imidazolinone compounds such as those described in u . s . pat . no . 4 , 188 , 487 , no . 4 , 798 , 619 and no . 5 , 334 , 576 are highly potent , broad spectrum , environmentally benign , herbicidal agents . chiral imidazolinone compounds having the ( r ) configuration demonstrate an increase in herbicidal activity over the corresponding racemic mixture . the preparation of said chiral compounds by the resolution of racemic 2 - amino - 2 , 3 - dimethylbutyronitrile , hydrolysis of the resultant ( r ) 2 - amino - 2 , 3 - dimethyl - butyronitrile to the corresponding ( r ) 2 - amino - 2 , 3 - dimethylbutyramide intermediate , and subsequent elaboration of this intermediate to the ( r ) imidazolinone herbicidal product is described in u . s . pat . no . 4 , 683 , 324 . the preparation of chiral imidazolinone compounds having substantially complete retention of enantiomeric purity directly from the ( r ) aminonitrile starting material to the final chiral imidazolinone herbicidal product is described in co - pending patent application ser . no . 09 / 304 , 401 , filed on may 3 , 1999 . in general , the two - step resolution described comprises a first resolution step in which racemic 2 - amino - 2 , 3 - dimethylbutyronitrile ( ii ) in c 1 - c 4 alkanol is treated with d - tartaric acid ( i ) to afford the d - tartrate salt of ( r ) 2 - amino - 2 , 4 - dimethylbutyronitrile ( iii ), which crystallizes from solution . because said aminonitrile partially decomposes in the process of this kinetic resolution , the methanol mother liquor contains varying amounts of ammonium d - tartrate ( iv ), which is ordinarily discarded . this first resolution step is shown in flow diagram i wherein the c 1 - c 4 alkanol is methanol . in the second resolution step , ( r ) 2 - amino - 2 , 3 - dimethylbutyronitrile ( vi ) is liberated from its d - tartrate salt ( iii ) by treatment with an alkali metal hydroxide in the presence of a minimum amount of water and a water - immiscible solvent , such as toluene . this second resolution step yields an aqueous phase containing a full equivalent of the di ( alkali metal ) salt of d - tartaric acid ( v ), which is also ordinarily discarded . this second resolution step is illustrated in flow diagram ii wherein m is an alkali metal and the water immiscible solvent is toluene . thus both resolution steps give rise to waste streams containing d - tartrate salts . although l - tartaric acid is natural tartaric acid which occurs widely in nature , either as the free acid or in combination with potassium , calcium or magnesium , d - tartaric acid does not occur widely in nature and is commercially available only in limited quantities . further , existing methods for producing d - tartaric acid are limited . surprisingly , it has now been found that d - tartaric acid may be recovered in high yield and in essentially enantiomerically pure form from the waste streams produced in the resolution of racemic 2 - amino - 2 , 3 - dimethylbutyronitrile . advantageously , the recovered d - tartaric acid may be recycled for use in the same resolution of said aminonitrile . beneficially , the processes of this invention may be run repetitively , i . e ., d - tartaric acid may be repeatedly recovered and recycled in a continuous resolution of racemic 2 - amino - 2 , 3 - dimethylbutyronitrile , allowing for a sustainable resolution process . in accordance with the process of the invention the di ( alkali metal ) d - tartrate or ammonium d - tartrate waste streams produced in the resolution of the above - said aminonitrile are acidified to a ph of about 2 . 5 to 4 . 5 , preferably 3 . 0 to 4 . 0 , most preferably about 3 . 0 . the acidification is preferably conducted with hydrochloric or sulfuric acid , to form the crystalline mono - basic hydrogen d - tartrate ( vii ) and said hydrogen d - tartrate is treated with at least one molar equivalent of an acid , optionally in the presence of a solvent , preferably an aliphatic alkanol , more preferably methanol or ethanol , to give essentially enantiomerically pure d - tartarc acid ( i ). the process of the invention is illustrated in flow diagram iii wherein m is an alkali metal . the recovered d - tartaric acid ( i ) may then be utilized directly in the first resolution step by adding the recovered d - tartaric acid to a solution of racemic 2 - amino - 2 , 4 - dimethylbutyronitrile in a water - immiscible solvent , such as toluene , to yield the corresponding d - tartrate salt ( iii ) as shown hereinabove in flow diagram i . it is also intended that the processes of this invention embrace the recovery and recycle of l - tartaric acid in a resolution of racemic 2 - amino - 2 , 3 - dimethyl - butyronitrile to produce ( s ) 2 - amino - 2 , 3 - dimethylbutyro - nitrile , such as that described in u . s . pat . no . 4 , 683 , 324 . acids suitable for use in the process of the invention include mineral acids such as hydrogen halides , sulfuric acid , phosphoric acid , or the like , preferably hydrochloric acid or sulfuric acid . solvents suitable for use in the inventive process include polar solvents , preferably water miscible . preferable solvents include aliphatic alkanols such as methanol , ethanol , propanol , isopropanol , or the like , preferably methanol or ethanol , more preferably ethanol . in general , reaction temperatures for the inventive process are directly related to reaction rate , that is increased reaction temperature leads to increased reaction rate . however , excessively high reaction temperatures are to be avoided . suitable reaction temperatures may be about 0 ° c . to 50 ° c ., preferably about 5 ° c . to 35 ° c ., more preferably about 10 ° to 30 ° c . in actual practice , waste streams from a 2 - step resolution of racemic 2 - amino - 2 , 3 - dimethylbutyronitrile , combined or individually , are acidified to a ph of about 3 to form crystalline alkali metal hydrogen d - tartrate and said hydrogen d - tartrate is reacted with at least one molar equivalent of acid , preferably hydrochloric acid or sulfuric acid , optionally in the presence of a solvent , preferably an aliphatic alkanol , more preferably methanol or ethanol , to give the desired essentially enantiomerically pure d - tartartic acid . advantageously , the crystalline alkali metal hydrogen d - tartrate may be isolated using conventional means such as filtration or , alternatively , may be carried on in the inventive process as is or as a concentrated slurry . similarly , the recovered d - tartaric acid may be isolated using conventional techniques or recycled as is or as a concentrated slurry . in order to facilitate a further understanding of the invention , the following examples are presented primarily for the purpose of illustrating certain more specific details thereof . the invention is not to be deemed limited thereby except as defined in the claims . hplc designates high performance liquid chromatography . unless otherwise indicated , all parts are parts by weight . a mixture of ( r )- 2 - amino - 2 , 3 - dimethylbutyronitrile ( 2s , 3s )- tartaric acid salt ( 89 . 9 g , 0 . 34 mmole ), toluene , ice and 50 % sodium hydroxide ( 68 . 5 g , 0 . 85 mmol ) is shaken until no solid particles are observed . the phases are separated and the aqueous disodium tartrate waste produced ( 343 . 3 g , 15 wt % d - tartaric acid ) is acidified to a ph of about 3 with concentrated hydrochloric acid over a 30 minute period at 8 - 13 ° c ., stirred for 15 minutes and filtered . the filtercake is washed with methanol and dried in vacuo to afford recovered d - sodium hydrogen tartrate as an off - white solid 53 . 2 g ( 91 % recovery ) in & gt ; 99 % purity and 100 % optical purity as determined by hplc analysis . using essentially the same procedure and employing concentrated sulfuric acid in place of concentrated hydrochloric acid , recovered d - sodium hydrogen tartrate is obtained as an off - white solid ( 279 g , 77 % recovery ) & gt ; 99 % purity and 100 % optical purity as determined by hplc analysis . recovery of d - sodium hydrogen tartrate from combined aqueous disodium tartrate and methanolic ammonium tartrate waste a mixture of aqueous disodium tartrate waste produced as described in example 1 ( 438 . 5 g , 14 . 5 wt % d - tartaric acid ) and a methanolic mother liquor waste slurry of ammonium tartrate ( 278 g , 2 . 6 wt % d - tartaric acid ) produced as described in u . s . pat . no . 4 , 683 , 324 is acidified to a ph of about 3 with concentrated hydrochloric acid over a 30 minute period at room temperature , stirred for 25 minutes and filtered . the filtercake is washed with methanol and dried in vacuo to afford recovered d - sodium hydrogen tartrate as an off - white solid , 73 . 7 g , ( 92 % recovery ) in & gt ; 99 % purity and 100 % optical purity as determined by hplc analysis . a slurry of recovered d - sodium hydrogen tartrate ( 20 . 6 g , 0 . 119 mol ) in methanol is treated with concentrated sulfuric acid ( 6 . 1 g , 0 . 059 mol ) at room temperature , stirred for one hour and filtered to remove inorganic salts . a portion of the filtrate is concentrated in vacuo to give a solution of recovered d - tartaric acid ( 13 . 6 g , 0 . 090 mol , 82 %) in methanol . this solution is treated with a solution of racemic - 2 - amino - 2 , 3 - dimethylbutyronitrile ( 12 . 4 g , 0 . 11 mol ) in toluene , stirred for 16 hours and filtered . the filtercake is washed with methanol and dried to afford ( r )- 2 - amino - 2 , 3 - dimethylbutyronitrile ( 2s , 3s )- tartrate , 22 . 3 g , ( 61 % yield ) 95 / 5 r / s isomer ratio as determined by hplc analysis . preparation of crystalline d - tartaric acid from recovered d - sodium hydrogen tartrate in ethanol a slurry of recovered d - sodium hydrogen tartrate ( 50 . 5 g , 0 . 293 mol ) in ethanol is treated with concentrated sulfuric acid ( 15 . 0 g , 0 . 147 mol ) at room temperature , stirred for 45 minutes and filtered to remove inorganic salts . the filtrate is concentrated in vacuo to afford a concentrated slurry of d - tartaric acid in ethanol . the slurry is diluted with toluene and filtered . the filtercake is washed with toluene and dried to afford d - tartaric acid as a crystalline solid , 37 . 3 g , ( 83 % yield ) 97 . 7 % purity 100 % optical purity as determined by hplc analysis . aqueous dipotassium tartrate waste ( 137 . 0 , 14 . 7 % wt % d - tartaric acid ) is acidified to a ph of about 3 with concentrated hydrochloric acid over a 30 minute period at 13 - 28 ° c ., stirred for about 15 minutes and filtered . the filtercake is washed with methanol and dried in vacuo to afford recovered d - potassium hydrogen tartrate 24 , 8 g , ( 98 % recovery ) in & gt ; 99 % purity as determined by hplc analysis . preparation of crystalline d - tartaric acid from recovered d - potassium hydrogen tartrate in ethanol a slurry of recovered d - potassium hydrogen tartrate ( 22 . 1 g , 0 . 117 mol ) in ethanol ( 148 g ) is treated with concentrated sulfuric acid ( 6 . 0 g , 0 . 0588 mol ) at room temperature , stirred for 45 minutes and filtered to remove inorganic salts . the filtrate is concentrated in vacuo to a viscous slurry . the slurry is diluted with acetonitrile and filtered . the filtercake is washed with acetonitrile and dried to afford crystalline d - tartaric acid , 4 . 7 g , ( 25 % recovery ), 92 . 3 % purity as determined by hplc analysis .