NOVELTY - Fractionating lignocellulosic biomass, comprises: (a) providing a feedstock comprising lignocellulosic biomass; (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; (c) substantially removing the cellulose-rich solids from the liquor; (d) introducing a solid additive to the liquor; and (e) separating a mixture comprising the lignin and the solid additive, each in free/combined form, from the liquor.    USE - The method is useful for fractionating lignocellulosic biomass (claimed) for converting biomass into fermentable sugars, cellulose, and lignin. The cellulose is useful in industrial applications to make paper or other pulp-derived products. The fermentable sugars are useful for making ethanol or other chemicals. The lignin is useful to produce liquid fuels, synthesis gas or hydrogen.    DETAILED DESCRIPTION - Fractionating lignocellulosic biomass, comprises: either (a) providing a feedstock comprising lignocellulosic biomass, (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin, (c) substantially removing the cellulose-rich solids from the liquor, (d) introducing a solid additive to the liquor, where the solid additive combines with at least a portion of the lignin, and (e) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the liquor; or (a1) providing a feedstock comprising lignocellulosic biomass, (b1) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin, (c1) substantially removing the cellulose-rich solids from the liquor, (d1) in a hydrolysis reactor, hydrolyzing the hemicellulose contained in the liquor, in the presence of a hydrolysis catalyst, to produce fermentable hemicellulosic sugars, (e1) introducing a solid additive to the hydrolysis reactor, where the solid additive combines with at least a portion of the lignin, (f1) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the fermentable hemicellulosic sugars and (g1) recovering the fermentable hemicellulosic sugars. An INDEPENDENT CLAIM is included for improving lignin separation during lignocellulosic biorefining, comprising (i) catalyzing fractionation or hydrolysis with an acid to release sugars into an acidified solution containing lignin, where the solution also contains a solvent for lignin, (ii) neutralizing the acidified solution with a base to form a salt in a neutralized solution, (iii) in a separation unit, separating the salt and the lignin, individually or in combination, from the neutralized solution and then (iv) recycling a portion of the salt and optionally a portion of the lignin to step (i) to combine, physically or chemically, with the lignin, to improve lignin separation in the separation unit.