ISSN 0967-0912, Steel in Translation, 2008, Vol. 38, No. 3, pp. 254–259. © Allerton Press, Inc., 2008. Original Russian Text © Yu.A. Galkin, 2008, published in “Stal’,” 2008, No. 3, pp. 92–96.
Profound Purification of Recycled Water from Hot-Rolling Mills and Continuous-Casting Machines in Sedimentation and Flocculation Units Yu. A. Galkin Eko-Proekt Scientific-Design Enterprise DOI: 10.3103/S0967091208030170
Most recycled-water systems for direct (contact) cooling of rolling equipment and metal were designed in the last century in accordance with the standards of the Soviet Ministry of Ferrous Metallurgy. To meet metallurgical requirements on water quality, a twostage purification system was employed, as a rule. This system would include primary sedimentation tanks (scale tanks) in the shop and secondary horizontal or radial sedimentation units and hydrocyclones outside the shop. Three-stage water purification on clarifying filters with different charges was only used for secondary billet cooling at continuous-casting machines and for certain rolling-shop needs (hydraulic scale removal, etc.) [1–3]. The basic structure of such systems is shown in Fig. 1A. Its main deficiencies include large size, frequent failure to meet quality requirements for the purified water, local pollution with overspill, transportation of inadequately processed scale- and oil-bearing residues, and an excess (imbalance) of recycled water returned to natural water bodies (in violation of ecological norms). Currently, with ever-more stringent requirements on purified-water quality for all technological consumers and on the environmental impact of recycling systems, and with a growing number of large systems (up to 20000 m3/h), modernized water-purification systems are under development. Those introduced in Russian ferrous metallurgy are mainly produced by foreign engineering firms. The most common system is shown in Fig. 1B. Characteristics of such systems include the following: profound removal of mechanical impurities (suspended particulates and petroleum products) from recycled water for all consumers; differentiation of the purification and cooling systems on the basis of the composition, temperature, and required pressure of the water; wide use of reagents for intensification of water treatment, prevention of biological and salt deposits and corrosion, and reduction of the oil content in scale;
topping-up of closed cycles with high-quality water from desalination units; dehydration of scale- and oil-bearing deposits in centrifugal and filtration units; a high level of automation. However, the solution of the remaining problems sometimes demands more than further complication of the processes and equipment, enlargement of the purification buildings, increase in their capital and running costs, and significant increase in staff training. The basic components of such systems (filter presses, centrifuges, scraper systems for sedimentation tanks, units for thickening the residue, large-diameter pipeline control gates with pneumatic drives, pressurized clarifying filters of diameter up to 5 m, unpressurized DynaSand filters, water-demineralization systems, a variety of pumps, and often fan-based cooling towers) are supplied by foreign firms, in accordance with their designs. As result, Russian enterprises are in a state of technological and economic dependency, and their competitive capabilities are restrained. This view is shared by other specialists in water preparation [4, 5] and in basic metallurgical technologies [6, 7]. In the present work, we describe the development of up-to-date water-recycling systems by the Eko-Proekt design enterprise and their introduction at Russian and Ukrainian steel plants, primarily in the Urals [8–10]. The basic water-supply system for hot-rolling shops and continuous-casting machines is shown in Fig. 1C. The possibility of creating highly efficient purification systems for recycled water is based on new equipment: sedimentation and flocculation units [11–13] and drainage bunkers [14, 15] for the dehydration of scale- and oil-bearing residues.
254
PROFOUND PURIFICATION OF RECYCLED WATER 2
6
II
A
255 9 VII
2
XII
X
X
VIII
8
2
XI
III
3
4
I
VI
5
1 7 B 2
~
II
14
V VIII
2
X
15
XII
X
IX
III
XI VII
I
4
1
XIII
10
~
C
V
11
12
6
13
16
18
17
19 2
II IX
2
XII
X
XI IV
22
VII
I
1 or 1*
20
16
21
6
Fig. 1. Systems of purifying recycled water from hot-rolling mills and continuous-casting machines: (A) traditional; (B) modified; C) modified according to Eko-Proekt specifications (using the example of the system for the 250/150 mill at NSMMP); (1) primary sedimentation tank, with pumping station and bunker for scale dehydration; (1*) equivalent installation with compact Eko-Proekt sedimentation tank; (2) grab bucket; (3) secondary sedimentation unit with oil-collection system; (4) bunker for dehydration of scale- and oil-bearing residue; (5) pumping station; (6) cooling tower; (7) reservoir for washing water from pumping stations for scrubbing filters and for supplying to consumers; (8) clarifying filters; (9) sedimentation unit for the washing water from the filters, with a pumping station for processed water; (10) secondary preliminary sedimentation unit, with a bridge-type scraper and an oil-removal unit; (11) pumping station for water supply to filter system; (12) clarifying filters; (13) pumping station with store for water supply to consumers and for washing filters; (14) coagulation unit for washing water; (15) pumping station for transport of clarified washing water; (16) slurry pump; (17) belt-type filter-press or decanter; (18) conveyer; (19) container for scale- and oil-bearing residue; (20) sedimentation and flocculation unit with an oil-collection unit; (21) pumping station for water supply to the cooling tower and to consumers; (22) Eko-Proekt drainage bunker with a pumping station for processed water and filtrate; (I) dirty scale- and mass-bearing water from consumers; (II) water supply to secondary sedimentation; (III) water beyond secondary and preliminary sedimentation tanks; (IV) water beyond sedimentation and flocculation units, purification to meet consumer requirements; (V) water supply from pumping station of the primary sedimentation unit of the continuous-casting machine, pumping stations for laminar and intense cooling of rolled product, and thermal sections (options with purification and cooling of all or part of water flux); (VI) supply of water after sedimentation and cooling to consumer; (VII) supply of profoundly purified cooled water to consumer; (VIII) washing water for filters; (IX) slurry for dehydration; (X) dehydrated scale; (XI) dehydrated scale- and oil-bearing residues; (XII) petroleum products; XIII) filtrate from filter press or centrifugate from decanter; R, reagents (not shown here).
The sedimentation and flocculation unit consists of a cylindrical metal housing (diameter 3–12 m) that contains a multipass eddy-type flocculation chamber, a system for water introduction and for hydrodynamic regulation of the mixing process, counterflow and directSTEEL IN TRANSLATION
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flow thin-layer sedimentational separators, an electrically driven scraper mechanism for transferring the residue at the bottom of the unit to the exhaust pipe (fitted with slurry pumps), oil-collection mechanisms, and a system for discharge of the clarified water. The charac-
256
GALKIN Particulate content, mg/l 700 600 500 1 400 300 200 2 100 0 14 16 18 20 22 24 26 28 30 Date (September 2006)
Particulate content, mg/l 50 Interval without dosing of flocculant
40 1
30
10 0
Interval without dosing of flocculant
2
20 Onset of flocculant dosing
1
2
3
4
5 Time, h
Fig. 2. Purification cycle for the 250/150 mill at NSMMP: variation in the content of suspended particulates after the preliminary sedimentation unit (1) and after sedimentation and flocculation unit 2 (2).
Fig. 3. Purification cycle for the 250/150 mill at NSMMP: variation in the quality of purified water after the sedimentation and flocculation unit with cyclic addition of Praestol65 flocculant in doses of 0.2 mg/l (1) and 0.5 mg/l (2).
teristics of the sedimentation and flocculation unit are as follows: increased hydraulic load: 11–15 m3/m2 h, as against 1.0–2.5 m3/m2 h for the secondary horizontal and radial sedimentation units in Fig. 1A and 5–9 m3/m2 h for the preliminary sedimentation units in Fig. 1B; small size and large unit power: thus, for a unit of working diameter 12 m, external diameter 14.5 m, and height 6.4 m, the productivity is 1600–2500 m3/h; an enclosed structure, ruling out the emission of water vapor and petroleum products; together with its compact design, this allows the unit to be located outdoors or indoors (in particular, in shops close to the equipment that consumes the water), improves the operating conditions in cold climates, and reduces the environmental impact; stability both in terms of the efficiency of removal of suspended particulates and petroleum products from the water and in terms of the effectiveness of the scraper mechanism with an increased content of particulates in the initial water supplied from the primary-sedimentation shop: consistently to 400–500 mg/l and periodically (after mechanical scale removal) to a few g/l; very efficient removal of suspended particulates (S) and petroleum products (P) from the water: their residual content with the given hydraulic load, in the absence of reagents, is S = 20–60 mg/l, P = 5–15 mg/l, depending on the content in the incoming water; when using reagents, strict consumer requirements may be met, down to S = 5 mg/l, P = 1 mg/l. Thanks to the controllability of the process in Fig. 1C, the water quality obtained may be optimized on an ongoing basis by modifying the operating conditions of the sedimentation and flocculation units and also the dose and type of reagent (usually a flocculant). Complete elimination of the reagent is an option, if this meets the consumers’ actual requirements (rather than
the elevated requirements often stated in design papers). Thus, operating costs may be considerably reduced, perhaps even to zero. In contrast, when using the system in Fig. 1B, which involves the use of granular clarifying filters, one of the main expenses cannot be eliminated: the power costs for pumping water through the filter, washing the filter, and recycling the washing water. An important feature of the system in Fig. 1C is the minimal power needs for water purification and dehydration of the residue, since gravity is employed [15]. In contrast to Fig. 1B, the system in Fig. 1C employs a small quantity of small-diameter (d = 100 mm) electrical hardware to link the slurry pumps (of productivity around 30 m3/h); the number of electric drives is smaller by an order of magnitude. This reduces the
Fig. 4. Purification units in the water cycle of bar mills of productivity 5000 m3/h at OAO MMK (located in the building of the dismantled blooming mill). STEEL IN TRANSLATION
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PROFOUND PURIFICATION OF RECYCLED WATER B
257 C
1
2
3 4
1
2 7
6
5
Fig. 5. Purification buildings with sedimentation and flocculation units in the recycled-water system for the thermal department at Vyksa metallurgical plant (productivity 2500 m3/h).
complexity and cost of the power-supply system and the automatic control system. In Fig. 2, we see the influence of fluctuation in the content of particulates beyond the primary sedimentation unit (1) on their content in the purified water (2) from the sedimentation and flocculation unit, in the recycled-water system for the 250/150 mill at NizhneSerginskii metalware and metallurgical plant (NSMMP), without reagents. With considerable increase in the particulate content in the water following the primary sedimentation unit as a result of the removal of the sedimenting scale, without interrupting the water flow, there is little increase (or even reduction) in particulate content in the clarified water from the sedimentation and flocculation units. According to Smolukhovskii–Muller theory [16], this may be attributed to sharp increase in the relative quantity of large particles in the water and increase in the rate constant of aggregation of the hydrophobic scale particles in the flocculation chamber. Together with the high reliability of the scraper mechanism in the sedimentation and flocculation unit, this permits briefer sedimentation in the first tanks without less efficient purification of the water in the sedimentation and flocculation units. In Fig. 3, we see the influence of treatment of the recycled water for the 250/150 mill with Praestol-650 flocculant (in doses of 0.2 and 0.5 mg/l) on the decrease in particulate content of the purified water from the sedimentation and flocculation units. The table presents research data for NSMMP and other plants. It is evident that profound removal of particulates and petroleum products from recycled water for modern hot-rolling mills and continuous-casting machines is possible in sedimentation and flocculation units, without subsequent use of granular clarifying filters. STEEL IN TRANSLATION
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Fig. 6. Design of purification buildings of types B and C for recycled water from hot-rolling mills, to be located outside the production shop (system productivity 12000 m3/h): (1) cooling tower; (2) pumping station; (3) secondary (preliminary) sedimentation units; (4) open area for dehydration of oily residues; (5) unit for washing-water filtration and coagulation and dehydration of the oily residue; (6) sedimentation and flocculation units (housed in buildings or outside); (7) section for dehydration of the oily scale by the Eko-Proekt method (housed in a building or outside).
On the basis of the results, an Eko-Proekt design for processing the recycled water from three new bar mills was adopted at OAO Magnitogorskii Metallurgicheskii Kombinat (MMK), with cancellation of the construction of a 5000-m3/h filtration station, at a cost of more than 200 million rubles. The new cycle (Fig. 4), corresponding to Fig. 1C, has operated for around two years, with the required water quality [17]. The water-purification building for the thermal department at Vyksa Metallurgical Plant designed by Eko-Proekt on the basis of Fig. 1C is shown in Fig. 5; the productivity of this system is 2500 m3/h. It includes three sedimentation and flocculation units of working diameter 10 m. There is no primary sedimentation unit within the shop. In accordance with the customer’s initial data regarding the water quality required in the heat treatment of pipe, provision is made for flocculant treatment. Operational experience shows that the water purified by sedimentation and flocculation units, with no reagent, meets the actual customer requirements; this minimizes the operating costs, leaving only the costs of water circulation, slurry transfer from the sedimentation and flocculation units to the existing horizontal sedimentation units, and operation of the fanequipped cooling towers. The design in Fig. 6 depicts water-purification buildings located outside the production shop. The system, of productivity 12000 m3/h, is used for direct cooling of equipment and metal in the steel-smelting and rolling shops at a steel plant, with water purification by systems of types B and C.
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GALKIN
Contents of recycled water (mg/l) Component of recycled water
Suspended particulates Petroleum products Phosphates (technological lubricants) Suspended particulates Petroleum products Suspended particulates Petroleum products Suspended particulates Petroleum products
Wire module at NSMMP
NSMMP 250/150 mill
HTMK continuouscasting machines 1–4
NLMK 2000 mill
Starting water 60–70 100–170
PNTZ continuous pipe mill
PNTZ group of pipe-rolling shops
MMK group of bar mills
150–600
200–250
100–150
100–150
150–450
6–8
8–10
35–40
15–30
30–100
115–174
–
–
–
–
–
20–30
16–18
–
≤60
≤20
≤15
≤5
80
–
10
–
≤20 ≤5
Purified-water requirements of equipment suppliers ≤50 ≤20 ≤20 ≤60 ≤5
≤10
≤5
≤15
Water after sedimentation and flocculation units without reagent treatment 20–30 30–40 30–40* 35–45 80 2–3
– –
2–3
5–15*
7–10
12
Water after sedimentation and flocculation units with flocculant treatment 5–15 12–15 17–19 10 12–18 1–2
2–3
1.5–2.5
2
<20
<2
<3
Type and dose (mg/l) of flocculant – Praestol-650; Nalco-7752-22; Praestol-853BC; Praestol-650BC; VPK-402; Nalco-7752 0.2–0.5 1–1.5 0.2 0.7 0.5–2.0 Content of moisture/petroleum products in dehydrated residues from Eko-Proekt drainage bunker, % 15/6 – – – 3.5/50 – * Purification of the water is completed in pressurized clarification filters.
Thus, the technology developed at Eko-Proekt, which has been introduced at numerous steel plants, has certain advantages over existing equipment and may be widely used in the creation of water-preparation and water-purification systems of any capacity at new or existing enterprises. REFERENCES 1. Levin, G.M., Pantelyat, G.S., Vainshtein, I.A., and Suprun, Yu.M., Zashchita vodoemov ot zagryaznenii stochnymi vodami predpriyatii chernoi metallurgii (Protecting Water Bodies from Pollution by Ferrous-Metallurgical Wastewater), Moscow: Metallurgiya, 1978. 2. Shabalin, A.F., Ochistka i ispol’zovanie stochnykh vod na predpriyatiyakh chernoi metallurgii (Wastewater Treatment and Use in Ferrous Metallurgy), Moscow: Metallurgiya, 1968.
3. Serikov, N.F., Krasavtsev, G.N., Il’ichev, Yu.M., and Rylov, A.V., Vodnoe khozyaistvo zavodov chernoi metallurgii (Water Management at Iron and Steel Plants), Moscow: Metallurgiya, 1983. 4. Shvetsov, V.N., Note to Readers, Vodosnabzhenie Sanitarnaya Tekhn., 2007, no. 11, p. 2. 5. Aksenov, V.I., Nikulin, V.A., Nichkova, I.I., et al., Improving Water Management at Industrial Enterprises, Vodosnabzhenie Sanitarnaya Tekhn., 2007, no. 12, pp. 25–28. 6. Sadykov, V.V. and Chikalov, S.G., Development of the Russian Pipe Market and Metallurgical Manufacturing, Stal’, 2007, no. 11, pp. 121–122. 7. Sivakov, D., Environmental Protection, Expert, 2007, no. 42(583), pp. 114–116. 8. Aksenov, V.I., Belichenko, Yu.P., and Galkin, Yu.A., Zamknutye sistemy vodoispol’zovaniya na trubnykh predpriyatiyakh (Closed Water-Treatment Systems at Pipe Enterprises), Moscow: Metallurgiya, 1987. STEEL IN TRANSLATION
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PROFOUND PURIFICATION OF RECYCLED WATER 9. Galkin, Yu.A., Physicochemical and Filtrational Properties of Residues of Scale- and Oil-Bearing Wastewater, Extended Abstract of Cand. Sci. Dissertation, Yekaterinburg, 1988. 10. Aksenov, V.I., Galkin, Yu.A., Ladygichev, M.G., et al., Vodnoe khozyaistvo promyshlennykh predpriyatii. Sprav. (Water Management at Industrial Enterprises: A Handbook), Moscow: Teplotekhnik, 2005, vol. 2. 11. Galkin, Yu.A., Russian Patent 2 234 357, Byull. Izobr. 2004, no. 12, p. 15. 12. Galkin, Yu.A., Russian Patent 2 182 838, Byull. Izobr. 2002, no. 15, p. 30.
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13. Galkin, Yu.A., Up-To-Date Technologies and Equipment for Wastewater and Recycled-Water Purification in Ferrous Metallurgy, Stal’, 2006, no. 5, pp. 131–133. 14. Galkin, Yu.A., Ulasovets, E.A., and Sidorova, I.A., Russian Patent 2 286 197, Byull. Izobr. 2006, no. 30, p. 29. 15. Galkin, Yu.A. and Sidorova, I.A., Dehydration Technology for Scale- and Oil-Bearing Residues, Stal’, 2007, no. 12, pp. 91–93. 16. Babenkov, E.D., Ochistka vody koagulyantami (Water Purification by Coagulants), Moscow: Nauka, 1977. 17. Rashnikov, V.F., Senichev, G.S., and Takhautdinov, R.S., Environmental Protection at OAO MMK, Stal’, 2007, no. 2, pp. 135–139.
