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The objective of clay processing (milling and sieving) is to remove impurities such as sand and organic matter. Dried, powdered clay will also absorb water more readily. Powdered clay is formed from raw clay by: 1) drying the clay in the sun; 2) precleaning the clay; 3) pulverizing the clay; and, 4) sieving the clay.

About 4.5 kilos (9.9 pounds) of powered clay is required for each filter, so a factory producing 50 filters a day will need to prepare 225 kilos (500 pounds) of dry, powered clay daily. Extra clay should be prepared and stored to prevent an interruption in production in the event of equipment breakdown, interruptions in the power supply, poor weather, or other unforeseen events. Prepared clay can be stored in 25-kilo (50 pound) sacks in a dry, raised, covered location, protected from moisture.

The materials needed for clay preparation include an area protected from rain, a shovel, a wheelbarrow, sacks, a hammermill, scoops, and a 25 or 30 mesh sieve (equivalent to 0.7-0.6 mm openings, see Tyler mesh equivalent chart in Annex B). It is important that equipment is inspected regularly and sieves are inspected daily. Employees should have access to

damaged parts or equipment. Worker safety items needed for clay preparation include fan(s), face masks, goggles, gloves, earplugs, and closed shoes.

Although it is possible to dry the clay (and burn-out materials) in the sun on a sheet of plastic, in practice, plastic will trap moisture and impede complete drying, which will make pulverizing difficult. It is therefore preferable to sun-dry the materials on a cement slab. Since clay put through the hammermill should not exceed 2

centimeters (3/4 inch) in size, large clumps of dry clay should be broken up with a hammer or tamper such as an “elephant’s foot”— a heavy metal plate attached to a heavy bamboo handle (Figure 4-1). Rocks and organic material should be removed from the clay. A hammermill is used to pulverize the clay. It pounds the clay pieces with turning metal hammers. When clay particles are small enough, they pass through the hammermill screen and discharge as fine powder. A flail type hammermill without fixed blades is preferable as the blades are less likely to get damaged if

nonmillable material is introduced. Hammers on each shaft must be adjusted relative to the others so that the mill will be balanced.

The hammermill should be located in a low-traffic area of the factory, preferably outdoors under a roof, so as not to pose a hazard to other workers. A fan can be used to blow away the dust generated.

Before operating the hammermill, the screen should be checked and replaced if broken or punctured, and a sack should be well secured to the outlet of the hammermill to collect the clay and minimize dust output. The hammermill should be running before adding material and only a small amount of material should be added at a time to prevent the machine from jamming. A lid (or a square wooden bat with hole in middle) can be used to cover the hopper to prevent clay from spitting out of the hammermill. Loud or strange noises that occur during operation of the hammermill may be due to rocks, wood chips, or other materials in the hammermill chamber. In this case, the machine should be turned off, disconnected from the power source, the cover opened, and the screens inspected. Any foreign material should be removed and discarded. If the screen or any other components have been damaged or punctured, they should be replaced before restarting the hammermill.

Figure 4-1: Elephant’s Foot Tamper (Hagan et al. 2009)

Figure 4-2: Hammermill Diagram (Mossman and Lam 2004) Since hammermill screens are generally larger than the required 30 mesh (0.55 mm), after being processed through the hammermill clay should be sieved using the appropriate mesh. To facilitate sieving, a 25 or 30 mesh screen can be attached to a wooden frame. Clay that does not pass through the sieve can be remilled and sieved again. Material that does not pass through the sieve the second time should not be used in filter production. Sieves should be inspected daily for holes and repaired or

replaced if damaged.

Health and safety measures while processing clay must be

strictly enforced as the inhalation of silica particles can cause silicosis (Section 4.3.1). In addition to following national or local health and safety guidelines, workers should wear face masks at all times while working with or near clay. While operating the hammermill, a

facemask, earplugs and protective eyewear should be worn, long hair should be tied back, and loose clothing, which could get caught in the rotating machinery, should be avoided. A large amount of fine dust is produced during operation of the hammermill, so collection bags should be properly secured to hammermills to minimize dust output and clay processing equipment should be strategically located. Airborne silica dust can travel and remain suspended for several hours, so nearby workers not directly involved in clay processing may be exposed to the silica dust and should take appropriate preventative measures. Preventative measures should be taken not only while processing clay but also during cleaning the production area. Sweeping (dry brushing) will cause settled particles to become airborne, so water should always be used while cleaning. Although wet processing of clay is possible, and more

considerate to worker health, wet processing guidelines for filter production have not yet been established.

4.3.1 Silicosis

Silicosis is a long-term health risk associated with the inhalation of crystalline silica dust present in clay. It is an irreversible lung disease and can progress even when exposure has

Figure 4-3: Hammermill Screen (source unknown)

stopped. The list of long-term health effects associated with the inhalation of silica dust

includes asthma, silicosis, lung cancer, cardiovascular issues, and premature death. Inhalation of any airborne particles should be minimized, but silica is particularly dangerous and can remain airborne for several hours. Although workers should be provided with facemasks and protective goggles and instructed to use them, in practice they may avoid doing so due to discomfort. It should be emphatically communicated to employees that the importance of avoiding the long-term health risks from inhaling fine silica dust on a daily basis outweighs the short-term discomfort of wearing a facemask.