Separation of Iron Hydroxide Precipitates by Settling
Abstract Generated abstract
This study examines the settling separation of iron hydroxide precipitates formed during water treatment with iron coagulants. Laboratory experiments measured clarification rates in 500 ml cylinders for suspensions of varying hydroxide concentration, and for a 0.8% suspension aged for different periods before settling. The results show that lower hydroxide concentrations settled more readily, with a 0.4% suspension reaching 97% clarification after 20 minutes, while a 10% suspension reached only 5%. Aging also improved clarification, with the settling behavior stabilizing after about 110 hours, indicating that both suspension concentration and precipitate aging should be considered in designing water treatment processes using iron coagulants.
Full Text
SEPARATION OF IRON HYDROXIDE PRECIPITATES BY SETTLING
S. A. BABENKO, O. I. POPOVA, L. V. MULLINA
(Presented by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
Iron coagulants are finding increasing application for the purification of water from oil, surfactants, and other organic substances [1]. The technology of their use includes neutralization of sulfate or hydrochloric-acid solutions of iron salts with the formation of hydroxides, which at the moment of formation are active sorbents of organic substances. The final stage of the water-purification process consists in separating iron hydroxide precipitates. Settling is most often used for these purposes.
In the present work, the influence on the settling process of iron hydroxide of its concentration and aging time was studied.
The experimental procedure was as follows: in a cylinder with a capacity of 500 ml, hydroxide was formed from a calculated amount of ferric sulfate and caustic soda. After thorough shaking of the precipitate in the cylinder, the clarification percentage was recorded at specified time intervals. At the end of the experiment, the contents of the cylinder were discarded, and in it a precipitate of iron hydroxide of another concentration was prepared, which was also studied with respect to the rate of clarification. The influence of aging of the hydroxide on the rate of its settling was studied at a constant concentration of the iron hydroxide suspension equal to 0.8%. The water temperature in all experiments was 20°C.
Table 1
Dependence of clarification of an iron hydroxide suspension on concentration
| Hydroxide concentration, % | Clarification percentage, after 1 min | Clarification percentage, after 3 min | Clarification percentage, after 5 min | Clarification percentage, after 10 min | Clarification percentage, after 15 min | Clarification percentage, after 20 min |
|---|---|---|---|---|---|---|
| 10.0 | 1 | 2 | 2 | 3 | 4 | 5 |
| 5.0 | 1 | 2 | 3 | 5 | 7 | 10 |
| 1.25 | 2 | 4 | 7 | 13 | 18 | 25 |
| 0.80 | 2 | 4 | 7 | 13 | 20 | 27 |
| 0.40 | 13 | 65 | 86 | 91 | 95 | 97 |
Iron hydroxides at concentrations of 0.8 and 1.25% (Table 1) give a percentage of clarification after 20 min (27% and 25%) five times greater than hydroxide at a concentration of 10%. Reducing the concentration to 0.4% increases the percentage of clarification after 20 min by a factor of 19.3.
The aging time, that is, the time elapsed from the moment the hydroxide is prepared (Table 2), has a favorable effect on the settling rate of iron hydroxide. Thus, after 10 min of settling, the percentage of clarification of freshly prepared hydroxide was 13%, whereas for hydroxide with an aging time of 17 h it was 31%. After 110 h after preparation of the suspension
Table 2
Dependence of clarification of an iron hydroxide suspension on aging time
(hydroxide concentration 0.8%)
| Aging time | Clarification percentage, after min | Clarification percentage, after min | Clarification percentage, after min | Clarification percentage, after min | Clarification percentage, after min | Clarification percentage, after min |
|---|---|---|---|---|---|---|
| Aging time | 1 | 3 | 5 | 10 | 15 | 20 |
| 20 min | 2 | 4 | 7 | 13 | 20 | 27 |
| 17 h | 7 | 13 | 31 | 45 | 54 | |
| 42 h | 10 | 20 | 35 | 47 | 56 | |
| 66 h | 3 | 10 | 20 | 37 | 52 | 61 |
| 110 h | 3 | 13 | 22 | 45 | 57 | 64 |
| 182 h | 3 | 13 | 23 | 45 | 57 | 65 |
of iron hydroxide at 0.8% concentration, its particles stabilized, as is evident from the constant clarification rate for this suspension and for suspensions with longer aging times.
Thus, the concentration of iron hydroxide and the aging time substantially influence the process of separating a finely dispersed suspension. This circumstance must be taken into account when developing water-treatment technology using iron coagulants.
LITERATURE
- V. G. Perevalov, V. A. Alekseeva. Treatment of wastewater from oil fields. “Nedra,” 1969.