Application of analytical chemistry to waste minimisation in the powder coating industry.
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Date
2005
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Abstract
A local company instituted a new chemical procedure in their spray phosphating system
used in the pretreatment of large components for industrial racking systems. An inorganic
conversion coating is deposited on the workpiece surface during phosphating and this
prepares the surface to receive an organic top-coat. The organic coating is applied to the
workpiece surface in the form of a powder and cured to form a continuous film about 80
u.m thick.
The solution chemistry of the phosphating system was monitored by sampling and
chemical analysis and taking direct reading instrumental measurements on the process
and rinse solutions. The process was also evaluated using the results of a waste
minimisation audit. This involved gathering data on composition, flow rates and costs of
inputs and outputs of the process. Two types of information were collected and used
during the audit, namely chemical monitoring (concentration levels of Na, Fe, Zn, Mo,
Mn and Cr and measurements of conductivity, TDS, SS and pH) and water usage data on
the Phosphating Line and existing data (raw materials, workpieces and utility inputs as
well as domestic waste, factory waste and scrap metal outputs). The data were analysed
using four established waste minimisation techniques. The Scoping Audit and the Water
Economy Assessment results were determined using empirically derived models. The
Mass Balance and the True Cost of Waste findings were obtained through more detailed
calculations using the results of the chemical analysis.
The results of the audit showed that the most important area for waste minimsation in the
Phosphating Line was the (dragged-out phosphating chemicals present in) wastewater
stream. According to the scoping audit, water usage had the third highest waste
minimisation potential behind powder and steel consumption for the entire powder
coating process. While the scoping audit and the specific water intake value showed that
water consumption for the process was not excessive, it did not indicate that the pollution
level in the rinse waters was high. Further, drag-out calculations showed that drag-out
volumes were typical of those found in the metal finishing industry. However the presence of high levels of metal species in the rinse waters was highlighted through the
chemical monitoring of the Phosphating Line. The True Cost of Waste Analysis
estimated potential financial savings for the effluent stream at about R8000 for a period
of 105 days. However this does not take into consideration the cost of the liability
associated with this stream when exceeding effluent discharge limits (given in the Trade
Effluent Bylaws) or of the chemical treatment necessary to render this stream suitable for
discharge to sewer. Intervention using only "low-cost-no-cost" waste minimisation
measures was recommended as a first step before contemplating further areas for
technical or economic feasibility studies. However, a further study involving monitoring
the sludge was recommended in order to establish the potential financial savings offered
by this waste stream.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
Keywords
Chemical industry--Waste disposal., Waste minimization., Phosphate coating., Coating processes., Metals--Finishing--Waste minimization., Theses--Chemistry.