Method and arrangement for process water treatment
A method for treating process water of a flotation arrangement is disclosed. The process comprising the steps of a) dewatering overflow of a mineral flotation circuit in a gravitational solid-liquid separator to separate a sediment from a supernatant comprising water, silica-containing particles and soluble SiO2, fine particles, microbes, and residual flotation chemicals; b) subjecting the supernatant to cleaning flotation, in which at least 90% of the flotation gas bubbles have a size from 0.2 to 250 μm, in a cleaning flotation unit for collecting at least silica-containing particles, for separating at least silica-containing particles from the supernatant into cleaning flotation overflow, and for forming purified process water as cleaning flotation underflow; c) removing cleaning flotation overflow as tailings; and d) recirculating purified process water into the mineral flotation circuit. A process water treatment arrangement is also disclosed.
1 . A method for treating process water of a flotation arrangement comprising a mineral flotation circuit arranged to treat ore particles comprising Fe suspended in a slurry by reverse flotation for the separation of the slurry into an underflow and an overflow, and a process water treatment arrangement for treating the overflow of the mineral flotation circuit, the method comprising the steps of:
a) dewatering the overflow of the mineral flotation circuit in a gravitational solid-liquid separator to separate a sediment from a supernatant that comprises water, silica-containing particles and soluble SiO2, fine particles, microbes, and residual flotation chemicals, wherein after step a), the supernatant is led directly into a separator overflow tank,
b) subjecting the supernatant received from the separator overflow tank to cleaning flotation, in which at least 90% of the flotation gas bubbles have a size from 0.2 to 250 μm, in a cleaning flotation unit for collecting at least silica-containing particles, for separating at least silica-containing particles from the supernatant into a cleaning flotation overflow, and for forming purified process water as a cleaning flotation underflow,
c) removing the cleaning flotation overflow as tailings, and
d) recirculating the purified process water into the mineral flotation circuit.
2 . The method according to claim 1 , wherein the cleaning flotation unit is a dissolved gas flotation unit.
3 . The method according to claim 1 , wherein prior to step b), the temperature of the supernatant is 2 to 70° C.
4 . The method according to claim 1 , wherein prior to step b), the pH of the supernatant is 5 to 14.
5 . The method according to claim 1 , wherein in step a), the residence time of the overflow in the gravitational solid-liquid separator is under 10 hours.
6 . The method according to claim 5 , wherein a residence time of the overflow in the gravitational solid-liquid separator is in the range of 2-8 hours.
7 . The method according to claim 1 , wherein the solids content of the sediment of the gravitational solid-liquid separator is at least 80 wt %.
8 . The method according to claim 1 , wherein prior to step b), the supernatant is chemically conditioned in a mixing unit by adding a coagulant and/or a flocculant to flocculate silica-containing particles in the supernatant.
9 . The method according to claim 8 , wherein the coagulant is chosen from a group consisting of inorganic collector, aluminium salts, iron salts, and organic coagulants.
10 . The method according to claim 8 , wherein a coagulant is added into the supernatant in an amount of 20 to 2000 ppm.
11 . The method according to claim 8 , wherein the flocculant is chosen from a group consisting of natural polymers, and synthetic flocculants.
12 . The method according to claim 8 , wherein a flocculant is added into the supernatant in an amount of 2 to 100 ppm.
13 . The method according to claim 1 , wherein in step b), at least 90% of the SiO2 of overflow from the flotation circuit is removed.
14 . The method according to claim 1 , wherein in step b), at least 70% of soluble SiO2 of overflow from the flotation circuit is removed.
15 . The method according to claim 1 , wherein prior to step d), the purified process water is subjected to filtration for removing chemicals promoting microbiological growth.
16 . The method according to claim 15 , wherein in filtration, a filtering unit comprising a ceramic filter is used.
17 . The method according to claim 1 , wherein hardness of the purified process water is unaffected by the process water treatment arrangement.