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A design is discussed for a horizontal filter having frontal and transverse irrigation for the combined treatment of ventilation discharges from electroplating and etching baths to remove aerosol particles and the vapors of mineral acids and alkalis. Nominal volume coefficients of absorption are presented for various conditions and modes of operation.NIIOGAZ has developed a fibrous filter of SNNK type (with frontal and transverse irrigation) for the combined treatment of ventilation discharges to remove suspended particles and gaseous impurities. The frontal irrigation is intended for the preliminary removal of suspended particles, while the transverse irrigation is intended to remove gaseous impurities (the vapors of mineral acids and alkalis). Instead of the usual packing, which is of considerable mass, use is made of light and convenient cassettes containing polypropylene grids of knitted fabric. The same material is used in the droplet trap.The horizontal gas flow enables one to locate the equipment in a low building, while the sections with packed layers (three or more) can be irrigated individually with solutions differing in composition in order to trap multicomponent impurities.The packing is of high porosity (up to 80-95%) and one can use highly efficient fibrous droplet traps, which means that the gases can be treated at speeds higher than in ordinary packed scrubbers.The SNNK fibrous filter (Fig. 1) of throughput 10000 m 3 /h has been installed for treating ventilation gases at the Krasnyi Oktyabr Company electroplating shop. The packing consisted of a corrugated grid of woven polypropylene filaments of diameter 0.3 mm.The irrigation was with a 10% solution of NaOH, spraying pressure not more than 300 kPa, and air temperature at the input to the filter about 25°C. Table 1 gives basic results on the operation of the SNNK fibrous filter in treating air containing vapor and droplets of HCl.The scrubber is more effective in trapping HCl vapor than it is on the aerosol phase (droplets of HCl mist). The vapor is trapped on average with an efficiency of 73.7%, while the drops are trapped to 56.5%. The overall performance in removing HCl is about 73%.There is an exceptionally high HCl concentration at the scrubber inlet. In some experiments, the total HCl concentrations at inlet and outlet attained 1.5-2.0 g/m 3 or more. In practice, the figure should be substantially less. According to recommendations made by GiproNIIaviaprom on the design of aeration and ventilation for electroplating and etching plants, the mean concentration of HCl mist at similar technological operations should not exceed 48 mg/m 3 .

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In etching and galvanic coating, chemical and electrochemical treatments are applied to metals in aqueous solutions of acids, alkalis, and salts. The chemical and electrochemical reactions and the stirring of the solutions with air give rise to gas bubbles, which produce mist droplets when they break.It is found [1] that the maximum rise speed for gas bubbles is 30 cm/sec. When they reach the surface, they break with the formation of droplets with sizes not more than 40 µm [2]. These droplets are of suspension or solution, and they are virtually completely carried off by the air current from the bath, whose speed considerably exceeds the suspension velocity of these droplets.To treat the discharges containing suspended droplets in galvanic processes, common use is made of fibrous filters [3], which provide highly effective trapping. However, sometimes when such filters are used with ventilation discharges, problems arise in regenerating the filter surface. Fibrous mist traps are either self-cleaning or regeneratable [3]. In the first case, droplets deposited on the filter surface accumulate and flow off it, i.e., there is ongoing removal of the trapped droplets.In regenerated filters, the filter surface needs ongoing or periodic flushing, which is usually due to the gases containing not only droplets but also suspended solid particles. The latter may be formed by the drying-up of droplets carried off from the bath.As a rule, the bath temperature is quite high (attains 100°C), while the temperature of the air flow taken from the bath is usually the room value (20-25°C). Then as the droplets are carried away from the source, their size decreases by evaporation.The evaporation rate (kg/sec) is given by an equation [4]:in which D v is the vapor diffusion coefficient in m 2 /sec; C d is the equilibrium concentration of the solvent vapor (water vapor) at the droplet temperature in kg/m 3 ; C g is the concentration of the solvent vapor (water vapor) in the gas flow in kg/m 3 ; Re = v g d d /υ g is Reynolds number; Sc = υ g /D v is the Schmidt number; v g is the speed of the gas flow in m/sec; d d is the droplet diameter in m; and υ g is the kinematic viscosity of the gas flow in m 2 /sec. We estimate the evaporation rate for droplets carried off by air from a bath filled with 10% NaOH solution (used for degreasing metals). The solution temperature has been taken as variable and having the values 80, 60, 45, and 25°C, while the temperature of the air taken off by a side pump [3] was constant at 20°C.The following assumptions were made to use (1):• the size of the transported solution droplets is maximal (d d = 40 µm);• the air is saturated with water vapor (at air temperature 20°C, C g = 17.3·10 -3 kg/m 3 [5]); and • the speed of the outgoing air flow is v g = 10 m/sec.