Essentially insoluble heavy metal sulfide slurry for wastewater treatment

#The present invention is directed to a product and a process for producing a “premade” heavy metal sulfide slurry, which can be used for the removal of heavy metal pollutants from and the reduction of hexavalent chromium in wastewater.

#Various chemical precipitation methods have been employed as one aspect of a complete system for the removal of heavy metal pollutants from aqueous solutions. Sulfide precipitation is often employed because sulfide salts may be used to remove heavy metal pollutants such as lead, copper, silver, cadmium, zinc, mercury, and nickel. Sulfides are also used as a reducing agent to convert hexavalent chromium to trivalent chromium. There are two basic types of sulfide precipitation processes—the soluble sulfide process and the insoluble sulfide process.

#By the soluble sulfide process, a soluble sulfide salt such as sodium sulfide or sodium hydrosulfide is added to a wastewater stream containing at least one heavy metal pollutant. The soluble salt quickly dissociates into sodium ions and sulfide ions and the heavy metal pollutant ions rapidly react with the sulfide ions to form a relatively insoluble heavy metal salt, which precipitates out of solution. There are two common problems associated with the soluble sulfide process. First, the relatively insoluble heavy metal pollutant sulfide salt often forms as very fine colloidal particles, which are not easily filtered or otherwise separated from the wastewater stream. Of even greater concern is the formation of odorous and highly toxic hydrogen sulfide gas, which invariably results from the high concentration of soluble sulfides present in the soluble sulfide process. Thus, the soluble sulfide process must be carefully monitored and controlled in order to avoid discomfort and harm to the treatment personnel.

#An essentially insoluble ferrous sulfide salt in accordance with the present invention was prepared as follows. Two parts of a stock solution of commercial ferrous chloride, FeCl2 (approximately 26% FeCl2) was diluted with one part water. Caustic (sodium hydroxide) was then added in excess to precipitate all of the iron as ferrous hydroxide. Sodium hydrosulfide was then added to convert the ferrous hydroxide to ferrous sulfide. Additional water was added to dilute the soluble salts. The ferrous sulfide was then separated from the solution containing soluble salts by means of a filter press. The ferrous sulfide was then dispersed in water to form a slurry having 6.8% percent by weight ferrous sulfide. At least 75 percent of the ferrous sulfide particles were less than 1 micron in diameter. Two percent by weight of a polyacrylic acid-based dispersant was then added.

#An essentially insoluble ferrous sulfide salt in accordance with the present invention was prepared essentially as set forth in Example 1. However, following separation by means of a filter press from the original solution, the ferrous sulfide was then dispersed in water to form a slurry having 20% by weight ferrous sulfide. At least 75 percent of the iron sulfide particles were less than 1 micron in diameter. Two percent by weight of a polyacrylic acid-based dispersant was then added.

#An essentially insoluble ferric sulfide salt in accordance with the present invention was prepared as follows. A stock solution of commercial ferric chloride, FeCl3 (approximately 30% FeCl3) was diluted 2:1 with water. Caustic was then added in excess to precipitate all of the iron as ferric hydroxide. Sodium hydrosulfide was then added to convert the ferric hydroxide to ferric sulfide. Additional water was added to dilute the soluble salts. The ferric sulfide was then separated from the solution by means of a filter press. The ferric sulfide was then dispersed in water to form a slurry having 6.8% percent by weight ferric sulfide. Two percent by weight of a polyacrylic acid-based dispersant was then added.