Solid-liquid leaching percolation

Often, the immobilized product has a structural strength sufficient to prevent fracturing over time. Solidification accomplishes the objective by changing a non-solid waste material into a solid, monolithic structure that ideally will not permit liquids to percolate into or leach materials out of the mass. Stabilization, on the other hand, binds the hazardous constituents into an insoluble matrix or changes the hazardous constituent to an insoluble form. Other objectives of solidiflcation/stabilization processes are to improve handling of the waste and pri uce a stable solid (no free liquid) for subsequent use as a construction material or for landfilling. 

Due to its great breadth of application and its importance to some ancient processes, leaching is known by many names including extraction, solid-liquid extraction, lixiviation, percolation, infusion, washing,... 

The Kennedy extractor (Fig. 18-87), also requiring little head-room, operates substantially as a percolator that moves the bed of solids through the solvent rather than the conventional opposite. It comprises a nearly horizontal line of chambers through each of which in succession the solids being leached are moved by a slow impeller enclosed in that section. There is an opportunity for drainage between stages when the impeller lifts sohds above the liquid level before dumping them into the next chamber. Solvent flows countercur-rently from chamber to chamber. Because the solids are subjected to mechanical action somewhat more intense than in other types of... 

The BoHman extractor [38] (Fig. 13.20) is one of several basket-type machines. Solids are conveyed in perforated baskets attached to a chain conveyor, down on the right and up on the left in the figure. As they descend, they are leached in parallel flow by a dilute solvent-oil solution ha f miscelld) pumped from the bottom of the vessel and sprayed over the baskets at the top. The liquid percolates through the solids from basket to basket, collects at the bottom as the final strong solution of the oil (/ // miscella)y and is removed. On the ascent, the solids are leached countercurrently by a spray of fresh solvent to provide the half miscella. A short drainage time is provided before the baskets are dumped at the top. There are many variants of this device, e.g., the horizontal arrangement of Fig. 13.21. 

Column test (described in Dutch standard NEN 7343) is intended to simulate the percolation of (acidic) rainwater through granular materials in a road construction or landfill. The material is placed in a column with a diameter of 5 cm and height of 20 cm, and acidified demineralized water is then passed upwards through it. The eluate is collected in seven fractions at a liquid/solid ratio of between 0.1 and 101 kg-1. The leaching is calculated and expressed in mg kg 1 as a function of time, based on the analysis results. 

It is classification by contacting method that provides the two principal categories into which leaching equipment is divided (1) that in which the leaching is accomplished oy percolation and (2) that in which particulate solids are dispersed into a liquid and subsequently separated from it. Each includes batch and continuous units. Materials which disintegrate during leaching are treated in equipment of the second class. 

Leaching is a unit operation where a solute molecule is removed from a solid using a fluid extractor. This is similar to liquid extraction, except that the solute to be removed comes from a solid rather than from a hquid as in the case of liquid extraction. Also, the fluid extractor may be a fluid or a gas. For example, poUutants can be leached out from solid wastes in a landhll as rain percolates down the heap. 

Whatever the scale of the extraction, leaching is performed in one of two ways. In the first, the raw material is placed in a vessel, forming a permeable bed through which the solvent or menstrum percolates. The wanted constituents are dissolved, and the solution issues from the bottom of the bed. This liquid is sometimes called the miscella and the exhausted solids, the marc. The process is called leaching by percolation. The second process employs immersion and consists of immersing the solid in the solvent and stirring. After a suitable period of time, solid and liquid are separated. 

In agitated tank leaching systems, finely ground ore must be mixed with the lixiviant until the reaction is complete. Liquid/solid separation processes are used to recover the solution and prepare the waste for disposal in a tailing impoundment. Unlike percolation leaching operations, these operations require a large impoundment area to not only handle the solids, but also to neutralize the chemistry and pH of the solutions used to extract the metal value. 

Percolation leach particle diameter >700 pm hquid concentration 0.8 to 20% relatively fragile solid (e.g., seeds). Immersion leach particle diameter <700 pm liquid concentration <20% relatively robust solids (e.g., minerals). Combo leach high feed concentration of solute, relatively robust solid. Supercritical solvent usually CO2 for small capacity of high value products, especially for temperature sensitive foods, cosmetics, and pharmaceuticals. 

Percolation leach particle diameter > 700 (xm liquid concentration 0.8 to 20 % relatively fragile solid (e.g. seeds)... 

In Fig. 12.8-2a an enclosed moving-bed bucket elevator device is shown. This is called the Bollman extractor. Dry flakes or solids are added at the upper right side to a perforated basket or bucket. As the buckets on the right side descend, they are leached by a dilute solution of oil in solvent called half miscella. This liquid percolates downward through the moving buckets and is collected at the bottom as the strong solution or full miscella. The buckets moving upward on the left are leached countercurrently by fresh solvent sprayed on the top bucket. The wet flakes are dumped as shown and removed continuously. 

For more fibrous solids, such as sugar cane, which is leached with water to remove the sugar, it has been shown [35] that leaching is generally more efficient in a thoroughly agitated vessel than by percolation, probably because the large amount of static liquid holdup (see Chap. 6) makes important amounts of solute unavailable. 

---