# US 5,346,557 A · Process for cleaning silicon mass and the recovery of nitric acid > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,346,557 A - Document ID: US5346557A - Status: Granted - Kind: A - Application number: 07/967,066 - Filed: 1992-10-28 - Granted: 1994-09-13 - Earliest claimed: 1991-10-29 - Assignees: Hi-Silicon Co., Ltd. - Inventors: Hideo Ito; Mitsutoshi Narukawa; Kazuhiro Sakai - Web page: https://patentgrep.com/patent/US5346557A ## CPC - H10P 70/12: {by dry cleaning only (H10P70/52 takes precedence)} - H10P 70/15: {by wet cleaning only (H10P70/52 takes precedence)} - H10P: GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10 ## Abstract In a process for cleaning a silicon mass with a acid washing solution, a waste solution containing a mixed acid composed of nitric acid and hydrofluoric acid is introduced to a distillation step, and hydrofluoric acid is introduced to a distillation cooling portion to prevent precipitation of silicon dioxide. The waste cleaning liquor is distilled to recover nitric acid. ## Claims ### Claim 1 A process comprising: sufficiently heating a liquid waste solution "obtained from cleaning a polycrystalline silicon mass" has been inserted after which comprises nitric acid, hydrofluoric acid, hydrosilicofluoric acid, and water so as to boil off a hydrofluoric acid containing vapor; transmitting said hydrofluoric acid containing vapor, said heating being effective to remove the hydrosilicofluoric acid to a condensation region; condensing said vapor in said condensation region to form a liquid condensate; and recovering said nitric acid as a substantially pure solution; wherein liquid hydrofluoric acid is added to said condensation region during said condensing step. ### Claim 2 (depends on 1) The process according to claim 1, wherein the addition of said liquid hydrofluoric acid to said condensation region begins when silicon dioxide precipitate is detected in said condensation region and is continued at least until said precipitate is no longer apparent. ### Claim 3 (depends on 1) The process according to claim 1, wherein the addition of said liquid hydrofluoric acid to said condensation region is continuous during said condensing step. ### Claim 4 (depends on 1) The process according to claim 1, wherein said heating step is carried out such that said hydrofluoric acid containing vapor is substantially a hydrofluoric acid-water system that does not substantially contain any nitric acid. ### Claim 5 (depends on 4) The process according to claim 4, wherein said recovering of said nitric acid comprises continuing to heat a remainder portion of said liquid waste solution after said hydrofluoric acid-water system has been boiled off, so as to boil off a predominantly nitric acid containing vapor; transmitting said nitric acid containing vapor to a condensation region; condensing said nitric acid containing vapor thereat and collecting said substantially pure nitric acid solution. ### Claim 6 (depends on 4) The process according to claim 4, wherein said recovering step comprises transferring a remainder portion of said liquid waste solution after said hydrofluoric acid-water system has been substantially boiled off, to a distillation apparatus; and subjecting said remainder portion to a distillation treatment to thereby obtain said substantially pure nitric acid solution. ### Claim 7 (depends on 4) The process according to claim 4, wherein said heating step is carried out at a temperature of about 110.degree. to 120.degree. C. ### Claim 8 (depends on 1) The process according to claim 1, wherein said heating step boils off simultaneously both said nitric acid and said hydrofluoric acid as said vapor. ### Claim 9 (depends on 8) The process according to claim 8, wherein said condensate contains both nitric and hydrofluoric acid and said recovering step comprises distilling out said hydrofluoric acid so as to form a substantially pure nitric acid solution from said condensate. ### Claim 10 (depends on 8) The process according to claim 8, wherein said condensation region has a lower boiling point condensation zone and a higher boiling point condensation zone; and said condensing and recovery steps comprise condensing said nitric acid at said higher boiling point condensation zone and collecting therefrom said substantially pure nitric acid solution, and condensing said hydrofluoric acid at said lower boiling point condensation zone. ### Claim 11 (depends on 8) The process according to claim 8, wherein said heating step is carried out at a temperature of about 120.degree. to 125.degree. C. ### Claim 12 (depends on 1) A process for cleaning silicon mass, which comprises: forming a cleaning solution by combining nitric acid, hydrofluoric acid, and water; employing said cleaning solution in an acid wash of silicon mass, thereby generating hydrosilicofluoric acid; and recovering the nitric acid contained in said hydrosilicofluoric acid-containing solution by the process of claim 1. ### Claim 13 (depends on 12) The process according to claim 12, wherein said nitric acid is continuously recovered. ### Claim 14 (depends on 13) The process according to claim 13, wherein the nitric acid used to form said cleaning solution comprises the recovered nitric acid. ### Claim 15 (depends on 12) The process according to claim 12, wherein said recovery step is carried out whenever the concentration of hydrosilicofluoric acid becomes sufficiently high such that the cleaning solution is no longer sufficiently effective in cleaning silicon mass. ### Claim 16 (depends on 15) The process according to claim 15, wherein the nitric acid used to form said cleaning solution comprises the recovered nitric acid. ### Claim 17 (depends on 12) The process according to claim 12, wherein after said acid wash, said washed silicon mass is retained in steam of high purity so that acid remaining on the surface of said silicon mass can be rinsed and removed with condensed water formed on the surface of said silicon mass. ### Claim 18 (depends on 12) The process according to claim 12, wherein after said acid wash, said washed silicon mass is immersed in purified water, and then retained in steam of high purity so that acid remaining of the surface of said silicon mass can be rinsed and removed with condensed water formed on the surface of said silicon mass.