Process for removing particulate impurities from aqueous phosphoric acid

A process for removing sludge-forming impurities from wet process produced phosphoric acid, said process comprising the steps of:

• (a) filtering wet process produced phosphoric acid containing sludge-forming particulate impurities through a fabric filter whereby particulate impurities are separated from the phosphoric acid and retained on the surface of the fabric filter to form a filter cake comprising said particulate impurities and entrained phosphoric acid;
• (b) contacting the filter cake with a wash fluid to remove a major portion of the entrained phosphoric acid from said filter cake, thereby forming a washed filter cake;
• (c) removing the washed filter cake to expose a fresh surface of the fabric filter by the introduction of a discharge fluid to the fabric filter on the side opposite said filter cake, for contacting with additional phosphoric acid containing particulate impurities;
• and
• (d) repeating step (a) using said fresh surface of said fabric filter as said fabric filter.

In the wet-process method for producing phosphoric acid by the acidulation with sulfuric acid of a solution or suspension of phosphorus-bearing mineral in phosphoric acid thereby to precipitate calcium sulfate and to release crude phosphoric acid containing sludge-forming impurities, separating calcium sulfate therefrom to produce a crude phosphoric acid, and thereafter concentrating said crude phosphoric acid to produce a crude, concentrated phosphoric acid containing particulate impurities and filtering the particulate impurities therefrom, the improvement wherein said filtering comprises the steps of:

• (a) contacting at least a portion of one side of a fabric filter overlaying the surface of a porous support with said crude concentrated phosphoric acid while maintaining reduced pressure on the other side of said fabric filter, thereby filtering said crude concentrated phosphoric acid to remove at least a portion of the particulate impurities therefrom and forming, directly on the surface of said fabric filter, a filter cake comprising said particulate impurities and retained phosphoric acid;
• (b) contacting at least a portion of said filter cake comprising said particulate impurities and retained phosphoric acid with a wash fluid comprising water to remove a major portion of the retained phosphoric acid and produce a washed filter cake;
• (c) removing said washed filter cake by directing pressurized air to the side of said fabric filter which is opposite to said washed filter cake, thereby exposing a fresh fabric filter surface for subsequent acid contacting;
• and
• (d) repeating step (a) using said fresh fabric filter surface as said fabric filter.

In the wet-process method for producing phosphoric acid by the acidulation with sulfuric acid of a solution or suspension of a phosphorus-bearing mineral in phosphoric acid, thereby to precipitate calcium sulfate and to release crude phosphoric acid containing sludge-forming impurities, separating calcium sulfate therefrom to produce a crude phosphoric acid, and thereafter concentrating said crude phosphoric acid to produce a crude, concentrated phosphoric acid analyzing in the range of about 46% to about 56% by weight P.sub.2 O.sub.5 and containing in the range of about 3% to about 15% by weight of particulate impurities, and filtering the particulate impurities therefrom, the improvement wherein said filtering comprises the steps of:

• (a) supporting separate segments of fabric filter over the porous outer surface of a cylindrical drum of a rotary vacuum filter, said drum having means for defining a plurality of separate chambers within the drum, with each chamber communicating with separate segments of the fabric filter, and fluid communication means for drawing a vacuum and, alternatively, pressurizing with fluid said separate chambers, said fabric filter being a felt comprising phosphoric acid resistant fibers with calendered outer surface;
• (b) contacting a portion of the outer calendered surface of a segment of fabric filter with said crude concentrated phosphoric acid at a temperature in the range of about 120.degree. F. to about 185.degree. F. while rotating said drum and maintaining reduced pressure on the inner surface of said segment, thereby filtering said crude concentrated phosphoric acid to remove at least a portion of the particulate impurities therefrom and forming directly on the calendered surface of said segment of fabric filter, a filter cake comprising retained phosphoric acid and at least about 96 weight % of said particulate impurities in said crude, concentrated phosphoric acid;
• (c) contacing a portion of said filter cake comprising said retained phosphoric acid and particulate impurities with a wash fluid comprising water to remove a majority of the retained phosphoric acid and produce a washed filter cake containing at least about 96 weight % of said particulate impurities in said crude, concentrated phosphoric acid;
• (d) removing said wahsed filter cake by directing pressurized air into the inner surface of said segment of fabric filter thereby exposing a fresh outer calendered surface of the segment for subsequent acid contacting;
• and
• (e) repeating step (b) using said fresh outer calendered surface as said portion of said outer calendered surface of said segment of fabric filter.

A process for removing sludge-forming particulate impurities from aqueous phosphoric acid containing sludge-forming particulate impurities, said process comprising:

• (a) passing an aqueous phosphoric acid containing sludge-forming particulate impurities at an elevated temperature through a phosphoric acid resistant fabric filter supported on a porous medium to cause at least a portion of said particulate impurities in said acid to be filtered therefrom and to be retained by said fabric filter, thereby forming, directly on the surface of said fabric filter, a filter cake comprising said particulate impurities and entrained phosphoric acid, wherein said fabric filter has a permeability to said aqueous phosphoric acid between about 2 and about 100 cfm, and wherein the openings in said fabric filter are significantly larger than the dominant size of said particulate impurities;
• (b) removing said filter cake from said fabric filter by the introduction of a discharge fluid to the fabric filter on the side opposite said filter cake, thereby exposing a fresh surface of said fabric filter for subsequent filtration;
• and
• (c) repeating step (a) using said fresh surface of said fabric filter as said phosphoric acid resistant fabric filter.

A process for removing sludge-forming particulate impurities from wet process produced phosphoric acid, said process comprising the steps of:

• (a) filtering wet process produced phosphoric acid containing sludge-forming particulate impurities through a fabric filter whereby particulate impurities are separated from said phosphoric acid and retained on the surface of said fabric filter to form a filter cake comprising said particulate impurities and entrained phosphoric acid, wherein said fabric filter has a permeability of said wet process produced phosphoric acid between about 2 and about 100 cfm, and wherein the openings in said fabric filter are significantly larger than the dominant size of said particulate impurities;
• (b) contacting said filter cake with a wash fluid to remove a major portion of said entrained phosphoric acid from said filter cake, thereby forming a washed filter cake;
• (c) removing said washed filter cake to expose a fresh surface of said fabric filter by the introduction of a discharge fluid to the fabric filter on the side opposite said filter cake, for contacting with additional wet processed phosphoric acid containing particulate impurities;
• and
• (d) repeating step (a) using said fresh surface of said fabric filter as said fabric filter.

In the wet process method for producing phosphoric acid by the acidulation with sulfuric acid of a solution or suspension of phosphorus-bearing mineral in phosphoric acid thereby to precipitate calcium sulfate and to release crude phosphoric acid containing sludge-forming impurities, separating calcium sulfate therefrom to produce a crude phosphoric acid, and thereafter concentrating said crude phosphoric acid to produce a crude, concentrated phosphoric acid containing particulate impurities and filtering the particulate impurities therefrom, the improvement wherein said filtering comprises the steps of:

• (a) contacting at least a portion of one side of a fabric filter overlaying the surface of a porous support with said crude concentrated phosphoric acid while maintaining reduced pressure on the other side of said fabric filter, thereby filtering said crude concentrated phosphoric acid to remove at least a portion of the particulate impurities therefrom and forming, directly on the surface of said fabric filter, a filter cake comprising said particulate impurities and retained phosphoric acid, wherein said fabric filter has a permeability to said crude concentrated phosphoric acid between about 2 and about 100 cfm, and wherein the openings in said fabric filter are significantly larger than the dominant size of said particulate impurities;
• (b) contacting at least a portion of said filter cake comprising said particulate impurities and said retained phosphoric acid with a wash fluid comprising water to remove a major portion of said retained phosphoric acid and produce a washed filter cake;
• (c) removing said washed filter cake by directing pressurized air to the side of said fabric filter which is opposite to said washed filter cake, thereby exposing a fresh fabric filter surface for subsequent crude concentrated phosphoric acid contacting;
• and
• (d) repeating step (a) using said fresh fabric filter surface as said fabric filter.

In the wet-process method for producing phosphoric acid by the acidulation with sulfuric acid of a solution or suspension of a phosphorus-bearing mineral in phosphoric acid, thereby to precipitate calcium sulfate and to release crude phosphoric acid containing sludge-forming impurities, separating calcium sulfate therefrom to produce a crude phosphoric acid, and thereafter concentrating said crude phosphoric acid to produce a crude, concentrated phosphoric acid analyzing in the range of about 46% to about 56% by weight P.sub.2 O.sub.5 and containing in the range of about 3% to about 15% by weight of particulate impurities, and filtering the particulate impurities therefrom, the improvement wherein said filtering comprises the steps of:

• (a) supporting separate segments of a fabric filter over the porous outer surface of a cylindrical drum of a rotary vacuum filter, said drum having means for defining a plurality of separate chambers within said drum, with each chamber communicating with separate segments of said fabric filter, and fluid communication means for drawing a vacuum and, alternatively, pressurizing with a fluid said separate chambers, said fabric filter being a felt comprising phosphoric acid resistant fibers with a calendered outer surface;
• (b) contacting a portion of said outer calendered surface of a segment of fabric filter with said crude concentrated phosphoric acid at a temperature in the range of about 120.degree. F. to about 185.degree. F. while rotating said drum and maintaining a reduced pressure on the inner surface of said segment, thereby filtering said crude concentrated phosphoric acid to remove at least a portion of said particulate impurities therefrom and forming directly on said calendered outer surface of said segment of fabric filter, a filter cake comprising retained phosphoric acid and at least about 96 weight % of said particulate impurities in said crude concentrated phosphoric acid, wherein said fabric filter has a permeability of said crude concentrated phosphoric acid between about 2 and about 100 cfm, and wherein the openings in said fabric filter are significantly larger than the dominant size of said particulate impurities;
• (c) contacting a portion of said filter cake comprising said retained phosphoric acid and said particulate impurities with a wash fluid comprising water to remove a majority of said retained phosphoric acid and to produce a washed filter cake containing at least about 96 weight % of said particulate impurities in said crude concentrated phosphoric acid;
• (d) removing said washed filter cake by directing pressurized air into the inner surface of said segment of fabric filter thereby exposing a fresh outer calendered surface of said segment of fabric filter for subsequent crude concentrated phosphoric acid contacting;
• and
• (e) repeating step (b) using said fresh outer calendered surface as said portion of said outer calendered surface of said segment of fabric filter.