US2760995A - Refinement of benzene hexachloride - Google Patents

Description

REFINENIENT OF BENZENE HEXACHLORIDE Application September 28, 1951, Serial No. 248,866

No Drawing.

6 Claims.

This invention relates to improvements in methods of treating crude benzene hexachloride, and more particularly to a new and improved continuous process for the isolation of a product containing an enhanced proportion of the gamma isomer.

Benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane), as is well known, is prepared by the chlorination of benzene under conditions that favor addition, 'but not substitution, reactions. The product so obtained comprises a mixture of stereoisomers, of which five, designated as alpha, beta, gamma, delta and epsilonthe alpha isomer being present in preponderant amount-have been isolated and characterized. Of these only the gamma isomer, M. P. 113 C., possesses a significant amount of insecticidal activity. Pure gamma benzene hexachloride, known as lindane is, therefore, an important article of commerce and processes for the isolation of products containing enhanced proportions of gamma benzene hexachloride have been the subject of recent intensive investigation. Several methods based upon relative solubilities of the isomers have been heretofore used for the isolation of such products. Since, in most solvents, the solubility of the gamma isomer is intermediate between the solubrhties of the other isomers, extraction techniques, wherein the solid mixture of benzene hexachloride isomers, hereafter designated as crude benzene hexachloride, is contacted with a solvent having a preferential solubility for the gamma over the alpha isomer, in proportions such that only a part of the crude benzene hexachloride is dissolved, have been most extensively investigated. Among the solvents used for this purpose are included lower aliphatic alcohols as described in U. S. Patent 2,553,956.

Thus, according to the teaching of the above patent, crude benzene hexachloride is treated with a solvent, e. g. methanol, in proportions such that substantially all the gamma benzene hexachloride, but not all the alpha and beta benzene hexachloride, is dissolved. The solution thereby formed is saturated with respect to the alpha and beta isomers, but contains less than the saturation amounts of the gamma, delta and epsilon isomers, together with small amounts of by-products, such as heptachlorocyclohexanes, which are formed in the chlorination reaction. The extract solution is separated from the undissolved solids, and a high-gamma fraction isolated and recovered by fractional crystallization, following which the highgamma fraction is separated from its mother liquor by conventional solid-liquid separation techniques.

Use of the foregoing procedure, it is true, results in isolation of a product contained an enhanced proportion of the gamma isomer. However, the yield of the highfraction, and the gamma content thereof, while at reasonably high levels, fall far short of those theoretically possible.

An important object of our 1nvent1on, therefore, 1s to provide a method for the refinement of benzene hexachloride in which a very high yield of a product extremely rich in gamma benzene hexachloride is obtained. An-

2,760,995 Patented Aug. 28, 1956 other object is to provide an efiicient continuous process for the concentration of gamma benzene hexachloride, characterized by a high degree of recovery of gamma isomer. Still another object is to provide a method for gamma concentration in which gamma isomer otherwise lost in a discard stream is recovered. Further objects will be apparent from the following description.

We have made the surprising discovery that when a portion, but not all, of the mother liquor from the crystallization step, as described above, is recycled to the partial dissolution or extraction step, as described above, we obtain in our crystallized product an enhancement in gamma content, and particularly a marked enhancement in recovery of gamma isomer. The copending application Serial No. 353,081 of George Calingaert (a division of his application Serial No. 135,022), now abandoned, discloses recycling of all this mother liquor and claims recycling thereof generally. Our present invention is concerned with recycling 2. portiononly of such mother liquor.

When our partial recycle is employed the proportion of gamma benzene hexachloride in the extract solution is lower than when either no recycle or complete recycle is used. In spite of this, yields and purity of the high gamma fraction obtained with partial recycle are markedly higher than those obtained with either no recycle or complete recycle.

The benefits of our invention are graphically illustrated in the following three examples in which results we obtain our contrasted with results obtained without recycle and with complete recycle of the crystallization filtrate. Example i shows results when our process is employed. All parts and percentages mentioned are on the weight basis.

Example I To an extraction vessel equipped with a mechanical stirrer, crude benzene hexachloride containing 12.2 per cent of gamma benzene hexachloride was fed continuously at a rate of 100 parts per hour. Simultaneously, methanol was fed to the extraction vessel at the rate of 300 parts per hour. Vigorous agitation was maintained, and the temperature was held by external cooling at 15 C. The extraction vessel was sized so that the average residence time of the mixture therein was approximately two hours. Under these conditions the two-hour period was sufiicient to achieve equilibrium distribution of isomers between the liquid and solid phases, as determined by infrared analyses of samples of the solution at varying time intervals from the time of initial contacting of benzene hexa chloride and methanol. A slurry of undissolved benzene hexachloride and methanol solution was continuously withdrawn from the extraction vessel and sent to one of two centrifuges mounted in parallel. In the centrifuge, separation of the liquid phase from the solid phase was accomplished. The centrifugate was sent to a vaporizer and the solid cake removed from the centrifuge. During the removal of the cake the slurry was sent to the alternate centrifuge to maintain continuous operation. After distillation of 60 per cent of the methanol from the solution, the residue was transferred to a crystallizer and cooled therein so that the temperature after two hours was 15 C. The slurry of crystals and mother liquor produced in the crystallizer was sent to one of a second pair of centrifuges operating in tandem. The solid cake on this centrifuge comprised the high-gamma fraction. The centrifugate from this second centrifugation was split into two portions, one comprising about per cent of the centrifugate and the other about 25 per cent. The larger portion was continuously recycled to the extraction vessel, and the smaller portion discarded. When equilibrium had been reached, as evidenced by constancy or isomer distribution in all liquid streams, determined by inethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, equally beneficial results are obtained.

The following two examples, in contrast, demonstrate '7 the results we obtainwith no recycle of the mother liquor,

and results obtained when -we recycle all the mother liquor, respectively.

Example .11

Crude benzene hexachloride containing 12.5 per cent of gamma'benzene hexachloride was processed in a manner identical with that in Example I, except that the centrifugate from the second centrifugation was completely discarded, and was'not recycled to the extraction vessel. By this -procedure,-only '37 percent of the gamma benzene'hexachloride present'in the crude charge was recovered, and the gamma content of the high-gamma fraction was only 73 per cent.

Example III Crude benzene hexachloride containing 13.6 per cent gamma benzene hexachloride wasprocessed in a manner identical with that in Example I, except that the centrifugate from the second centrifugation was com pletely recycled, and none was discarded. The only removal of methanol from the system was that accomplished in'the-vaporizer. aOnly 49 per cent of the gamma benzene hexachloride. presentin thecrnde benzene hexachloride charge was recovered, and the gammacontent of the high-gamma fraction was only 76 per cent.

From the above examples it can readily be seen that use of our invention results notonly in enchancement of the gamma content of the high-gamma product but particularly in a marked enhancement of the yield of gamma isomer. In the ordinary processonly 37 per cent of ,the gammaisomer is recovered whereas 63 per cent, or almost two-thirds,.of this valuable insecticide is discarded and lostoutright. 'When our process is employed 83 per cent, or more than twice as much, of the gamma isomer is recovered in a marketable condition. Even with-complete recycleof the filtrate more than half of the gammadsomer isdiscarded. 'On this basis itcan readily be .seen that our process-can lead to significant increases in the capacity: of lindane'manufacturing equipment; that is, withthe same equipment wecan turn out more than twice as much lindane or high-gamma benzene hexachloride'as can those employing theold method.

As mentioned above,,in most solvents the. delta benzene hexachloride isomer is more soluble than the alpha and gamma isomers. This is .true in the case of methanol. -When theconcentration of delta benzene'hexachloride in methanol solution builds upimproved results in terms of yieldand purity of the high-gamma fraction are obtained. However, if thedelta concentration continues to build up to too high concentrations then yields and purity of the high-gamma fraction begin to fall off. The reason-for this is not completely understood and we cannot state quantitatively .these concentration limits of delta benzene hexachloride. This is because analytical procedures for determining the concentration of any particular benzene hexachlorideisomer in solution are complicated by thezpresenee of other isomers and other impurities which are present .in our mixtures. We have found, however, that if v we discard enough of the recycle strearmto make the :amountofdelta isomer leaving the systemequal to the amount entering thesystem,thus preventing .any buildup vabove the optimum range of delta con en a ion, w obtain zenchanc yields 10f high gam- .between 0-per. cent and aboutper cent.

ma material. In fact, we find that we obtain some benefit'as long as we recycle any portion of the filtrate so long as some of the filtrate is discarded. The amount discarded can be very small relative to the amount recycled. For example, we obtain good results when only about 5 per cent of our recycle stream is discarded. Generally speaking, we prefer to discard 15 to 60 per cent of our recycle stream'because within this range, depending up the delta content of the original crude benzene hexachloride, upon the extraction and crystallization temperatures, and upon the ratio of methanol to benzene hexachloride in the extraction step, build-up of deltaisomer in the extract solutionis avoided.

7 An important variable in our process is the ratio of alcohol to crude benzene hexachloride in our extractor. In general, we can operate with a rather wide range of ratios of alcohol to benzene hexachloride. For example, we can use methanol in amounts just insufiicient to completely dissolve all-the benzene'hexachloride present. On the other hand, we can use ratios of methanol to-benzene 'hexachloride so low as to just permitliquid-solid separation to be made.

The preferred range of ratios of alcohol to benzene hexachloride depends on the temperature of extraction. In our preferred extraction temperature range, as discussed below, we prefer to'use ratios of methanol to freshly'fed benzenehex-achloride of about 2.6/1 to about 5.0/1 and in addition we-prefer that the ratio of methanol-to all-benzene hexachloride present-in the extraction step be approximately =1.5/1 to 2.5/1. Similar ratios are preferred when other alcohols are employed.

The benefits-of ourinvention can berealized with-extractiontemperatures ranging from about 0 C. to as high as the refiux temperature of thesolution employed.

'Howevenfor maximum yields and recoveries as well as the most economic operation-weprefer to extract at temperatures ranging from about 10 C. to about 35 C. It

is-w-ithin thispreferred temperatureregionthatwe utilize our prefered ratios of methanolto benzene hexachloride anol with about 3 per cent water. Under certain operating-conditions-we-obtain increased benefits when use of-this solventm-ixture is combined with our partial recycle.

The benefits of ourinvention are obtained when weremove by evaporation a portion of the solvent varying However, for best -yields and-recoveries We prefer to evaporate from about 30'per cent to'about 80 per cent of the-methanol prior to crystallization.

-Wecrystallize our highgammafraction from this concentrated solution at temperatures ranging from about 20C. to about60'C. We prefer to crystallize in the temperature range lying between about 0 C. and 40 C. since this temperature-range enables us toobtain our best yields and purities of'thehigh-gamrna fraction and in addition-permitsthe most economical modeof operation.

From the foregoing, we have found that the amount of the-recycle stream to be discarded can be expressed as :a-function of thespe'cificgravity of the recycle solution according :to the following equation:

where -.y equals parts of recycle solution discarded per unittimeper partof fresh benzene hexachloride charged perunitrtime and X equals the specific gravity of the recycle solution. .Useof this equation permits us to adjustourrecycle ratio to counteract natural fluctuations in our operation and permits us to discard the recycle .streamin proportions such that the delta isomer is discardedrasvfastastit isbeing introduced intothe system.

The followingexample illustrates .thedegree of control and the purity :and vyield of product we obtain by use of our preferred conditions together with the above equation.

Example IV Crude benzene hexachloride containing 13.1 per cent of gamma benezene hexachloride was processed in a manner identical with that of Example I, except that the specific gravity of the centrifugate from the second centrifuge was continuously determined by a conventional hydrometer, and the amount of centrifugate recycled to the extraction vessel continuously adjusted according to the equation:

where y equals parts of recycle solution discarded per unit time per part of fresh crude benzene hexachloride charged per unit time, and X equals the specific gravity of the recycle solution. During this operation the spe' cific gravity of the recycle solution varied between the limit of about 0.990 and 1.080. However, by continually adjusting the recycle flow according to the above equation, operation was carried out smoothly, with optimum results. Sixty-seven per cent of the gamma benzene hexachloride charged in the feed was recovered as a 90 per cent concentrate.

We employ any of the customary means of solid-liquid separation, such as filtration, centrifugation, decantation, etc. In general, centrifugation is preferred, giving rise to the most clean-cut separation of liquids from solids.

We claim:

1. In a process for the production of a product containing an enhanced proportion of gamma benzene hexachloride by partialy dissolving crude benzene hexachloride in a lower aliphatic alcohol, separating the therebyformed solution from the undissolved solids, and crystallizing a product containing an enhanced proportion of gamma benzene hexachloride from said solution, the improvement comprising recycling a portion of the mother liquor from said crystallization step to said partial dissolution step, the portion recycled having essentially the same composition as the Whole quantity of the mother liquor.

2. The process of claim 1 in which the lower aliphatic alcohol is methanol.

3. In a process for the production of a product containing an enhanced proportion of gamma benzene hexachloride by partially dissolving crude benzene hexachloride in a lower aliphatic alcohol, separating the therebyformed solution from the undissolved solids, and crystallizing a product containing an enhanced proportion of the gamma isomer from said solution, the improvement comprising recycling from about 40 per cent to about per cent of the mother liquor from said crystallization step to said partial dissolution step, the portion recycled having essentially the same composition as the whole quantity of the mother liquor.

4. In a process for the production of a product containing an enhanced proportion of gamma benzene hexachloride by partially dissolving crude benzene hexachloride in a lower aliphatic alcohol, separating the therebyformed solution from the undissolved solids, and crystallizing a product containing an enhanced proportion of gamma benzene hexachloride from said solution, the improvement comprising recycling a portion of the mother liquor from said crystallization step to said partial dissolution step, the portion of mother liquor to be recycled being determined by the equation:

where y equals parts of mother liquor discarded per unit time per part of crude benzene hexachloride fed per unit time, and X equals the specific gravity of the mother liquor, the portion recycled having essentially the same composition as the whole quantity of the mother liquor.

5. In a process for the production of a product containing an enhanced proportion of gamma benzene hexachloride by partially dissolving crude benzene hexachloride in an aliphatic alcohol having from 1 to 4 carbon atoms, separating the thereby-formed solution from the undissolved solids, and crystallizing a product containing an enhanced proportion of gamma benzene hexachloride from said solution, the improvement comprising recycling a portion of the mother liquor from said crystallization step to said partial dissolution step, the portion recycled having the same composition as the whole quantity of the mother liquor.

6. The process of claim 1 wherein up to percent of the mother liquor is recycled.

References Cited in the file of this patent UNITED STATES PATENTS 2,486,688 Thomas Nov. 1, 1949 2,585,898 Kauer Feb. 12, 1952 FOREIGN PATENTS 491,132 Belgium Mar. 16, 1950

Claims (1)

1. IN A PROCESS FOR THE PRODUCTION OF A PRODUCT CONTAINING AN ENHANCED PORPORTION OF GAMMA BENZENE HEXACHLORIDE BY PARTIALY DISSOLVING CRUDE BENZENE HEXACHLORIDE IN A LOWER ALIPHATIC ALOCHOL, SEPARATING THE THEREBYFORMED SOLUTION FROM THE UNDISSOLVED SOLIDS, AND CRYSTALLIZING A PRODUCT CONTAINING AN ENHANCED PROPORTION OF PROVEMENT COMPRISING RECYCLING A PORTION OF THE MOTHER PROVEMENT COMPRISING RECYCLING A PORTION OF THE MOTHER LIQUOR FROM SAID CRYSTALLIZATION STEP TO SAID PARTIAL DISSOLUTION STEP, THE PORTION RECYCLED HAVING ESSENTIALLY THE SAME COMPOSITION AS THE WHOLE QUANTITY OF THE MOTHER LIQUOR.