Magnesium Hydroxide Slurry Metering

Magnesium Hydroxide Slurry Metering

The key to magnesium hydroxide slurry handling is to keep the slurry moving so solid particles cannot deposit on the bottom of the tank or pipe. Mechanical agitators are used in storage tanks to keep solids off the bottom and ensure a uniform product. However, preventing the slurry from settling in the feed lines from the tank to the application point is just as essential. This article will address considerations when designing a system for slurry metering.

The first step is determining the required flow rate, which will then be used to select a metering pump. Positive displacement peristaltic hose pumps are the most common choice for metering slurry. They provide accurate metering, deal well with mildly abrasive materials, and can be run dry. Pump hoses must be changed periodically. To maximize hose life, the pump may be “oversized” so that the rotor spins slower and wears the hose down at a slower rate. However, be conscious of the turndown of the pump. Some peristaltic pumps may not run slower than 10% speed. A good rule of thumb is that the required flow rate should be about 25% of the pump's maximum capacity. However, if slurry flow is variable, that must be considered when sizing the pump.

The second step is determining the appropriate line size. Martin Marietta Magnesia Specialties recommends an ideal line velocity of 0.5 ft/s for slurry metering. The minimum recommended line velocity is 0.3 ft/s. Line velocity equals the volumetric flow rate divided by the pipe's cross-sectional area. Minimum flow rates for standard line sizes are listed below:

 

Line Size	Minimum Flow Rate (GPM)
3/8"	0.10
1/2"	0.18
3/4"	0.41
1"	0.73

 

MMMS does not recommend lines smaller than 3/8”. This includes pump hoses, hose barbs, and fittings on the pump. If a customer’s flow rate is less than 0.1 GPM, MMMS recommends flushing the line of solids with water regularly. The system must also be flushed with water any time it is down for at least 12 hours.

There is a tradeoff between line velocity and pressure at the metering pump. The faster the fluid flows, the higher the line losses will be and, therefore, the higher the pressure at the metering pump. It is essential to verify whether the selected metering pump can handle the pressure at the given line size and feed rate.

Carrier water can help alleviate issues related to high pressure in the metering lines. It will drastically reduce slurry viscosity and, therefore, line losses. However, when the slurry is diluted, solids will settle out of solution faster. Thus, the recommended line velocity when using carrier water is 5.0 ft/s. It is also important to use soft water as carrier water. Water with high calcium levels can cause scaling when mixed with magnesium hydroxide slurry.

Some customers install basket strainers on the suction lines before the metering pumps. This offers extra protection against plugging, as it prevents solids formed in the tank from splashing or poor agitation from reaching the pump. On the suction side of the metering pump is typically a hose barb, a common place for plugging, as it is the narrowest point in the feed line. It is vital to check and rinse basket strainers on at least a weekly basis, as solids may accumulate over time.

Finally, it is vital to avoid dead spots in the lines. These are most commonly seen when customers have a spare pump piped into the system. The spare pump will be on the other side of a closed valve. Any space between the main feed line and that closed valve will be “dead,” and solids will accumulate over time. Minimizing that space and running both pumps or alternating regularly is important so solids cannot collect. Using sweeping elbows in the feed lines is also recommended, as slurry can accumulate in the corners of 90° bends.

Because every application is unique, Martin Marietta Magnesia Specialties application engineers are available to discuss your particular system. You are encouraged to contact us at the phone number below.