(633a) In-Situ Precious Metal Recovery From Silicone Streams

Silicones are used as additives in a wide variety of products and generally provide or enhance specific attributes.  Silicones can serve as surface-tension lowering agents, stabilizers for polyurethane foams, additives for coating applications, antifoams, emulsifiers, sealants, etc.  One of the key reactions for making organofunctional silicon-containing products (silanes and siloxanes) is the hydrosilylation reaction between a Si-H functionality and an unsaturated substrate.  Currently, Speier’s chloroplatinic acid and Karstedt’s platinum-siloxane complex are commonly used homogeneous hydrosilylation catalysts for silicone manufacturing.  As a result, platinum remains either in the silicone product or silane heavy wastes after distillation.  This causes significant precious metal waste, high color in silicone products due to Pt presence, and black particle formation in products due to Pt precipitation.

In this work, we describe IN-SITU fixed-bed adsorption technology for Pt recovery from a variety of siloxane and silane streams.  In this IN-SITU recovery process, fixed-bed columns have been designed directly between a continuous silicone production unit and a product storage container or directly between a continuous distillation unit and distillation heavies waste containers.  As a result, several significant process advantages can be recognized: (1) the Pt-recovery is IN-SITU and continuous, with no additional operational costs; (2) hot process streams enter the Pt-recovery unit, with no additional heating requirements; (3) minimal modifications of the existing processes are required; (4) much higher Pt recovery from silicone streams and much higher Pt loading on scavengers can be achieved through fixed-bed continuous recovery process compared to batch-wise recovery process.  With the IN-SITU Pt-recovery process, a Pt recovery greater than 90% can be obtained both from silane distillation heavy wastes (with initial Pt concentration of around 50 ppm) and from silicone products (with initial Pt concentration of around 5 ppm), with suitable temperature and residence time.  Long-term Bed Volume (BV = amount of treated liquid streams/amount of packed scavengers) experiments were also carried out to evaluate the process efficiencies.  As a result, very high BV operations of the corresponding streams could be achieved, still with reasonably high Pt recovery at the end the long-term run, and a Pt loading of around 2% could be obtained on scavengers.  In addition, after Pt removal, the silicone products showed much reduced color, which is preferable in the silicone industry.  One limitation of this process is that it is not efficient for high viscosity streams, due to limited mass transfer.

Because of the high Pt recovery from various silicone streams, high Pt loading obtained on scavengers, and low related manufacturing costs, this new IN-SITU fixed-bed adsorption technology has shown very significant benefits in silicone manufacturing.  These benefits include not only precious metal economic savings but also product quality improvements (reduced product color and reduced potential black particle formation).