Is there any way to restore a pail of Alconox to its original form after it has become solid like a rock?
Regretfully, once solidified, the phosphates in Alconox have been hydrolyzed by exposure to water or humidity and then have re-condensed and hydrated to form a phosphate cement. It is certainly possible to scrape and pound to break it up in to chunks and release powder and use these chunks for general purpose cleaning. It is not recommended to use the chunks for critical cleaning. Once the phosphates have hydrolyzed, they are chemically different than the original chemical and will not perform all of the dispersing, chelating and sequestering functions as well as the original formula did. Certainly the hydrolyzed phosphates still have significant detergency, but for the most critical cleaning, do not use the solidified and scraped Alconox.
Alconox should be stored is a closed container in a cool dry place (60 - 90 deg F). Alconox is actually fairly robust and can tolerate a fair amount of excess humidity and moisture, although eventually it will harden.
To view the technical bulletin for Alconox, Powdered Precision Cleaner, click the link http://www.alconox.com/downloads/pdf/techbull_alconox.pdf.