A process for producing a carboxylic acid composition, said process comprising: oxidizing in a primary oxidation zone, at least one oxidizable compound comprising at least one selected from the group consisting of 5-(hydroxymethyl)furfural (5-HMF), 5-(chloromethyl)furfural (5-CMF), 2,5-dimethylfuran (2,5 -DMF), 5-HMF esters (5-R OCH 2 -furfural where R = alkyl, cycloalkyl and aryl), 5-HMF ethers (5-R'OCH 2 -furfural, where R' = alkyl, cycloalkyl and aryl), 5-alkyl furfurals (5-R"-furfural, where R" = alkyl, cycloalkyl and aryl), alkylcarboxylate of methylfuran (5-R′′′O -methylfuran, where R′′′ = alkyl, cycloalkyl), alkylcarboxylate of alkoxyfuran (5-RʺʺO -OR′′′ʺfuran, where Rʺʺ = alkyl, cycloalkyl and aryl and R′′′ʺ = alkyl, cycloalkyl and aryl), mixed feed-stocks of 5-HMF and 5-HMF esters and mixed feed-stocks of 5-HMF and 5-HMF ethers, mixed feed-stocks of 5-HMF and 5-alkyl furfurals, mixed feed-stocks of 5-HMF and 5-CMF, mixed feed-stocks of 5-HMF and 2,5-DMF, mixed feed-stocks of 5-HMF and alkylcarboxylate of methylfuran mixed feed-stocks of 5-HMF and alkylcarboxylate of alkoxyfuran, in the presence of a solvent stream, an oxidizing gas, and a catalyst system at a temperature of about 110°C to about 220°C to produce said carboxylic acid composition comprising furandicarboxylic acid at a yield of 80% or more; wherein said primary oxidation zone comprises at least one oxidation reactor, wherein said at least one oxidation reactor in said oxidation zone has been previously used in an terephthalic acid and/or isophthalic acid process; generating a cooled crude carboxylic acid slurry stream; isolating, washing, and dewatering solids present in the cooled crude carboxylic acid slurry stream in a solid-liquid separation zone to generate (i) a crude carboxylic acid wet cake stream comprising FDCA and (ii) a mother liquor stream comprising oxidation solvent, catalyst, and impurities, wherein from 5 wt.% to 95 wt.% of mother liquor present in the crude carboxylic acid slurry is isolated in the solid-liquid separation zone to generate the mother liquor stream, and wherein the solid-liquid separation zone comprises at least one solid-liquid separation device capable of separating solids and liquids, washing solids with a wash solvent stream, and reducing the % moisture in the washed solids to less than 30 wt.%; routing a portion of the mother liquor stream to a mother liquor purge zone, wherein a portion is at least 5 wt.%, and wherein the mother liquor purge zone comprises an evaporative step to separate oxidation solvent from the mother liquor stream by evaporation. oxidizing in a primary oxidation zone, at least one oxidizable compound comprising at least one selected from the group consisting of 5-(hydroxymethyl)furfural (5-HMF), 5-(chloromethyl)furfural (5-CMF), 2,5-dimethylfuran (2,5 -DMF), 5-HMF esters (5-R OCH 2 -furfural where R = alkyl, cycloalkyl and aryl), 5-HMF ethers (5-R'OCH 2 -furfural, where R' = alkyl, cycloalkyl and aryl), 5-alkyl furfurals (5-R"-furfural, where R" = alkyl, cycloalkyl and aryl), alkylcarboxylate of methylfuran (5-R′′′O -methylfuran, where R′′′ = alkyl, cycloalkyl), alkylcarboxylate of alkoxyfuran (5-RʺʺO -OR′′′ʺfuran, where Rʺʺ = alkyl, cycloalkyl and aryl and R′′′ʺ = alkyl, cycloalkyl and aryl), mixed feed-stocks of 5-HMF and 5-HMF esters and mixed feed-stocks of 5-HMF and 5-HMF ethers, mixed feed-stocks of 5-HMF and 5-alkyl furfurals, mixed feed-stocks of 5-HMF and 5-CMF, mixed feed-stocks of 5-HMF and 2,5-DMF, mixed feed-stocks of 5-HMF and alkylcarboxylate of methylfuran mixed feed-stocks of 5-HMF and alkylcarboxylate of alkoxyfuran, in the presence of a solvent stream, an oxidizing gas, and a catalyst system at a temperature of about 110°C to about 220°C to produce said carboxylic acid composition comprising furandicarboxylic acid at a yield of 80% or more; wherein said primary oxidation zone comprises at least one oxidation reactor, wherein said at least one oxidation reactor in said oxidation zone has been previously used in an terephthalic acid and/or isophthalic acid process; generating a cooled crude carboxylic acid slurry stream; isolating, washing, and dewatering solids present in the cooled crude carboxylic acid slurry stream in a solid-liquid separation zone to generate (i) a crude carboxylic acid wet cake stream comprising FDCA and (ii) a mother liquor stream comprising oxidation solvent, catalyst, and impurities, wherein from 5 wt.% to 95 wt.% of mother liquor present in the crude carboxylic acid slurry is isolated in the solid-liquid separation zone to generate the mother liquor stream, and wherein the solid-liquid separation zone comprises at least one solid-liquid separation device capable of separating solids and liquids, washing solids with a wash solvent stream, and reducing the % moisture in the washed solids to less than 30 wt.%; routing a portion of the mother liquor stream to a mother liquor purge zone, wherein a portion is at least 5 wt.%, and wherein the mother liquor purge zone comprises an evaporative step to separate oxidation solvent from the mother liquor stream by evaporation.