Application of analytical chemistry to waste minimisation in the powder coating industry.

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.