THE ELECTROPLATING INDUSTRY
2.4 WASTE MINIMISATION OPTIONS AND MEASURES
2.4.3 MAINTENANCE OF PROCESS SOLUTION
Further flowing rinse systems, in contrast to static rinses, should not have to be dumped and recharged and so production time is not lost because a tank is out of commission. However, there are restrictions on the application of these equations. These equations (Equations 2.5 to 2.7) do not hold under conditions of low flow rates and incomplete rinsing. It has been found that when the rinse tank volume is under ten times the drag-out volume, Equation 2.8 must be used.128 This allows for the dilution effect that the drag-out volume will make at low rinse volumes. Incomplete rinsing is found when the drag-out film on the workpiece surface is not washed off immediately and then well mixed into the bulk rinse water. These equations will be used in the water usage analysis in Chapter 7.
Agitation has been used to ensure equilibrium rinsing i.e. where good mixing of the drag-in and the water in the rinsing tank is achieved quickly. This is carried out either by moving the workpieces in the solution or by moving the solution over the workpieces. The former can be achieved manually or mechanically, by moving the jig backwards and forwards or/and up and down or rotating the barrel in the tank solution. In the latter case, sometimes the natural movement of the water in a flowing rinse system will be adequate. This may be the case when the water enters the tank at the bottom, travels diagonally across the tank and exits at the top of the other side and vice versa. However often pumping is needed over and above gravitational flow. Air agitation is often used. A perforated pipe runs across the bottom of the tank and compressed air is forced through it generating streams of bubbles through the rinse. Barrelling and jigging will be discussed more fully in Chapter 3.
Table 2.4 Methods of detecting and removing pollutants from process solution766, l28 1) monitoring of solution chemistry, conductivity levels, pH, temperature and density 2) monitoring the current consumed over a specified time period (ampere-hours) 3) filtration and carbon treatment of the plating solution
4) addition of chemicals to precipitate out impurities 5) use of electricity to plate out impurities (dummying)
6) removal of the contaminant source e.g. workpieces dropped from racks and left lying on tank bottom
Various solution monitoring techniques have been developed to detect the buildup of impurities in the bath. These include simple measurements like pH, density, conductivity, ampere-hours as well as elemental and other chemical analysis of the process solution and the consumption of active plating and processing chemicals.7' Monitoring allows the correct amount of raw materials and treatment chemicals to be added before any plating problems occur.
A conductivity measurement is one of the simplest ways of determining the quantity of inorganic substances present in effluent and flowing rinse water tanks.7 Monitoring the conductivity of rinse water helps to maintain chemical concentrations of pollutants at levels that provide adequate rinsing and prevent excessive drag-in to subsequent process tanks.133 By checking conductivity measurements the amount of water used for rinsing can be also reduced using a conductivity based control system to automatically adjust the rinse water flow rate.14 Chemical changes taking place in the plating solution itself can be monitored by recording specific density (Baume reading), pH and ampere-hour measurements. The densities of the process solution can be measured directly using a hydrometer. The Baume hydrometer scale is calibrated in units of degree Baume (°Be). These readings can be converted to specific density (see Equations 2.10 and 2.11) or to C1O3 concentration (using conversion tables):89
D= 1 4 4 3 144.3-N g . 1 4 4 3
134.3-N
D = density for liquids heavier than water 5 = density for liquids lighter than water N = Baume scale reading
Equation 2.10
Equation 2.11
The Cr03 levels range from 10.5 °Be (specific gravity 1.080) to 32.0 °Be (specific gravity 1.285) and a reading of 21.5 °Be is equal to a C1O3 concentration of 250 g/L.23'134 The presence of impurities including iron and copper can lead to erroneously high Baume readings.7'23
Current, like chemicals, is consumed in the plating process. By monitoring the amperes passed through the solution over time, the amount of chemicals used up can be estimated. This has its basis in Faraday's Law (see Equation 1.15). Chemical suppliers quote the values of ampere- hours at which chemical additions should be made. This is particularly useful for monitoring additive levels e.g. brighteners. These substances are present in small amounts and are not easily chemically analysed. This means chemicals can be added to the solutions at the appropriate time to prevent the bath running below the specified level as chemicals are used up in the plating process.
Filtration is one of the common methods used to remove insoluble suspended solids from a plating solution. These substances have been found to enter the tank from many sources e.g.
airborne dust, anode corrosion, drag-in on the workpiece and impurities in make-up chemicals.
Suspended solids may cause roughness, porosity, poor adhesion and burning on workpieces.14'63 Filtration of some plating solutions is carried out continuously e.g. Ni while for other solutions it is less frequent (annual) e.g. Cr, Cu, Zn. In both cases the solution is usually pumped out of the tank, through the filter into an empty tank, and the cleaned solution pumped back into the tank. Various specialised pieces of equipment have been developed for filtration of electroplating solutions. The most widely used are wound cartridge filters and disc plate or precoat (diatomaceous earth) filters (see Figures 2.4-2.6 and Figure 3.3 in Chapter 3).I28 Cartridge filters can be utilised for both small and larger tanks while precoat filters are used mostly for large tanks. The filter media are selected depending on the chemical composition of the solution. Size of filtration systems are based on solid loading and flow rate of the plating solution.128
Figure 2.4 Types of filters used in electroplating
Figure 2.5 Cartridge filter assembly Figure 2.6 Disc and plate clarifying filter assembly Soluble organic and inorganic bath contaminants cannot be removed by ordinary filtration. The organic contaminants are removed by treatment with activated carbon.7'I36 The carbon adsorbs these contaminants and removes them from the solution. Carbon treatment can be applied by various methods such as carbon filtration cartridges, carbon canisters and packing between precoat filters.128
The inorganic contaminants, such as dissolved base metals e.g. Fe, Zn and Pb, dragged-in treatment chemicals containing Cr, Ca as well as P and Cu from wiring workpieces are removed by chemical methods such as precipitation or by dummy electroplating.136 Precipitation is usually a batch process which is frequently performed by pumping the solution into a spare tank where it is chemically treated and filtered and then pumped through the filter back to its original tank.128 The precipitated sludge is left behind in the spare tank for disposal.
Chemicals which can be used for the precipitation purpose include barium hydroxide or
carbonate for the removal of sulfate in a chromium bath (see Equation 3.6 and 3.7 in Chapter 3), silver oxide or silver carbonate to remove chlorides from chromium plating baths (see Equation 2.11), lime addition for the removal of carbonate in silver cyanide baths (see Equation 2.12) and nickel carbonate treatment of nickel plating baths to remove metal contaminants such as iron and aluminum (see Equation 2.13). " • Iron, zinc and copper can be precipitated out of the solution by treatment with NaOH at high pH (see example Equation 2.14).
Ag20(s) + 2HCl(aq) - * 2AgCl(s) + H20(1) Equation 2.11 CaO(s) + H2C03(aq) — CaC03(s) + H20(1) Equation 2.12 3NiC03(s) + 2Fe3+(aq) — 3Ni2+(aq) + Fe2(CC-3)3(s) Equation 2.13 Zn2+(aq) + 2NaOH(aq) — 2Na+(aq) + Zn(OH)2(s) Equation 2.14 Dummy plating is an electrolytic treatment method used to plate out or oxidise metallic ion contaminants from a process solution.128 The process uses low current density usually between 0.2-0.5 A/dm"2 to plate out metallic ion contaminants such as copper, iron and zinc from a process solution onto scrap metal.7' I38 This has been found particularly useful for purifying nickel plating baths where hydroxide precipitation leads to loss of nickel as Ni(OH)2. High current density between 10-30 A/dm"2 is applied to oxidise Cr(III) to Cr(VI) in chromic acid baths used in chromium plating or chromic acid based anodising operations.128
When workpieces hung on a rack move from one tank to the other in the plating line, they may drop into the process solution and dissolve. This may cause Fe or Zn contamination of the solution. Regular removal of these dropped workpieces can help to prevent contamination of the process solution.
Maintenance of process chemical solutions helps to extend their useful lives and improves their operating efficiencies and effectiveness.7 The former reduces the dumping frequency and re- charging of process and rinse solutions and the latter reduces reject rates from poor quality plate e.g. those with cracks, inclusions and satins.