ENCO ENGINEERING
GmbH
Sägenstrasse 97 CH-7001 Chur Telefon 081 254 12 12 Telefax 081 254 12 13 e-mail
[email protected] Internet http://www.enco.ch
Polyaluminium chloride (PAC) is increasingly used in water treatment. Against the conventional use of aluminium sulphate it is showing distinct advantages. A polyaluminium chloride plant is an investment opportunity with attractive returns in a growing market. Enco Engineering is presenting in the following basic information for a polyaluminium chloride project. The process technology is licensed by a European producer. The process as offered is following newest technologies and achieving products of excellent quality. It has substantial advantages against a conventional PAC process. The technology is presently applied in seven plants in Europe producing 20'000 tons per annum each and one producing 100'000 tons per annum.
ENCO
ENGINEERING GmbH
PROCESS FEATURES The proposed improved PAC process is giving the following advantages against a conventional process: • • • • •
PAC with a concentration of 18 % Al2O3 (1-2 % higher than most other products) PAC with a concentration of 9 % Al2O3 PAC with a higher basicity Low production cost No by-products (no acidic calcium sulphates difficult to dispose of).
PLANT CAPACITY A typical plant capacity for a polyaluminium chloride plant is 30'000 tons/ annum with 90 tons/day in 24 hours operation.
PRODUCT SPECIFICATION Polyaluminium Chloride 18 % Al2O3 Appearance: liquid with straw-yellow colour General Formulae [Al2(OH)2.7 Cl3.3]n Al2O3 17 – 19 % Cl21 – 22 % Basicity 36 – 42 % Specific Weight (20° C) 1.36 – 1.38 g/ml Viscosity (20° C) 40 – 60 cP pH (29° C) 0.4 – 1.2 Solidification point - 10 °C The primary product resulting from the polymerisation process at standard reaction conditions has a content of 18-19 %. This product is adjusted to the commercial assay of 17 - 17.5% Al2O3 and sold as such for most applications. Polyaluminium Chloride 9 % Al2O3 For drinking water treatment plants without units for addition of silicates, fungicides or polyelectrolytes preferably a PAC of 9 to 10% Al2O3 is used. By adjustment of the assay it is possible to change the size of the resulting flocks and consequently the sedimentation velocity. The corresponding PAC product has also a high so called "Basicity".
The second product produced in the plant will therefore be a polyaluminium chloride with a Al2O3-content of 9 % and a basicity of approx. 66 %. It is produced out of poly aluminium chloride 18 % by a special process achieving the requested high basicity. Appearance: liquid with straw yellow colour Al2O3 8.8 – 9.2 % Cl9.8 – 11 % SO421.2-1.4 % Basicity 62 – 68 % Spec. Weight(20°C) 1.18 – 1.22 g/ml Solidification point - 10°C
PROCESS DESCRIPTION The process for the manufacturing of polyaluminium chloride consists of the reaction and polymerization between alumina (aluminium oxide hydrate) and hydrochloric acid at high temperature and pressure. The hydrochloric acid is fed to the reactor as first ingredient. The amount depends on the batch capacity and on the concentration of the raw material. Alumina is fed to the reactor as second ingredient, under continuous stirring. After completion of feeding the reactor system is closed and heating is started. The heating of the reactor is carried out in two steps. Low pressure steam is used first, to avoid thermal shock that may damage the internal glass lining of the reactor. As the reaction temperature is reached the polymerisation step starts under stirring and constant temperature. At the end of the reaction the product is cooled again step wise with a warm water circuit to avoid thermal shock. The product is then fed by a high pressure pump to the filter-press. The filtered product is transferred to a storage tank.
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ENCO
ENGINEERING GmbH
CONSUMPTION FIGURES
depend on the quality of the alumina raw material. It is usually below 1 %.
The consumption figures for 1000 kg of Polyaluminium chloride 18 % are as follows:
Off-gas from reactors and tanks containing hydrochloric acid is washed in a scrubber.
Raw Materials Alumina Hydrochloric Acid (33 %)
kg kg
280 700
kg kWh m3 Nm3 m3
180 15 20 3 0.3
Utilities Steam Power Cooling water Compressed air Demineralized water
RAW MATERIALS, CHEMICALS AND UTILITIES Alumina Specification (typical analysis) Formula Al2O3 content Humidity Impurities
Al2O3.3H2O or Al(OH)3 61 – 63 % 3–6% SiO2 10 – 150 ppm Fe2O3 5 – 120 ppm CaO 10 – 150 ppm ZnO 5 – 35 ppm
Hydrochloric Acid Specification (typical ) HCl content Specific Weight HF Cl2 content max. Hg content
32 – 35 % 1.165 – 1.175 kg/l absent 0.1 % absent
For the production of PAC ideally hydrochloric acid of 35 % is used. With such a concentration the product will be a PAC with a concentration of 19 % Al2O3, with an important economical advantage. Using hydrochloric acid of 32 % a PAC of 18 % Al2O3 will be obtained, with hydrochloric acid of 31 % a PAC of 17.7 % Al2O3.
OFFSITES To serve the needs of the process units, the following utility units are required: -
Storage facilities for raw materials, chemicals, consumables and product Power supply Steam boiler Cooling tower Demineralized water supply Instrument air supply Compressed air supply
SPACE REQUIREMENTS The main part of the plant is foreseen to be installed in an open air structure. Only the control room will be installed inside a building. For the process plant an area of approx. 20 x 20 m will be required, without storage silos.
LABOUR REQUIREMENTS For operation of the process plant including supervision the following staff will be required: per shift Supervisor Operator
total
1
1 4
The supervisor could partly be used for other plant sections within the complex.
PROJECT TIME SCHEDULE The estimated time from start of the engineering to plant completion and operation is in the range of 15 months.
Impurities in the hydrochloric acid do not disturb, if the produced PAC will be used in treatment of urban or industrial waste water or for the treatment of sludge.
EFFLUENTS The process is not producing any liquid effluents. Acidic washing water from the filter press etc. will be recycled to the process. Crude polyaluminium chloride is filtered in a filter press, where small amounts of solids are removed. The quantity of the solids, mostly silicates, will Page 3/4
ENCO
ENGINEERING GmbH
APPLICATIONS OF PAC Polyaluminium chlorides are synthetic polymers dissolved in water. They react to form insoluble aluminium poly-hydroxides which precipitate in big volumetric flocks. The flocks absorb suspended pollutants in the water which are precipitated with the PAC and can together be easily removed. Stable PAC resulting from defined reaction conditions can be stored at ambient temperature and easily be transported by normal transport facilities like trucks or railway. Dosing of PAC can also be achieved easily. Flocculent in Drinking Water Treatment In the production of drinking water PAC is replacing aluminium sulphate. Due to its polymer structure treatment with PAC achieves bigger flocks and higher sedimentation velocity, easily separated by filtration through a sand or an activated carbon filter. PAC flocks are able to adsorb acidic biomaterials as usually present in river water or in rain water collecting pits. Treatment with PAC results in a very clean drinking water with low values for TOC (total organic carbon) and an extremely low aluminium ion content (less than 0.3 ppm). Additionally PAC treatment replaces treatment with iron salts. It is producing a drinking water which is perfectly colourless and is removing any material with so called "after flocculating" properties, i.e. after PAC filtration no sediments will reappear as sometimes occurring with other flocculants. The possible slightly higher unit price of PAC is compensated by a • • •
lower dosage requirement no requirement for any neutralising agent (soda, lime) higher quality of the treated water
Using PAC instead of aluminium sulphate results in getting a • • •
shorter flocculation time smaller amount of sludge reduced number of back washing steps with a significant saving of already treated water.
Against the aluminium sulphate in powder form, PAC solution is a product ready to use and with constant assay.
Industrial Process Water Treatment Textile and paper industries as well laundries require a pre-treated raw water. PAC allows elimination of any suspended materials without use of alkalisation compounds. Industrial Waste Water Treatment PAC has excellent absorption capabilities for coloured materials and other pollutants. It is widely applied in treatment of waste waters from small, medium or big industries, without using any alkalisation compounds. The obtained sludge can easily be dewatered in dryers, centrifuges or a filter press. Urban Waste Water In urban waste water treatment PAC is used in the primary sedimentation step, in the biological oxidation step and also in the fourth step, filtration after phosphate precipitation. With PAC treatment it possible to keep a waste water treatment unit in operation without biological oxidation step. A further special application is the dewatering of the sludge in filter press without application of polyelectrolytes. Paper Industry In the paper industry PAC has three important applications: PAC is applied in the preparation of cellulose batches with inorganic loads, especially with recycling paper. It has the property of dispersing resins, avoids breaking of the paper on the drying cylinder and helps in dewatering. PAC is used for a better distribution of the cellulose fibres, a better distribution and fixing of pigments added to the paper mass and consequently less turbid effluents after filtration. Aluminium sulphate has to be used at a pH of appr. 4.5 and allows only the production of acidic papers, whereas PAC allows to work at a pH of 6.5 -7.5, producing papers with neutral pH, which is a high advantage for papers for books, drawing paper and other documents. Such a paper will not get any yellow colour also after years. PAC is used in the water effluents obtaining good results without the necessity to use a biological oxidation step. PAC produced in the process as offered is not containing any sulphates and therefore there are no incrustation of calcium sulphate. Such sulphate- less PAC develops at the presence of reducing bacteria no corrosive and bad smelling hydrogen sulphide.
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