Chemical process

This investigation was aimed at determining the achievable composition of treated water during the lime process and the behaviour of the precipitate obtained in the process.

As the main part of magnesium is present in form of magnesium sulphate, the chemical process can be described as following:

MgSO4 + Ca(OH)2 = Mg(OH)2 + CaSO4

A small portion of magnesium is present in form of magnesium chloride:

MgCl2 + Ca(OH)2 = Mg(OH)2 + CaCl2

In the case of total precipitation of magnesium ions, soluble calcium chloride will be formed in the corresponding quantity. The bulk quantity of precipitate obtained will consist of magnesium hydroxide and gypsum.

It should be noted that the precipitation of gypsum is dependent on time and temperature, therefore in a real process system, the composition of the precipitate can be enriched in magnesium hydroxide. Because the process of the precipitation is extremely important from the operational point of view, this question was investigated in more detail.

1.3.1. Kinetics of the precipitation of magnesium hydroxide and gypsum Batch experiments

In general terms, the kinetics of the precipitation of components present in water is important for designing the process. Because the main components in our case are magnesium hydroxide and calcium sulphate, the kinetic characteristics of the precipitation of these compounds was investigated.

The experiment was carried out as follows. 500 ml of seepage water from dams was neutralised with lime milk to pH ~10.7. After 30 min of mixing at 18ºC the mixture obtained was left in the reaction vessel without mixing, and in 0.5-hours time a sample was taken for assaying. Results of the measurements are presented in Figure 4.

Kinetic curves of the precipitation of magnesium hydroxide and gypsum

0 0,5 1 1,5 2 2,5

0 1 2 3 4 time, hours

concentration, g/l

Ca Mg

FIG. 4. Kinetic curves of the precipitation for magnesium and gypsum.

Table I. Composition of water to be treated

Type of water _Na K Ca Mg Cl SO ₄ CO₃

mg/l

Process water* 1100 180 550 2800 2400 12800 <10

Pond water 520 210 610 1520 2322 5632 <10

Seepage from the

dams 560 133 485 2244 1489 9185 <10

Contaminated shallow ground

water 741 <5 493 1608 1564 6668 <10

* average for mill operation period

HCO₃ TDS pH Spec. cond. U Ra-226

mg/l g/l microS/cm mg/l Bq/l

<10 22000 7.2 <0.5 5-20

49 14 7.2 14765 0.03 5-20

159 17 7.2 12600 2-5 0.2-0.4

635 15 7.2 10550 0.01-0.04 <0.2

Mn NH₄ NO₃ NO₂ As

mg/l

1200 <100 200 <1

180 6 208 1.56 <1

100 6 186 1.78 <1

0.36 n.d. n.d. n.d. <0.1

It can be seen that the concentration of magnesium decreased very sharply to 0.1–0.2 g/l, but the concentration of calcium remained for the whole period of the experiment (3.5 hours) much higher (2-1.6 g/l) than the equilibrium concentration for calcium (0.7–0.8 g/l).

As it can be seen, the kinetics of the precipitation for the two main components is different. If the separation of magnesium and calcium is not required, this situation is unfavourable because a very long residence time is needed for the precipitation of gypsum (more than 4 hours).

Precipitation of the gypsum can be substantially accelerated by adding solid gypsum to the water, prior to or during the neutralisation, that is by gypsum recycle. The positive effect of the gypsum recycle is presented in Figure 5. Experimental data presented in Figure 5 show that using recycle, the precipitation rate of the gypsum is much quicker and the concentration of the calcium drops below 1 g/l in two hours.

Kinetic curves of the precipitation of gypsum

0 0,5 1 1,5 2 2,5

-1 0 1 2 3 4 5

precipitation time, hours

Ca conc. g/l

0%, 17 C0 100 %, 17 C0 200 %, 17 C0 200 % 7 C0

FIG. 5. Effect of the temperature and recycle for the precipitation of gypsum.

The experiments were carried out at temperatures of 17ºC and 7ºC. There was no difference between the data obtained at these two temperatures. Thus the gypsum can be precipitated much more rapidly if gypsum recycle is used. On the other hand if the separation of magnesium hydroxide and gypsum is required during the water treatment process, the recycle should be omitted.

Continuous laboratory experiment

Kinetic curves of the precipitation of gypsum were also determined in continuous laboratory experiments. The experimental set-up is shown in Figure 6.

Continuous laboratory-scale experiment for lime water treatment process

V=3,8 l/h polyelektrolyte

1 2

V=1.8 l V=1.9 l

t=0.5 h t=0.5 h

V=210 ml/h 3 ^{t=1.1 h}

V=4.5 l

Ca(OH)₂=100 g/l recycle*

* only in the second stage of the experiment

pH

ground water

Ca(OH)₂

FIG. 6. The experimental set-up for continuous experiments.

The experiments using this rig aimed at proving the possibility of decreasing the TDS of the contaminated tailings ponds-originating water and undertaking more detailed investigation of the precipitation process.

Two sets of experiments were carried out, one without recycle and the other with the recycle of gypsum. The circuit operating time was the same for each experiment.

Results of the experiments are shown in Table 2. The experiment lasted 7 hours, but samples were also taken after 14 hours.

It can be seen that the calcium, in accordance with the results of batch experiments, precipitates much more effectively if recycle is used: without recycle about 1.5 g/l was measured while with recycle the concentration decreased up to 0.7g/l.

It can be seen that the TDS of the treated water can be reduced to 8.5 g/l without recycle, and with recycle to 6.5 g/l. The differences between the two data can be explained by the greater extent of precipitation of the calcium in the case of recycle.

Of course, in a real industrial process the precipitation of gypsum may differ from that obtained in a laboratory experiment, but the general conclusions will remain valid.

Table II. Results of continuous laboratory experiments

1. Without recycle

standing 1.45 9.82 1.47 0.14 9.82 1.37 0.12 9.8 7.13

Volume of the

reactor 1.8 l 1.9 l 4.5 l

2. With recycle (50-100 % for gypsum)

1** 2** 3**

standing 0.85 10.21 0.77 0.24 9.92 0.74 0.19 9.7 6.13

Volume of the

reactor 1.8 l 1.9 l 4.5 l

Composition of the contaminated ground water ** Number of the reactors used in the experiments