Calcium Carbonate Settling Observations
Abstract
We ran several experiments to determine the amount of precipitate settling out of Calcium Hypochlorite solution at 10g/L, 20g/L, and 30g/L concentrations. From these experiments, we were able to come up with a safetly height of 6 cm, which should be the minimum height the exit valve is placed at in the stock tank to mimimize clogging at the stock tank exit. Also from these experiments, it was found that by allowing the solution to settle out for 3 days before running it through the hypochlorinator, a lot of the precipitate will just sit in the bottom of the stock tank. And the precipitate that settles in the stock tank won't interfere with the rest of the hypochlorinator and cause clogging in other areas, such as the float valve.
Introduction and Objectives
These experiments were done in order to determine the amount of precipitate forming at various concentrations of Calcium Hypochlorite solution after allowing the solution to settle for varying amounts of time. From this data, the residual masses of calcium left in solution with potential to clog the hypochlorinator were calculated. Additionally, the effects of starting with a higher concentration of calcium hypochlorite and then reducing that concentration by the addition of water, were examined to determine whether that method changes the amount of precipitate settling out.
Procedures
In this experiment, we filled three 1 liter graduated cylinders with varying concentrations of Calcium Hypochlorite. The graduated cylinders consisted of a 10g/L, 20g/L, and 30g/L solution of Calcium Hypochlorite solution. The solutions were mixed well and allowed to sit for up to 3 days in order to observe the height and weight of precipitate forming in the bottom of each cylinder after each day.
The weight of precipitate in each solution was found using vacuum filtration. The contents of each cylinder were poured through a filter and the liquid was vacuumed out. The precipitate left on the filter paper was then weighed.
In the last part of this experiment, a 100g/L solution of Calcium Hypochlorite was made and allowed to settle for one day. The heights of precipitate after one day were determined and then the concentration was brought down to 30g/L with the addition of tap water to the supernatent of this solution. The new height readings were taken and compared to those resulting from starting with 30g/L initially.
Results and Discussion
After allowing the cylinders to stand for a three days, most of the precipitate settled to the bottom, but some did float as a film on the very top of the solution. Some flocs of precipitate could be seen floating throughout the solution. After three days the amount of precipitate settling to the bottom of the cylinders did not increase.
Small bubbles were observed floating to the top of the cylinder, which sometimes would attach to flocs of the precipitant. These flocs would either float to the top or stay suspended somewhere between the bottom and the top of the graduated cylinder depending on the floc size.
For the first three days, the height of precipitate in the bottom of each cylinder increased. At day four, the amount of precipitate did not increase from day three.
In the future, the 50 gallon drums that hold the Calcium Hypochlorite solution should have exit flow valves installed at a height high enough from the bottom, so that any precipitant formed and settled does not flow out of the tank at this valve.
Measuring the height from the bottom of a container, to the top of the settled precipitant that forms, helped us determine the height to place the exit valve. This height was calculated to be approximately 6 cm, which is the result of maximum precipitate height found to settle out of a 30g/L solution multiplied by a safetly factor of 4.
The fact that after 3 days no more precipitate settles out, gave us a good estimate of time to wait before sending the calcium hypochlorite solution into the plant.
The masses of Calcium Carbonate precipitate resulting from vacuum filtering varying concentrations of Calcium Hypochlorite solution are graphed below. The masses were found to be .3, .72, and 1.93 grams at 10g/L, 20g/L, and 30g/L, respectively. The mass of precipitate was found to increase with increasing concentration of solution. From these resulting masses and the known molecular weights of Calcium Hypochlorite going in, the amount of Calcium Carbonate that could precipitate out was calculated. And based on the amount of Calcium Carbonate we found that actually did precipitate out of solution, we were able to find the residual mass of Calcium left in solution that could potentially settle out at a later time and clog the hypochlorinators. These masses were found to be 2.68, 5.32, and 7.64 g at concentrations of 10g/L, 20g/L, and 30g/L, respectively. This also revealed an increase in potential for clogging with an increase in concentration of solution.
When starting with an initially high concentration of Calcium Hypochlorite and reducing the concentration after initial settling by adding tap water, the resulting amount of precipitate in solution was found to decrease from 1.4 cm in height to .32 cm in a 30g/L solution. This is a 80% reduction in precipitate found to accumulate. The full results are tabulated below.
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From this data, it can be inferred that in the first day that the 100g/L solution was allowed to sit, a lot of the precipitate did settle out. Most of the precipitate found in the diluted 30g/L solution probably came from the precipitate that remains suspended on the top of 100g/L solution due to the CO2 gas rising.
Conclusions
With increasing concentration of Calcium Hypochlorite Solution, the threat of clogging becomes greater due to increased amount of precipitate forming. There are several ways however to reduce this threat.
Waiting 3 days to use the solution and pouring out only the liquid at the top of the precipitant and bellow the film formed at the top of the container should decrease the amount of precipitate in the system and help to eliminate clogging. Further we would recommend some type of filtration of the solution to remove the flocs of precipitant that are levitating in the middle of the holding container.
The 50 gallon drums that hold the Calcium Hypochlorite solution should have exit flow valves installed at a height greater that approximately 6 cm in order to be above the line of precipitate forming on the bottom of the container. This should reduce clogging at the bulkhead fitting at the exit of the 50 gallon drum.