(Meth)acrylic acid crystal and process for producing aqueous (meth)acrylic acid

1. A process for purifying a composition comprising (meth)acrylic acid, at least one impurity and water, wherein the composition has a water content in the range of about 10 to about 85% by weight, based on the composition, to form a purified phase comprising (meth)acrylic acid and at least one impurity, wherein, in the purified phase, the quantity of at least one impurity is less than about 7 by weight, based on (meth)acrylic acid in the purified phase, comprising a stage of the process which includes the following process steps:

a) (meth)acrylic acid is crystallized from the composition while forming a suspension comprising a mother liquor and (meth)acrylic acid crystals;

b) (meth)acrylic acid crystals are separated from the mother liquor;

c) at least a portion of the separated (meth)acrylic acid crystals is melted to form a melt; and

d) a portion of the melt is recycled to step a) or step b) and wherein the portion of melt which is not recycled is in the form of a separated (meth)acrylic acid.

2. The process according to claim 1 , wherein, in step a), (meth)acrylic acid crystallizes at least in part to form a crystal with a crystal structure having a surface with at least one recess located on the surface, the crystal structure having an orthorhombic Bravais crystal lattice with an Ibam space group, crystallographic data a=about 9.952 Å, b=about 11.767 Å and c=about 6.206 Å.

3. The process according to claim 1 , wherein, in step a), the mother liquor comprises at least about 60% by weight of (meth)acrylic acid and water, wherein the water concentration of the mother liquor is in the range of about 10 and about 90% by weight.

4. The process according to claim 1 , wherein the (meth)acrylic acid crystals are washed in the countercurrent of the recycled melt.

5. The process according to claim 1 , wherein the melt is purified in a separate purification process.

6. The process according to claim 1 , wherein the (meth)acrylic acid crystals from step b) are supplied at least in part to step a).

7. The process according to claim 1 , wherein the mother liquor separated in step b) is recycled at least in part to step a).

8. The process according to claim 1 , wherein the process comprises at least two stages, which each comprise steps a) to d), wherein at least one of the following features (α1)to (α4) is fulfilled:

(α1) separate (meth)acrylic acid from a first stage of the process is supplied at least in part to a second stage of the process;

(α2) separate (meth)acrylic acid from a second stage of the process is supplied at least in part to a first stage of the process;

(α3) mother liquor from a first stage of the process is supplied at least in part to a second stage of the process; and

(α4) mother liquor from a second stage of the process is supplied at least in part to a first stage of the process.