to each other. Thus, it is possible to maintain the cathode stationary and to move the stripping means or to fix the stripping means and to move the cathode. However, it is equally possible to effect the removal of the aluminum from the cathode by means of hydraulically working stripping means. At the required higher current densities the aluminum is deposited at the cathode in-finely divided form and by selecting an appropriate flow rate of the catholyte, the finely divided aluminum can be separated from the cathode and removed from the electrolysis device by the flowing electrolyte. This hydraulic effect may, if necessary, be enhanced by adding mechanically acting stripping elements, such as small balls of glass or metal to the electrolyte which are then carried along in the circulating stream. During this circulation, these balls strike the loose aluminum powder at the cathode and remove it. These mechanically working stripping elements are separated from the aluminum particles contained in the electrolyte outside the electrolysis device.

Occasional troubles connected with the removal of the aluminum from the cathode which might still occur, can be corrected by cleaning the cathode completely by means of a temporary reversal of the direction of the current and without necessitating any complicated mechanical measures, such as the dismantling of the electrolysis device.

In order to facilitate the removal of the aluminum from the cathode, the cathode is suitably so developed that larger amounts of aluminum can only be deposited at the cathode with difficulty. Thus, continuous metal plates are suitably not used as cathode, but the cathode is preferably constructed in such a manner that it consists of individual metal segments inserted in a carrier plate. According to the invention, the cathode may, for example, consist of metal wires embedded in a frame plate, preferably in oblique direction. The use of these metal wires provides for a particularly high current density in the conductive parts of the cathode thereby increasing the deposition of aluminum in finely divided form. Moreover, the aluminum can accumulate only at these metal wires. If, for instance, mechanical scrapers are used, this finely divided aluminum can be spread over the whole carrier plate. But by appropriate choice of the material of which the carrier plate consists the adherence between aluminum and carrier plate can be so minimized that the aluminum is easily removed therefrom.

Any materials may be used for the construction of cathode plate and the electrolysis device as a whole, which are not electrically conductive and which permit the safe carrying out of the electrolysis with the highly reactive and self-inflammatory electrolytes without giving any chemical change in the materials themselves or in the electrolytes. Particularly suitable materials for the electrolytic cells and the carrier plate of the cathode are laminated press materials of synthetic resins, especially Bakelite (phenol formaldehyde) resins, which may be strengthened with fabrics in order to increase their elasticity. This material permits the easy construction of the individual parts of the cells which have to be very carefully executed. Another suitable material is for example wood impregnated with polyethylene or with high melting paraflines. According to the invention, annealing lacquers of an epoxy resin have proven themselves as particularly suitable protection for those metal parts not designed for current transfer to the electrolyte.

As has already been pointed out, the conductivity of the electrolyte according to the invention increases with increasing temperatures. However, in the process of the invention the conductivity of the electrolyte mixture is restricted by the mentioned limitation of the reaction temperatures to about 70 C. At the reaction temperatures to be used in accordance with the invention, the resistance of the electrolyte is still comparatively high. Inasmuch as the voltage in the electrolysis cell should not exceed 2-10 volts, and preferably not volts for reasons of limiting the consumption of electrical energy and furthermore since for various reasons higher current densities of about 315 amperes/dmF, preferably 5-10 amperes/dm. are preferred (separation of aluminum in form of finely divided powder, restriction of size of cells, etc.) it is clear that one must Work with electrodes which are spaced relatively closely together. Otherwise, it is either necessary to increase the voltage excessively or to reduce the current density too much. According to the invention, a distance between the electrodes of about 1 cm. is particularly suitable. At the reaction temperatures to be used one can then work with a voltage of about 5 volts and a current density of about 5 amperes/dm. It is equally possible to select smaller distances between the electrodes for the electrolysis. In the technical application of the process according to the invention the combination of the largest possible number of individual electrolytic cells in such a manner that they can be simultaneously operated in a simple way is recommended. Practically all devices which permit the simultaneous operation of any desired number of electrolysis cells are thus suitable. spaces are connected with each other and anode spaces are connected with each other in such a manner, that electrolyte liquid from all of the cathode spaces is passed to and recycled back from a common supply and in the same manner electrolyte liquid from all of the anode spaces is passed to and recycled from a common supply. The cooling and if necessary treatment of the electrolyte can be effected at these common supplies. In a fully con tinuous process in which a certain proportion of anolyte and catholyte is continuously withdrawn and passed after appropriate preliminary treatment to the other electrode space, the electrolyte proportions to be exchanged are suitably each taken from a different one of the common supplies, the anolyte freed of tetraethyl lead; the catholyte mixed with the necessary amount of aluminum triethyl and each, subsequently introduced into the other common supply.

In this combination of individual electrolysis cells into one large electrolysis device the parts which require Preferably the cathode 1.2 mechanical movement are suitably moved together for example by a lever system.

An electrolysis device suitable for the technical execution of the process is diagrammatically shown in FIG. 1. This device has the anode 1 and cathode 2 in the form of frame shaped plates which are stacked in alignment side-by-side, separated by the diaphragms 3 and 4. A whole series of alternately positioned anode and cathode plates 1 and 2, each separated by a diaphragm, such as diaphragms 3 and 4, may thus be positioned together allowing a combination of individual electrolysis cells. The plates may be pressed together in position in any desired manner as, for example, in the manner of a filter press, and at each end there may be positioned an end plate such as the end plate 5 made of insulating material as, for example, synthetic resin orthe like. The hollow spaces in the interior of the frames serve to hold the electrolyte liquid and suitable openings or conduits may be provided through the tops and bottoms of the frames for the introduction and withdrawal of the electrolyte. The anode plate is, of course, a lead-containing plate, and the cathode plate may be formed, for example, of an insulating material, such as synthetic resin, or the like having bodies such as wires, or the like, of the conducting metal of the cathode, such as iron, aluminum, copper, brass, or the like, embedded therein. It is also possible to press suitable gaskets or packings of electric insulating material between the individual frame plates, such gaskets serving to prevent leakage of the electrolyte, and to insulate the plates one from another.

As has already been mentioned, the carrying out of the process according to the invention is however not limited to an electrolysis device of this type. Any device may be used which prevents the comingling of anolyte and catholyte.

Inasmuch as the electrolyte used in accordance with the invention is extremely oxygen-sensitive and self-igniting it is preferably to carry out all operations in an inert gas.

The invention will be described in further detail in the following examples which are given by way of illustration and not limitation. In these examples the following compounds are used for the electrolytes:

(a) Sodium aluminum triethyl fluoride (NaAl(C H F) (b) Sodium fluoride aluminum triethyl (1:2 compound) (NaF.2Al(C H (0) Sodium aluminum tetraethyl (NaAl(C H These three compounds are self-igniting and must therefore be handled in an inert atmosphere. The compounds (at) and (b) are obtained by mixing sodium fluoride and 1 mole or 2 moles aluminum triethyl at C. for one hour with stirring. Compound (0) is obtained by mixing aluminum triethyl at C. with metallic sodium in a molar ratio of 4:3 with stirring followed by separation of the deposited aluminum from the melt (cf. A. von Grosse and J. M. Mavity, Journal of Organic Chemistry 5, 1940, page 111).

The electrolysis device used in the examples consists in the simplest case as shown in FIG. 2 merely of a lead plate 5 as anode and an equally large plate 6 consisting of a different metal,-such as iron, aluminum, copper or brass as cathode. Anode and cathode are disposed parallel to each other and spaced 1 to 2 cm. apart. The plates are surrounded by an electrically insulating material corresponding to the plates in shape and form forming for example the container 8. In the middle between the two plates and parallel to them there is a thin diaphragm 7, dividing the container into spaces 9 and M. It is suitably formed of filter paper which is protected on both sides by a fabric consisting of cellulose or glass fibers. In the space M between diaphragm and cathode there is arranged a frame 11 which is covered with electrically insulating stripping wires 12, this frame being moved to and fro when in operation. Anode and cathode spaces have openings through which liquid electrolytes may be introduced from the top and discharged at the bottom. In the case of larger runs several individual cells are suit ably combined as shown in FIG. 1. In that case, anodes and cathodes are utilized on both sides (apart from the first and the last element). The frames may be provided with apertures at the top through which it is possible to move the stripping means in the cathode spaces and to replace the lead plates in the anode spaces. It the lead plates have a thickness of 0.5 to 1 mm. it is easily possible to replenish the lead continuously. It was found to be suitable to use in place of cathodes consisting of massive metal plates equally large cathodes consisting of electrically insulating material in which obliquely disposed metal wires are embedded.

All runs described in Examples 1 to 4 are carried out in an inert atmosphere of nitrogen or argon.

Example 1 The catholyte consists of the compound NaF.2Al C H 3 The anolyte is obtained by mixing 14.85 kg. NaAl(C H (c) 13.87 kg. NaA1(C H F (a) and 40.78 kg. NaF.2Al(C H (b) Anolyte (69.5 kg.) and catholyte (64.8 kg.) are filled into two storage chambers (13 and 14) above the container 8 and heated to 70 C. The anode 5 has a free surface of 25 dm. Anolyte and catholyte are allowed to flow through the anode space and the cathode space respectively of the electrolysis device. The flow rate amounts to approximately 4 liters per minute. The aluminum formed at the cathode 6 is scraped by the mechanically moved stripping means 11 and carried out of the device in the liquid stream. The electrolysis is carried out with an amperage of 200 amperes, or a current density of 8 amperes/dmF. The terminal voltage amounts to 8 volts. The resulting heat due to the current causes an increase in temperature of 3 to 4 C. in the electrolyte efiluent, as compared to the entering electrolyte. The anolyte fiows through the line 15 into a storage vessel 16 from .where it is pumped by the pump 17 through line 18 into the upper storage chamber 13. Upon leaving the electrolysis device the catholyte is first led through line 19 into a cylindrical separating chamber or settling tower 20 where the electrolytically separated aluminum which is carried along in the flowing catholyte is separated. The catholyte freed of the aluminum flows through line 21 into a storage chamber 22 from which it is recycled through line 23 into the upper storage chamber 14 by means of pump 24. The heat loss in the entire device is so considerable that a special cooling of the electrolyte is in general not required. However, if necessary, it may be effected in the upper storage chambers 13 and 14.

At a current passage of 200 amperes, the anolyte is exhausted after 12 hours of electrolysis. The 72.0 kg. anolyte are mixed with 10 liters dry isooctane and cooled to C. Hereby, the anolyte separates into two phases. The lower layer of 9.65 kg. consists of 5.1 kg. lead tetraethyl and 4.55 kg. isooctane. The upper layer is composed as follows:

64.8 kg. NaF.2Al(C H 1.8 kg. Pb(C:, H and 2.5 kg. isooctane Upon addition of isooctane and cooling to 0 C. it is thus possible to separate 75% of the formed lead tetraethyl from the anolyte. The electrolyte which still contains lead is now extracted with 32.4 liters isooctane in a 3-step extraction centrifuge in a countercurrent. Upon completion of the extraction, the electrolyte still contains about 3 g. Pb(C H /65 liters. it is thus practically 14 free of any lead compound. The electrolyte now has the composition NaF.2Al(C H and may be safely used as catholyte for a new electrolysis.

The extract containing 1.8 kg. lead tetraethyl is combined with the lower phase obtained by the cooling of the anolyte. Its content of organo-aluminum compounds is low and amounts to about 600 g. NaF.2Al(C I-I The isooctane used as extraction agent is distilled off in vacuo at a pressure of 70 mm. Hg at 50 C. and can be used for further extractions. The lead tetraethyl is subsequently distilled oil in vacuo at 45 C. and 0.5 mm. Hg; the distillation residue of about 600 g. NaF.2Al(C H is recycled into the next extraction.

At the end of the electrolysis the catholyte has the following composition:

14.84 kg. NaAl(C H 18.50 kg. NaAl(C- H F amounting to a total of 66.15 kg. This mixture is stirred with 3.4 kg. aluminum triethyl; the thus obtained reaction product consists of 14.85 kg.NaAl(C H 2400 amperes/ hr.

Example 2 The process of Example 1 is repeated, the catholyte being again NaF.2Al(C H The anolyte, however, has the following composition:

29.7 kg. NaAl(C H 27.7 2 5)s 12.1 kg. NaF.2Al(C H At a current passage of amperes and a terminal voltage of 6 volts the anolyte is exhausted after 48 hours. The isolation of the lead tetraethyl is effected in the same Way as in Example 1. The lead tetraethyl yield amounts to 14.1 kg. or 98% of the amount calculated for 4800 ampere hr.

Upon completion of the run any liquid electrolyte still present in the separation vessel 20 is siphoned oif from the deposited finely divided aluminum. The aluminum is suspended in 13.5 kg. aluminum triethyl and transferred into a rotating autoclave with a capacity of 50 liters.

A pressure of about 250 atmospheres electrolytic hydrogen is applied and the autoclave heated to C. and rotated. After about 2 to 3 hours the pressure has decreased by 200 atmospheres and remains constant after that. The reaction is finished. After cooling, excess hydrogen is blown off. Subsequently a pressure of 20 to 30 atmospheres ethylene is applied and the autoclave again heated with rotation to 60 to 65 C. The application of ethylene pressure is repeated several times. After three hours the addition of ethylene is finished. After blowing oil the excess ethylene a small sample of aluminum triethyl is taken from the autoclave and mixed with a few drops of lead tetraethyl. If this sample does not become dark in color as a result of the precipitation of lead, then the aluminum triethyl obtained does not contain any diethyl aluminum hydride and can be used for the regeneration of the electrolyte. If the sample still becomes dark colored the treatment with ethylene has to be repeated. There are obtained 20.23 kg. aluminum triethyl, i.e. 6.73 kg. or 100% of the theoretical value have been formed.

15 The 6.73 kg. aluminum triethyl are admixed to the catholyte which has then the following composition:

29.7 kg. NaAl(C H )4 12.1 kg. NaF.2Al(C I-I it is then ready to be used as anolyte during the next electrolysis.

Example 3 Example 1 is repeated using the arrangement shown in FIG. 3. fter passage of 2400 ampere hr., i.e. of 200 amperes during an electrolysis of 12 hours a liquid stream is branched otf from the cathode cycle at 25 at a rate of 5.5 kg./ hr. To this a stream of aluminum triethyl is admixed from container 26 at a rate of 284 g./hr. The mixture is allowed to flow through line 27 into the upper storage tank 13. Simultaneously a liquid stream of 6.0 kg./ hr. is removed from the anode cycle at 28 and cooled to C. in cooler 29. The lower phase formed is separated and subjected to a continuous counter-current extraction in the extraction centrifuge St The electrolyte now free of any lead is led through line 31 to the upper storage chamber 14 for the catholyte. Under these circumstances the electrolysis may be continued as long as desired.

Example 4 If the electrolytically deposited aluminum is not to be used for the production of aluminum triethyl but is to be recovered as refined aluminum, the following modification of the process is possible:

The process is carried out as described in Examples 1 to 3. Upon extraction of the anolyte with isooctane the electrolyte still contains about 40 mg. lead tetraethyl per liter. If this electrolyte is now used as catholyte the separated aluminum will always contain traces of lead. In order to free the electrolyte of the small amounts of lead compound, the anolyte is intensely stirred about half an hour at 70 C. after the extraction with about 80 g. of the electrolytic aluminum. Subsequently the aluminum is allowed to settle. The electrolyte is siphoned off. No lead earl be found in the electrolyte by chemical-analytical determination; this may then be used in the subsequent electrolysis as catholyte. The aluminum used for the precipitation of the last traces of lead can be repeatedly used for the same process. This operating step is suitable also if the aluminum is to be converted again to aluminum triethyl since an aluminum completely free of any lead reacts more quickly with aluminum triethyl and hydrogen.

We claim:

1. In the process for the electrolytic production of tetraethyl lead by passing an electrolysis current between a cathode and lead containing anode through an alumi num triethyl containing electrolyte, the improvement which comprises maintaining a diaphragm between the cathode and anode to thereby form separate cathode and anode spaces and at least initially maintaining as the electrolyte in said anode space an anolyte essentially consisting of NaAl(C H and NaF.Al(C H said NaAl(C H and NaF.1 il(C H being present in substantially equivalent amounts, said diaphragm substantially preventing migration of electrolyte between the cathode and anode spaces without substantially interfering with the passage of said current.

2. Improvement according to claim 1 in which said anolyte additionally contains NaF.2Al(C H 3. Improvement according to claim 2 in which NaAl(C H and NaF.Al(C H are present in substantially equivalent amounts and in which about 0.16 to 1.70 moles of NaF. 2Al(C H are present per mole of NaF.Al(C H 4. Improvement according to claim 3 in which not more than about 33 /366 /s ampere-hours/kg. electrolyte are used for the electrolysis.

5. Improvement according to claim 4 in which about 0.671.70 moles of NaF.2Al(C I-I are present per mole of NaF.Al(C H and in which the electrolysis is effected with about 50 ampere-hours/kg. electrolyte.

6. Improvement according to claim 1 in which the electrolysis is continued until one of said NaAl(C H and NaF.Al(C H has been substantially completely exhausted.

7. Improvement according to claim 1 which includes substantially continuously removing anolyte from said anode space and substantially continuously replacing the removed anolyte with fresh anolyte.

8. Improvement according to claim 1 which includes maintaining as the electrolyte in said cathode space a catholyte comprising NaF.2Al(C H I v V 9. Improvement according to claim 1 which includes at least initially maintaining as the electrolyte in said cathode space a catholyte essentially consisting of a mixtlll'fi 'Of and NaF.Al(C H 10. Improvement according to claim 1 in which an aluminum triethyl containing electrolyte is maintained in said cathode space as catholyte and which includes after at least partial exhaustion thereof regenerating the catholyte by the addition of an amount of aluminum triethyl substantially equivalent to the amount of aluminum deposited at the cathode.

11. Improvement according to claim 1 which includes removing exhausted anolyte from said anode space, freeing the same of tetraethyl lead and recycling the same to said cathode space .as catholyte.

12. Improvement according to claim 1 which includes substantially continuously removing anolyte from said anode space passing the same to a body of excess anolyte and recycling anolyte from said body back to staid anode space and which includes substantially continuously removing catholyte from said cathode space passing the same to a body of excess catholyte and substantially continuously recycling catholyte from said body back to said cathode space.

13. Improvement according to claim 1 in which the catholyte maintained in the cathode space is a sodium aluminum tetraethyl-sodium fluoride aluminum triethyl containing catholyte and which includes substantially continuously removing catholyte from the cathode space, and mixing an amount of aluminum triethyl therewith substantially equivalent to the amount of aluminum separated therefrom during the electrolysis and thereafter passing the removed catholyte with the added aluminum triethyl to the anode space and which includes substantially continuously removing a corresponding amount of anolyte from said anode space freeing the same of tetraethyl lead and passing the same into the cathode space.

14. Improvement according to claim 1 which includes removing formed tetraethyl lead from the anolyte by cooing to a temperature of at least about 0 C.

15. Improvement according to claim 1 which includes removing tetraethyl lead formed during the electrolysis from the anolyte by cooling to a temperature of at least 0 C. followed by extraction with an extraction agent.

16. Improvement according to claim 15 in which said extraction agent is isooctane.

17. Improvement according to claim 16 which includes maintaining an amount of isooctane up to the saturation quantity thereof in the anolyte during said electrolysis.

18. Improvement according to claim 1 in which the electrolyte maintained in the cathode space is an aluminum triethyl containing catholyte, and which includes stripping deposited aluminum from the cathode and substantially continuously removing catholyte from the cathode space carrying therewith entrained, the stripped aluminum.

19. Improvement according to claim 1 in which said l c y is is e fected under an inert gasatmosphere.

20. Improvement according to claim 1 in which said References Cited in the file of this patent electrolysis is effected with a current density of between UNITED STATES PATENTS about 3 to 15 amp./dm.

21. Improvement according to claim 1 in which s i 733 315 Hgopes Apr 25 1905 anode and cathode are frame-shaped plates pos tioned 5 2 3 3 3 7 Blcck N0 21 194 side by side in alignment with said diaphragm sand 2 737 4 Bodamm- Man 6, 5 Wiched therebetween- 2,844,615 Ziegler et al. July 22, 1958 22. Improvement according t0 claim 21, 111 which 2 49 349 Ziegler et aL Aug. 2 19 said frame plates are at least partially formed of fiber 2 3 3 394 smith Dec. 9 195g reinforced synthetic resin.

Claims (1)

1. IN THE PROCESS FOR THE ELECTROLYTIC PRODUCTION OF TETRAETHYL LEAD BY PASSING AN ELECTROLYSIS CURRENT BETWEEN A CATHODE AND LEAD CONTAINING ANODE THROUGH AN ALUMINUM TRIETHYL CONTAINING ELECTROLYTE, THE IMPROVEMENT WHICH COMPRISES MAINTAINING A DIAPHRAGM BETWEEN THE CATHODE AND ANODE TO THEREBY FORM SEPARATE CATHODE AND ANODE SPACES AN AT LEAST INITIALLY MAINTAINING AS THE ELECTROLYTE IN SAID ANODE SPACE AN ANOLYTE ESSENTIALLY CONSISTING OF NAAI(C2H5)4 AND NAF.AI(C2H5)3, SAID NAAI(C2H5)4 AND NAF.A1(C2H5)3 BEING PRESENT IN SUBSTANTIALLY EQUIVALENT AMOUNTS, SAID DIAPHRAMG SUBSTANTIALLY PREVENTING MIGRATION OF ELECTROLYTE BETWEEN THE CATHODE AND ANODE SPACE WITHOUT SUBSTANTIALLY INTERFERING WITH THE PASSAGE OF SAID CURRENT.

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