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Explore the importance of chemistry and motion in acid polishing, featuring Waterford Crystal case study and benefits of different polishing systems.
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What is More Important in Acid PolishingChemistry or Motion ? Chris Sandby-Thomas B. Eng. ICF Conference, Waterford 2002
People’s Perceptions of Acid Polishing • Still consider polishing a “black” art • Viewed as a “potentially” hazardous place to work • Working with toxic/corrosive chemicals that have possible health/environmental implications • Chemistry is complex • Unpredictable
What benefits have the current polishing systems brought to Waterford ? • Consistency of polished product • Better understanding of the chemical process • Better control of the process through monitoring and measuring • Increased awareness of relevant health, safety and environmental issues • Improved “working” environment • Increased productivity • Lower unit cost.
History of Polishing Systems Within Waterford Crystal Auto acid – now replaced. • Based on the carousel type system • Composition 37 – 42% HF 15 – 20% H2SO44 - 5% H2SiF6 • 2 minute cycle time • Visual monitoring “grey edge” • High acid repair and scrap levels • High acid usage (HF) • Inconsistent performance in relation to quality
History of Polishing Systems Within Waterford Crystal Robotic acid – now replaced. • Same composition and cycle times as auto acid • Constant breakdowns • High losses • High levels of acid related repair • High productivity, poor yield • Now operate two processes: • Salzle plant • Drum polishing system
Current Polishing System : “Salzle” • Three double and two single plants • Series of polishing and rinse tanks • Open system • Composition 65% – H2SO4 3% - HF 1 – 3% H2SiF6 • Cycle time 40 minutes (26 minutes acid time) • Dipping arm works in vertical motion, moves between acid/rinse tanks • Patented chemical system
Benefits of the Salzle Plants to Waterford Crystal • Clarity of the blank glass • Definition of the polished cuts • Consistency of polish • Low levels of acid related repair • Operator friendly • Suitable for stemware and hollowware products • Acid parameters easily controlled • Significant reduction in acid consumption • Lower scrubber emissions • Lower unit cost per glass
Current Polishing System : “Drum” Plant • Currently 1 system, with 3 drums • Closed system • Rotating drum in horizontal mode • Acid / rinse acids pumped from working tanks to reaction tanks as required • Cycle time 60 minutes (19 minutes acid time) • Works at higher temperature than Salzle (60 degrees versus 52)
Benefits of the Drum Process to Waterford Crystal • Suitable for polishing heavy solid giftware items and narrow necked products • High volume throughout • Flexibility in relation to the loading of product • Low repair figures
Basic Chemical Reactions ApplicableTo Both Systems: • Formation of silica tetra-fluorideSi02 + 4HF SiF4 + 2H20 • Formation of lead sulphatePb0 + H2S04 PbS04 + H20 • Formation of soluble salts e.g. (KF)K20 + 2HF 2KF + H20 • Formation of hydrofluorosilicic acidSiF4 + 2HF H2SiF6
What are the main reasons behind “Acid Related Problems” Each system has its own distinctive “acid related” problems – why do they occur ? • Is it simply because the acid conditions vary and can be out of balance ? • Or can each problem be resolved logically without adjusting the acid parameters ? • i.e. is it a chemical problem or a physical problem or both ?
What Causes Acid Related Problems ? We can determine by assessing the mechanisms that surround the polishing process : • How the glassware fills and empties • The effect the dipping motion has on the polishing process • The effect of pressure within the polishing bath It may be shown that acid related problems are dictated more by external forces than the chemistry of the system
Back of Basket Front of Basket
Acid Related Problems - “The Effect of Pressure within the Polishing Tank”
Conclusion • By applying logical principles to both polishing systems under constant chemical conditions, it has been shown that acid related problems can be more systematic of “Motion” rather than “Chemistry” • As new designs emerge, motion has become even more critical, especially to counteract the effect of • Heavy hollowware (the W Collection) • Thin, tall contemporary stemware.
Chemistry or Motion ? • As a chemical engineer, I am not stating that motion is more important than chemistry. • It is important to understand the possible chemical reactions that occur within a polishing tank, especially in respect to new glass compositions or even if new acid formulations are used. • However, Waterford’s experience suggests that “Motion” has an equally important part to play in the modern polishing system, to allow us to continue to operating at consistently high quality and low cost.
