India rank today as the fourth largest Nitrogenous fertiliser producer in the world. For producing nitrogenous fertilisers a variety, of feed stocks may be used—lignite, coal, coke oven gas, electrolysis of water, natural gas, associated gas, naphtha, fuel oil, etc. Considering the size of the country and diversity of the resources fertiliser plants have been located at the point nearest to the feed stocks.
Fertilisers can be divided into three broad categories:
(i) Nitrogenous fertilisers like urea, ammonium sulphate, ammonium sulphate nitrate and ammonium chloride.
(ii) Complex fertilisers like nitrophosphate (NP/NPK), diammonium phosphate (DAP), urea ammonium phosphate (UAP), ammonium phosphate sulphates (APS).
(iii) Phosphatic fertilisers like single super phosphates (SSP) and triple super phosphate (TSP). The production of these fertilisers requires a variety of raw materials and intermediates.
The raw materials and intermediates required to produce the above mentioned fertilisers are as follows:
(a) Natural gas, fuel oil/coal/naphtha.
(c) Carbon dioxide.
(d) Nitric acid.
(f) Sulphuric acid.
(g) Phosphoric acid.
(h) Rock phosphate.
Today about 52% of the fertiliser plants use natural gas as a raw material, 24% use naphtha, 13% use fuel oil, 4% use coal, 6% used imported ammonia and 1% use other sources. This mix will change with more and more units using associated gas.
Like any other chemical industry, fertiliser industry too produces some gaseous/liquid/solid effluents depending upon the technology adopted, feed stock used, location of the industry etc.
The major air emissions in this industry are as follows:
The major liquid effluents generated are as follows:
Various solid wastes generated in fertiliser industry are mentioned as follows:
(i) Carbon sludge.
(ii) Hard coke.
(iii) Lime sludge.
(iv) Inerts generated in nitrophosphate plant.
(vi) Calcium carbonate.
(vi) Chemical sludge from nitro ETP.
(viii) Gypsum (CaSO4).
The disposal of waste would cause damage to underground water and endanger ecosystem and the development per se had no meaning unless it is combined with sound environment management. Environment protection should be looked upon as a matter of faith rather than additional cost since both faith and environment were invaluable.
There was no limit for ammonia emission. The leakage of ammonia should be reduced to save on both energy and cost. Fugitive emission of fluoride in SSP plants expose workers to high level of fluoride which is a health hazard and safety should be accorded top priority. Noise pollution still remains a neglected area.
Through an effective water management programme the industry as a whole can reduce their water consumption. The average water consumption of urea plants, including ammonia plants, had come down from 18 m3/te urea during 1987 to 11 m3/te urea during 1991-92. In the phosphatic sector water consumption of plants with captive phos-acid had decreased from 30.4 m3/te P2O5 during 1988-89 to 16.4 m3/te P2O5 during 1991-92.
The industry, however, still faced the problem of utilisation/disposal of waste generated in fertiliser production namely phospho-gypsum, hydrofluosilicic acid chromium sludge, waste catalysts, etc. There was a need to develop suitable technology for recycling/utilisation of waste and for recovery of useful/ valuable material from waste.
Environment management has been organised to focus attention on the efforts made by the fertiliser industry in this direction, achievements and what further remains to be done.
Over the years, the technology adopted has been upgraded on the basis of improved systems available in advanced countries. With the energy crisis, greater attention is being paid to this aspect along with stricter emission controls adopted due to increasing environmental consciousness.
All the plants, therefore, have incorporated the most modern pollution control systems, minimising emissions to practicable limits with the best of technologies available to meet the standards. However, there are many old plants which have adopted older technologies where the emission levels are higher and so also is energy consumption.
Control of Sulphur Dioxide and Acid Mist Emissions from Sulphuric Acid Plants:
Sulphuric acid is produced by the oxidation of sulphur to sulphur dioxide (SO2) and conversion of this to sulphur trioxide (SO3). This is absorbed to produce sulphuric acid. All plants utilise a vanadium catalyst to convert SO2 to SO3 according to the contact process.
In the past 10 years all new sulphuric acid plants that have come up are of DCDA type which gives a conversion efficiency of 99.5-99.7%.
Table 17.1 shows the amount of SO2 emitted by sulphuric acid plants operating at several gas strengths and conversion levels:
In a typical sulphuric acid plant, the conversion of SO2 to SO3 is achieved in a four bed vanadium pentoxide catalyst bed. After 4th bed, when SO2 content has been reduced from 10% to 0.06% corresponding to 600 ppm, the sulphur trioxide is absorbed in 98.5-99% sulphuric acid in the Final Absorption Tower (FAT).
Before being vented to the atmosphere, the gases pass through a Brink Mist eliminator located after the IAT and FAT where acid mist is removed. The catalyst bed temperature, catalyst activity, pressure drop and concentration of the circulating acid are vital parameters affecting the performance.
In spite of adopting DCDA system, during initial start-up or start up after a shut-down of more than 4 hours when catalyst beds are cold or relatively cold the required temperature for conversion of SO2 to SO3 is not present resulting in higher emissions of SO2 during start up.
Sulphuric acid plants are therefore equipped with either a start-up heater to increase catalyst bed temperature to the required level or a start-up scrubber (which is a packed tower using caustic soda to absorb the SO2). In the case of start-up scrubbers Na2SO3/NaHSO3 is formed depending on the pH of the scrubbing solution.
Control of Sulphuric Acid Mist:
Acid mist comprises of liquid droplets which range in size from 10 microns down to 0.07 microns. It is a well-known problem in sulphuric acid production. This not only produces a visible and persistent plume in the stack gases but also causes equipment corrosion. Of course, this is a pollution hazard.
Mist is formed when water vapour in the air combines with SO3 in the converter and in the absorption circuit. By and large, this is controlled by drying the air thoroughly before feeding it to the converter and by proper maintenance of temperatures and acid concentration in the absorption towers.
Efficient drying of air is achieved by circulating 93-99% acid such that water vapour in the outlet gas is around 50-100 mg/nm3. At higher concentration of acid, higher operating temperatures can be tolerated while the reverse is true for lower acid concentrations.
The type of mist eliminator used in the drying tower is either the impaction type fibre bed mist eliminator or the mesh pad type. Wire mesh pad require periodic replacement in drying towers. Although plastic mesh pad offer better resistance to corrosion there have been cases where back flow of hot gases during a shut-down past a leaking shut off damper have melted the plastic mesh pad.
The absorption of SO2 in acid gives rise to acid mist. The acid concentration should be maintained between 98-99% to keep the H2SO4 + H2O + SO3 vapour pressure at a minimum. At lower concentrations, the partial pressure of water vapour is high enough to give rise to production of acid mist, by combination of this water vapour with the sulphur trioxide in the gas steam.
Typical mist loadings in the IAT are 500-1500 mg/nm3 with an average particle size diameter of 1-2 µm. Brownian diffusion filter elements are hollow cylinders which look like candles containing hydrophobic glass mattress packed between two stainless steel cages. Most mist particles 1 µm in size and larger are removed upto 96% of those smaller than 1 µm are collected.
The comparative efficiencies of filters are given in Table 17.2.
Advances are being made in mist collection equipment. High efficiency fibre beds are being developed with different efficiencies for various submicron particle sizes and various companies offer mist eliminators for sulphuric acid plant service (Table 17.3)
Control of Particulate Matter:
Urea Prilling Tower Dust:
Urea is produced by reacting NH3 and CO2 at elevated temperatures and pressures. Traditionally urea has been transformed into solid form by prilling techniques. The urea solution leaving the synthesis section has a concentration of 72-76% and is treated in a vacuum concentration section to obtain a urea melt to feed to the prilling systems.
It is during this prilling that particulate matter is emitted. The conversion of urea to solid form is achieved in a prilling tower by cooling the solution with air to expel its water content.
The quantity of air required has been estimated to be 10-1200 nm3/tonne of urea produced. In a typical 1000 tpd plant the total quantity of air is 500000 nm3/hr and it contains around 200 – 500 mg/nm3 of urea dust which has to be controlled, otherwise it would cause air pollution and result in loss of the product.
Most of the plants use wet scrubbers for particulate removal. Dust laden air rising through the prilling tower enters the annular duct. Air is drawn from this annular duct by a ring of liquid jets consisting of nozzles arranged in annular space which provides the energy required to overcome the pressure drop in the system. The liquid droplets act as spherical collectors for the urea dust whose size ranges from 2 – 200 microns. The water is sprayed at a pressure of 4.5-6 kg/cm2.
It is finally discharged into the atmosphere after passing through demister packings. Fresh make up water is sprayed into the demisters for washing purposes while the jet nozzles are fed with the urea solution which collects in the annular basin. After the concentration of urea in the recirculating solution reaches around 10-15% (Table 17.4), it is drained and sent to a tank for further processing.
Control of Particulate Matter in Complex Fertiliser Plants:
Dust is emitted daring rock grinding, product cooling and granulation operations. Normally, fabric filters are used for rock grinding and wet scrubbers like venturi scrubbers for granulation (i.e. drying of slurry in an equipment called the “Spherodizer”).
The hot gases are sucked out by an exhauster and enter the Venturi scrubber whose essential feature is the presence of a constricted cross-section or “throat”, through which the gas is forced to flow at high velocity. Water introduced ahead of the throat. The atomised liquid drops act as collectors.
The concentration of dust at the outlet is 60 mg/nm3.
Dust is emitted during the grinding of rock phosphate. Bag houses are commonly used for dust control. The dust concentration to the inlet of the bag house is normally in the range of 20-30 gms/nm3 which is brought down to less than 150 mg/nm3.
The fabric usually used is polypropylene or polyester with a special coating. Pulse jet cleaning is adopted when the pressure drop is around 6″ (150 mm) of water.
Control of Oxides of Nitrogen:
Nitric Acid Plants:
Nitric acid is commercially produced by reacting ammonia ‘with air to produce nitrogen oxides which are then absorbed in water to yield the acid.
Despite the many variations in operating details among the plants producing nitric acid from ammonia, three basic steps are common to all:
(i) Reaction of ammonia with air over a catalyst (platinum, radium) at a high temperature and moderate pressure to produce nitric oxide.
(ii) Oxidation of the nitric oxide by oxygen remaining in the gas stream to produce nitrogen dioxide.
(iii) Absorption of the nitrogen dioxide in water to produce nitric acid releasing additional nitric oxide which must be re-oxidised.
After absorption, the tail gas containing, namely, nitrogen, water vapour and oxides of nitrogen (commonly known as NOx) is normally preheated and expanded through a turbine to recover energy, which is used to power the nitric acid plant’s compressor.
The NOx content in the tail gas varies from 1000-3000 ppm of which approximately 60% is NO2 and the rest is NO. The NO2 component imparts a yellowish colour to the tail gas.
In general, there are four types of processes which can be used to reduce or control emissions:
(a) Catalytic reduction (selective and non-selective).
(b) Extended absorption
(c) Chemical absorption
(a) Catalytic Combustion:
The oxides of nitrogen formed during the combustion of ammonia are absorbed in water to form nitric acid. The unabsorbed portion of oxides of nitrogen and inerts are emitted to atmosphere which generally contain 0.2-0.4% of NOx and 2-3% free oxygen.
The tail gas leaving the absorber is mixed with a fuel gas like natural gas (or refinery gas or purge gas from the ammonia plant) and the mixture is passed over a catalyst (platinum vanadium, iron oxide or titanium based) bed. This converts the NO2 to NO rendering the exit gas colourless. This step is called the decolourisation reaction.
This alone may not be sufficient and complete destruction of NO by converting it to N2 may be required. To achieve this the oxygen present in the tail gas (2-3%) must first react with the fuel which must be present in slight excess of the stoichiometric amount. The reaction with oxygen results in a large amount of heat of combustion which results in temperatures of nearly of 650-700°C.
An improved process is adopted in some of the plants where selecting reduction of NOx (i.e. NO and NO2) in the gas system is done with slight excess of stoichiometric quantities of ammonia over a mixed catalyst.
The catalyst is selective and the ammonia consumption is almost stoichiometric and virtually no ammonia reacts with oxygen present in the gas which results in much lower exit gas temperatures allowing use of simpler and cheaper equipment.
Two catalysts are used in the reaction, the first is based on precious metals mixed with metallic oxides, noble or iron group or V2O5 on alumina which promotes the NOx reduction and the second is platinum based which destroys or converts any excess ammonia fed to the system into harmless nitrogen.
The exit temperature raised by 20-30°C due to exothermic reaction. The only care to be taken is to see that ammonia gas to the reactor is not fed when the tail gas temperature is less than 200°C, in order to avoid the formation of ammonium nitrite and nitrate. The NOx could be brought down to less than 300 ppm to NOx This technique has become widely used in recent years.
(b) Extended Absorption:
The extended absorption processes aims to continuing the process of absorption of the NOx in water beyond the level at which it normally ends, i.e. 2000-3000 ppm NOx. The extended absorption is made possible by provision of large absorption columns.
The additional absorber produces nitric acid. Owing to the large size of the absorbers which have to be necessarily of stainless steel construction, the investment costs are the highest as compared to the other NOx control processes. However, there is no additional operating cost involved.
(c) Chemical Absorption:
The tail gas could be cleaned by scrubbing with a liquid containing caustic soda or urea. In case of the caustic, it results in the formation of NaNO2 and NaNO3. A means must be found to recover and reuse the NaNO2/NaNO3 from the solution otherwise it could be a source of pollution.
Urea also reacts with the nitrogen oxides resulting in the liberation of N2/CO2 and formation of ammonium nitrate in solution. The scrubbing yields a steamy but colourless plume.
Here again, the byproduct ammonium nitrate solution must be utilised otherwise, it would result in the total use of urea as well as a source of water pollution. Liquid scrubbing systems are best adopted in combination with extended absorption.
After cleaning the tail gas substantially in additional absorber, liquid scrubbing could be adopted to reduce the pollution levels. This would result in considerable reduction in the absorber ‘volume while, at the same time, keeping chemical consumption reasonably low.
Control of NOx in the Nitrophosphate Plants:
Rockphosphate (fluoropatite 3Ca3 (PO4) CaF2), which is naturally available contains phosphorus in the form of tricalcium phosphate which is not soluble and hence not available to the plants. To convert this phosphorus to the soluble forms, i.e. dicalcium phosphate and monocalcium phosphate the CaO/P2Os ratio has to be reduced.
This is done either by removing part of the calcium nitrate formed (after addition of nitric acid to rock phosphate) by chilling and crystallisation or by the additional external P2O5 or by both. By this fertiliser of required grade containing nitrogen and phosphorous (NP) is produced.
The emission of NOx takes place during the treatment of rockphosphate with nitric acid. In theory no NOx, should be formed as the nitric acid is being used purely as an acid to dissolve phosphate rock.
In practice, however, the rock phosphate contains some oxidisable impurities such as sulphides and organic material and it is these which reacts with nitric acid to produce NOx.
The NOx emission can be minimised but not eliminated by maintaining proper temperature in the acidulation reactor. To combat the emission of NOx, urea is added at the digestion stage to yield nitrogen, ammonium nitrate and water.
The operation and maintenance of this system is very simple provided urea addition is continuously done. This is achieved through a urea solution tank equipped with a pump to add urea at the rock digestion stage. Good results have been obtained with apparently no adverse effects on product quality.
The absorption of NOx on a fixed bed of solids has not been extensively commercialised as other processes. Normal (dry) absorption on activated carbon or molecular sieves is very efficient but the absorbents must be regenerated either by thermal desorption or by pressure reduction.
Control of Fluorine Emissions:
Fluoropatite ores are used for the production of fertilisers such as single superphosphates, triple superphosphate or converted to wet process phosphoric acid and there on to other fertilisers.
Depending upon the source of origin the fluorine content in the rock varies between 1 and 5% which is released as silicon tetrafluoride (SiF4) and hydrogen fluoride (HF) in the presence of active silica contained in the rock when rock is acidulated and ammoniated for fertiliser manufacture.
To circumvent this problem it has become common to fix volatile fluorine compounds as H2SiF6 (hydrofluorisilicic acid) by scrubbing with water which then forms a base for production of a variety of fluorine related compounds.
Fluorine is released during the attack on rock by sulphuric or phosphoric acids. In the manufacture of SSP 12-25% of total fluorine in the rock is evolved as gaseous vapours in the form of hydrogen fluoride and silicon tetra fluoride.
The hydrogen fluoride liberated during acidulation reacts with silica to produce SiF4 which reacts with the water vapour generated during acidulation and with water in the scrubber towers to form hydrofluoride acid and finely divided silica.
In the TSP manufacture 5-15% of the total fluorine contained in the acid and rock is evolved during the acid attack and 20-40% of the total fluorine during drying. The reaction is nearly the same as above with a part of fluorine getting converted to H2SiF6 and the rest remaining as HF and SiF4 which is removed by further scrubbing. Phosphoric acid manufactured by action of sulphuric acids on rock at 70-80°C evolves fluorine during the acidulation and concentration steps.
Total volatalisation of F during acidulation and concentration of phosphoric acid is approximately 40%. The fluorine emissions are controlled by scrubbing these gases in either Venturi scrubbers, packed towers, Cross-flow scrubbers or a spray chamber.
Water is used as scrubbing liquor which converts the HF and SiF4 to H2SiF6. Efficient scrubbing system can operate upto 25% H2SiF6 but above this point the SiF4 vapour pressure rises steeply. Many towers operate at 15-20% H2SiF6 or less.
One of the commonly encountered problems is deposition of silica when concentration of phosphoric acid is less than 50%. This leads to choking of scrubber nozzles. Hence nozzles have to be periodically cleaned and specially designed for this service.
Control of Liquid Effluents:
Common Methods of Treatment and Control:
Liquid effluents generated in fertiliser plants vary in nature, quality and quantity depending upon various factors. The effluents generated in various sections of the plants are conveyed to the effluent treatment plant through segregated sewers.
This is because some of the effluents need separate specific treatment before common treatment, while others are used to meet the specific chemical requirement for the treatment of effluents in general.
Some common treatment and control measures are listed below:
(i) Process condensate strippers.
(ii) Air stripping followed by biological nitrification and denitrification.
(iii) Air strippers followed by recovery, biological nitrification and denitrifications.
(iv) Hydrolyser strippers.
(i) Process Condensate Strippers:
These are generally used in ammonia plants. The condensate produced from the shift conversion, CO2 absorption and methanation sections of a typical ammonia plant contains high concentration of ammonia, methanol and CO2. A typical quantity of condensate for a plant producing 900 tpd ammonia is around 50 m3/hr.
The condensate first flows to a CO2 flash vessel where most of the dissolved CO2 flashes off. From there this is sent to condensate stripper where mainly ammonia and methanol are stripped off in a distillation tower by means of stream at 4 kg/cm2 and at 180°C. The stripped condensate leaving the bottom of the tower contains only 10 ppm of ammonia and is used back in the plant completely.
(ii) Air Stripping Followed by Nitrification and Denitrification:
Air stripping to remove ammonical nitrogen is commonly used in most fertiliser units in the country. This works on the principle of dissociation of ammonium ion to free ammonia at a pH of 10–11.
The pH of the effluent is raised by the addition of alkali usually lime and the free ammonia is then stripped with air in a counter current stripping tower. This is followed by biological treatment viz. nitrification – denitrification. In this process, effluent is sent to the nitrification tank where residual ammonical nitrogen is converted to nitrates.
The process destroys alkalinity and lowers the pH which has to be adjusted by addition of Mg CO3. Aerators are provided in the nitrification tank for oxidation of ammonical nitrogen to nitrates. Effluent than goes to the denitrification tank where nitrates are anaerobically converted to nitrogen in the presence of carbon source like methanol or molasses.
Sludge removal is done in the final clarifier where polyelectrolyte is added to improve the settling characteristics. The sludge is sent to the biological reactor to maintain the required concentration of the micro-organisms and the rest sent to sludge lagoons.
(iii) Air Stripping with Recovery:
Air stripping invariably leads to contamination of the environment with the ammonia. When the ammonia emissions are high both qualitatively and quantitatively, in such situation, the alternative is to recover the ammonia as either ammonium sulphate or nitrate by scrubbing with the sulphuric or nitric acid. Packed absorbers can be used for the purpose.
For a plant generating 100 m3/hr. of effluent containing 1500 mg/litre of ammonical nitrogen the stripper-absorber utilises 1.5 – 2.5 lakh cu.m. of air/hr. for stripping the ammonia and 14 tonnes per day of nitric acid or 11 tonnes per day of sulphuric acid for recovery. Use of (NH4)2SO4 and NH4NO3 has to be found within the plant or has to be marketed.
The liquid effluent containing residual ammonical nitrogen (normally less than 250 mg/litre) can now be sent for biological treatment viz. nitrification.
Nitrification – denitrification is not attractive for high strength wastes, especially from the nitrophosphate plants whose effluents contain 1000-1500 mg/l of ammonical nitrogen and 1000 – 1500 mg/l of nitrate nitrogen.
(iv) Hydrolyser Stripper:
Urea is produced according to the reaction.
2NH3 + CO2 → CO (NH2) + H2O
It can be seen that urea production in all industrial plants is accompanied by an unavoidable formation of water.
Around 450 to 480 kg of effluent is generated for each metric tonne of urea produced. A typical composition (by wt.) of the effluent is 4-5% ammonia, 1.5-2% CO2 and 0.5-1% urea.
This effluent is pumped from a wastewater tank to a distillation tower operating at 2.5 kg/cm2. Before entering the distillation column, the effluent is preheated by means of the purified water flowing out of the bottom of the tower.
Since this wastewater contains urea, after stripping of ammonia in the upper part of the tower, it is pumped into the hydrolyser where urea is decomposed into ammonia and carbon dioxide by means of saturated steam at a pressure of around 26 kg/cm2g.
The hydrolyser is horizontal to provide plug flow, thus avoiding back mixing and also the continuous removal of the hydrolyser reaction products, to encourage the decomposition of urea to proceed further to completion.
Before entering the hydrolyser, the wastewater is preheated with that coming out of the hydrolyser. The vapours produced in the hydrolyser are sent to an overhead condenser, while the solution returns to the lower part of the tower where the remaining ammonia is stripped out by means of vapour produced in the reboiler which is fed by 4.5 kg/cm2 saturated steam.
The vapour leaving the top of the tower is condensed in the overhead condenser from where the carbonate solution flows to the reflux accumulator. Part of this solution is recycled back to the top of the tower as reflux. The remaining part goes to the low pressure condenser. The treated water containing only traces of ammonia is cooled at a temperature of 40°C and reused.
Depending on the operating pressures and temperatures, the processes have been classified into two groups:
(a) Hydrolysis i.e. operating pressure of 18 kg/cm2.
(b) Deep hydrolysis i.e. operating pressures of 40 kg/cm2.
The hydrolyser stripper system is highly energy intensive requiring large quantity of steam. Also the capital cost of the hydrolyser-stripper and associated equipment is also high. As an alternative to this, biological method of treatment of high urea concentration using enriched mixed culture is of microorganisms appear attractive. In this urea is converted biologically to ammonia and carbon dioxide.
Depending on the location, this can be either stripped with air or stripped with steam in a distillation column and the condensed liquid reused in the process. However, its reuse in the process has to be evaluated carefully to ensure that there is no carryover of the micro-organisms in the treated effluent which in turn can lead to bio-fouling of the equipment.
However, although not popular, it appears to be a process of the future. Considerable progress and R&D efforts in this direction are being made by a few fertiliser companies in our country.
Control of Fluorides and Phosphates:
Fluorides and phosphates in the liquid effluents are generated from the phosphoric acid and the complex fertiliser plants. These are normally removed by adding lime. This results in precipitation of fluoride as calcium fluoride and phosphates as mono and dicalcium phosphate/calcium hydro-xylapatite.
A two-stage treatment process involving the following steps is used to achieve low value of fluorides and phosphates:
(a) Mixing of wastewater with lime to raise the pH first to 8 and then to around 11- 12 to precipitate the fluorides and phosphates which are removed in two clariflocculators.
(b) Removal of the sludge and dewatering by solar evaporation in sludge drying beds.
(c) pH correction by acid dosing by acidic effluents.
Some other variances of this method have been reported in literature for the removal of fluorides to low levels:
(a) Lime treatment to pH 11 followed by adjustment to pH 6-7 with alum and polyelectrolyte coagulation or combined alum and sodium hexametaphosphate coagulation at pH 6.5 reportedly achieves 1-2 mg/l of fluorides.
(b) Lime and CaCl2 combinations can achieve 10-20 mg/l fluoride at approximately pH 8 as compared to lime treatment only achieving the same levels at pH 11.
(c) Activated alumina has reportedly been able to polish effluents to below 1 mg/l fluoride.
Control Reduction of Chromates:
Chromates in the form of sodium chromate or dichromate are added as a corrosion inhibitor in circulator cooling waters for heat exchange equipment. To maintain a certain level of total dissolved solids in the cooling water, a small quantity has to be discharged termed as “blow-down”. This blow-down contains 15-20 mg/l of chromates in hexavalent form.
The removal of this is achieved by lowering the pH of the effluent to 2-3 and then reducing the hexavalent chromate to its trivalent form. This is then precipitated using lime or caustic soda as Cr (OH)3. However, due to the recent regulations disposal of this Cr (OH)3 is also posing problems.
It is, therefore, likely that non-chromate cooling water treatment will become popular. Alternatively where chromates continue to be used, selective ion exchange system for treating the blow-down may have to be installed. Also methods to dispose of Cr (OH)3 sludge are to be exposed. Both these alternatives are currently available.
Discharge of Effluents:
Once the minimum national standards (MINAS) are met, the effluents can be safely discharged into the receiving body or public sewers. However, in some cases (for coastally located plants) a marine outfall discharge system with diffusers for better dispersion may become necessary.
In any industry waste is bound to generate and it cannot be avoided. It is more important to utilise these wastes in such a way that value product can be made out of it. This not only eliminates disposal of wastes but at the same time generates profit for the company.
A number of innovative projects were undertaken and are coming up as a result of which today GNFC is able to utilise most of its pollutants generated for making value added products. Many outside parties have also developed some projects to utilise pollutants from GNFC into useful products.
Details regarding utilisation of solid wastes are as follows:
1. Utilisation of Carbon Sludge in Boilers as Fuel:
During startup and shutdown and in the blow down, little quantity of carbon slurry is generated. This slurry is stored in carbon slurry pond, where settling of carbon takes place and overflow is treated for removal of ammonical nitrogen. The carbon which gets accumulated is required to be removed periodically.
The calorific value of this carbon sludge is 8547 K.cal./kg and it can be utilised in boilers for generating steam.
2. Utilisation of Spent Hard Coke in Boilers:
The effluent coming from ammonia plant is expected to contain 200 ppm of oil. To remove this oil from effluent, gravity separation followed by hard coke adsorption facility can be used.
As hard coke gets soaked with oil, it is required to be replaced by fresh hard coke. The oil soaked coke removed from effluent treatment plant is utilised in boiler which results in saving equivalent quantity of coal.
3. Utilisation of Lime Sludge:
Lime sludge is generated while treating effluent for removal of ammonical nitrogen. The effluent coming from plant is treated with hydrated lime to increase the pH and thereafter effluent is taken to stripping tower to remove the ammonical nitrogen.
During the reaction, lime sludge is generated, which can be stored in brick lined pond from where it is supplied to customers which is ultimately used for manufacturing of sagol and neutralisation. This lime sludge is also used for flooring and tiles making purpose. Along with ash, lime sludge will also be utilised for making building materials.
4. Utilisation of Inerts Generated in Ammonium Nitro Phosphate:
During digestion of rock phosphate some portion of rock phosphate remains as inert which is an unavoidable by-product in ammonium nitro phosphate plant.
The inert generated can be utilised as filler material to manufacture 20:20:0 grade ANP. This not only eliminates disposal problem of inert material, but also avoids expenditure on ingredient required for manufacture of 20:20:0 grade ANP.
5. Utilisation of Fly-Ash:
Ash is produced as a result of burning of pulverised coal in fired boilers. The ash is conveyed to slurry dump and is converted into slurry and transported to ash pond through pipeline.