Spent dust removal

Two versions of the ACP have been developed (1) an open-loop configuration in which the dry spent absorbent is simply removed from the system for disposal and fresh alkali is continuously fed to the spray dryer, and (2) a closed-loop process in which the spent absorbent is regenerated and reused. The open-loop system was installed on the 410 MW, Coyote Station at Beulah, North Dakota (Botts et al., 1978). The plant used a spray dryer followed by a fabric filter for simultaneous SO2 and dust removal. As indicated by the flow diagram of Figure 7-32, the process is extremely simple. Because the particulate collected on the fabric filter bags remains on the fobric for a period of time, the gas-solids contact time is extended beyond the particulate residence time in the spray diyo so additional absorption of SO2 by the alkaline material occurs. After this system went conunercial in 1981. no other large soda ash spray dryers were built. About 1990, this scrubber was converted finm soda ash to lime to eliminate solids build-up in the dryer vessel and to use lower cost lime. The... 

The reducing gas is distributed in reactor 4 by an ahoy grid, passes through the fluid bed, then exits the reactor via cyclones. The gas passes through reactors 3 and 2 so that a counter flow between gas and soHds is estabUshed. The spent reducing gas is scmbbed to remove dust and water vapor. Part of the cleaned top gas is recycled and the remainder is used as fuel. 

Air pollution problems and labor costs have led to the closing of older pyrometaHurgical plants, and to increased electrolytic production. On a worldwide basis, 77% of total 2inc production in 1985 was by the electrolytic process (4). In electrolytic 2inc plants, the calcined material is dissolved in aqueous sulfuric acid, usually spent electrolyte from the electrolytic cells. Residual soHds are generally separated from the leach solution by decantation and the clarified solution is then treated with 2inc dust to remove cadmium and other impurities. 

Based on dryer cost alone, indirect-heat dryers are more expensive to build and install than direct-heat dryers designed for the same duty. As environmental concerns and resulting restrictions on process emissions increase, however, indirect-heat dryers are more attractive because they employ purge gas only to remove vapor and not to transport heat as well. Dust and vapor recovery systems for indirect-heat dryers are smaller and less cosdy to supply heat for drying, gas throughput in direct-heat dryers is 3—10 kg/kg of water evaporated indirect-heat dryers require only 1—1.5 kg/kg of vapor removed. System costs vary directly with size, so whereas more money may be spent for the dryer, much more is saved in recovery costs. Wet scmbbers ate employed for dust recovery on indirect-heat dryers because dryer exit gas usually is close to saturation. Where dry systems are employed, all external surfaces must be insulated and traced to prevent vapor condensation inside. 

Emission problems in a causticization operation arise chiefly in the area of dust control of the exit gases from the lime kiln since the water circuit is virtually self-contained. Effective containment is obtained by the use of scrubbers, which achieve some 99% mass removal efficiency from the exhaust gases. Spent scrubber liquor may be returned to the causticization circuit for recycle. Using spent scrubber liquor, either for the slaking of lime or to prepare fresh sodium carbonate solutions for causticization, avoids creating a water emission problem from this aqueous waste stream. It also improves the raw material balance of the process. 

Basic alkyl sulfonates, Ca(OH)S03R, are also used to contribute acidity neutralizing and dispersing activity to the oil. These detergents (dispersants), help to keep particulate impurities, such as carbon, dust, or metal fines (e.g., lead or other metal oxides and salts) and water or acid droplets suspended in the oil to prevent deposition on or attack of critical moving parts. At the appropriate oil change interval most of the accumulated suspension is removed from the engine with the spent oil. 

The traditional oxide leach plant consisted of an acid leaching step, several neutral leaching steps and, until recently, a zinc dust purification step to remove cadmium, as shown in Figure 3. In the old scheme, most of the fume and various recycle streams were leached in spent acid in the acid leach step. The resulting acid slurry was thickened, and the lead-rich solid residues were filtered, repulped in water, combined with the residue slurry from the calcine leaching plant and pumped to the smelter. 

Solid Waste - The oxidation process uses vanadium catalyst that must be periodically removed and screened for dusts. A 1,500-tpd plant produces 20 m of the spent catalyst annually. 

After coarse particulate removal by cyclones, the off-gas scrubber uses spent electrolyte for scrubbing to maximize copper/cobalt recovery for the fines. The use of water for scrubbing of off-gas dust would dilute the concentration of copper and cobalt in the spent electrolyte and have a negative impact on the acid balance in the plant. 

All dust and spent abrasive must be removed from the surface by vacuum cleaning or brushing. After blasting all workers coming into contact with the clean surface should wear clean, protective gloves and clothing to prevent contamination of the cleaned surface. Any contamination may cause premature fail-... 

In the heat-treatment process, three distinct layers develop after there is water loss of approximately 12 percent through evaporation.The top layer consists of reusable oil, amounting to approximately 24 percent of the original scum volume the middle layer, containing approximately 16 percent of the scum, is called the rag layer and consists of flocculated particulate matter the bottom layer contains approximately 48 percent of the total scum and appears as relatively clear water. This bottom water layer is returned to the spent-coolant holding tanks for retreatment, and the rag, or intermediate, layer is combined with the swarf-ing dust and other heavy soUds for removal by haulage. 

If activated carbon is accidentally released, remove all sources of ignition. Ventilate area of leak or spill. Wear appropriate personal protective equipment. Clean up spills in a manner that does not disperse dust into the air. Use nonsparking tools and equipment. Reduce airborne dust and prevent scattering by moistening with water. Pick up spill for recovery or disposal and place in a closed container. Spent product may have absorbed hazardous materials. 

The process is practiced at 220 to 650 F, but most refiners contact diluted cylinder stocks at 475 F and neutral oils at 220 to 320°F. Occasionally lower temperatures are used, but such an operation should be classed as neutralization. The acid-stage oil is mixed with 200 or 200 to 300 mesh clay, and the mixture is heated in a pipestill to the aforementioned temperatures. The hot oil is then allowed to settle for a short time, and the fines (clay dust) are filtered from the oil in Sweetland or similar presses. Sometimes a short percolation filter is used to aid in the removal of the fine clay. R. C. Davidson has made a complete summary of the methods of applying contact treating. The clay is usually not recovered, but Chenault and Miller report the commercial use at 90 to 100 F of a 20 per cent acetone (naphtha) solution for extracting impurities from contact clay. A low contact temperature is employed so that the adsorbed materials will not be too tightly attached to the clay. The reactivated clay has an efficiency of 80 to 85 per cent. Spent clay is mixed with solvent, filtered in a closed rotary-type filter, and washed on the filter with naphtha. The recovery of acetone by distillation from the naphtha and from water constitutes a major part of the process. 

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