Mixer Selection Guide, ACUMIX Industrial Mixing

Solid Suspension
ACUMIX Industrial Mixers:
We have mixer selection tables for suspending solids however it would not be wise to publish them because they have limitations. In other words, and more importantly, you need to know when to apply them. In most cases, adding solids can turn the mixture into a Slurry, which may act independently from an insoluble solid for example. Solids Suspension Tables, for insoluble solids, generally encompass, the degree of suspension, the percent by weight solids, the settling velocity of the solids, versus the tank volume, but they do not encompass many other parameters. The following is a general discussion of solid-liquid mixing and the problems related to sizing a mixer based upon these tables.

INFORMATION REQUIRED TO DESIGN A MIXER:

Percent by Weight of Solids

Solubility of the solid for which the Solids are to be Suspended

The Tank Dimensions

The Type of Tank Bottom to be Used:

Cone: (Angle or Cone Vertical Height)

Dish Type: ASME (1/6 Tank Diameter) or 2:1 or Semi-Elliptical (1/4 Tank Diameter) or Hemispherical (1/2 Tank Diameter)

Flat Bottom

Sloped Bottom & vertical Height of Slope

Special

Maximum Solid Size or Size Distribution based percentages)

Degree of Suspension Required

Degree of Suspension:
There generally are four degrees of suspension of free setting solids, which include on-bottom suspension, off-bottom suspension, mid-depth suspension, and uniform suspension. All degrees refer to the largest solids that must be suspended. Solids of smaller size are suspended throughout each level.

	Uniform Suspension. Uniform suspension is required when an overflow is used, where the largest solids must be suspended to the level of the overflow weir. Generally speaking this is very inefficient because it takes approximately five times the amount of horsepower to achieve uniform suspension as compared to off-bottom suspension. The preferred method is to utilize a down-comer or a draw-off pipe or tube located at or near the tank bottom for draw-off purposes in combination with off-bottom suspension.
	Off-Bottom Suspension. Off-Bottom Suspension is where the largest solids are suspended just off the tank bottom. This is the most common level of suspension used. Generally speaking, it is more efficient power-wise to drive the flow of fluid downward to the solid catalyst at the bottom of the tank, than it would be to try to suspend these solids.
	On-Bottom Suspension. On-Bottom Suspension is also very popular in that the largest solids are not suspended per se, but are allowed to move upon the tank bottom, with the smaller sized solids suspended at various degrees throughout the tank. The reasons for this design parameter is again for economic reasons, as approximately 1/2 the horsepower is required as compared to off-bottom suspension.

Particle size directly influences the mixer design or the initial cost of the mixer. Think of a grain of sand. Due to its density & size it is quite difficult to suspend as compared to the overall requirement of primary goal of the mixing application. Said another way, in waste & water treatment applications it is acceptable to allow a very small percentage of sand accumulate in a large basin to continuously process say anoxic sludge which has a significantly lower suspension requirement. Over time, say several years, the basin would need to be shut down where the sand would be manually removed rather than to incur the continual & initial design to remove the sand.

Said another way, it is quite common to process solids that are processed to 200 to 400 microns for slurry mixing applications where the aggregate size is reduced significantly prior to processing. Think of gold or copper ore that is ground down prior to processing to remove the valuable pay dirt from the tailings. In other words, size reduction is performed not just for diffusion but for mixing pruposes (solid suspension) as well.

06.22.20