Productization of Raw Water Treat- Ment Process for Mineral Processing Industry

Jussi-Pekka Huttunen Productization of Raw Water Treat- ment Process for Mineral Processing Industry

Metropolia University of Applied Sciences Bachelor of Engineering Chemical Engineering Bachelor’s Thesis 15 May 2020

Abstract

Author Jussi-Pekka Huttunen Title Productization of Raw Water Treatment Process for Mineral Processing Industry Number of Pages 28 pages + 1 Appendix Date 15 May 2020

Degree Bachelor of Engineering

Degree Programme Biotechnology and Chemical Engineering

Professional Major Chemical Engineering

Instructors Sakari Hiidenheimo, Product Manager Timo Seuranen, Senior Lecturer

This bachelor’s thesis was commissioned by Outotec (Finland) Oy. The aim of the thesis was to produce standardized process flow diagrams of a raw water treatment process. The process flow diagrams can be used as references and models when designing a water treatment plant. In addition, the written part of this thesis can be used as a design guide to support the process flow diagrams.

In order to draft the process flow diagrams, the subject of raw water treatment was studied from several written sources. The process was divided into parts according to the uses of specific water types. Most commonly used unit processes and equipment were chosen for each water type and the operating principles of these equipment were studied to draft the process flow diagrams.

A process flow diagram was drafted for each of the unit processes covered in the text. A single diagram only represents a part of a larger process for water treatment. The process flow diagrams were drafted with MicroStation which is a CAD software widely used process design tasks. The diagrams were drafted based on the knowledge learned from written sources as well as according to reference diagrams from older plant projects. The diagrams were drafted so that they could be easily copied or modified for the project being worked on.

The goals of this bachelor’s thesis were accomplished well. As a result of the thesis, eight individual process flow diagrams were drafted, all of which can be easily utilized during process designing. In the future the process flow diagrams could be used as a basis for drafting piping and instrumentation diagrams (P&I-diagrams).

Keywords raw water, water treatment, process flow diagram

Tiivistelmä

Tekijä Jussi-Pekka Huttunen Otsikko Raakavesiprosessin tuotteistaminen mineraalien jalostusteol- lisuutta varten Sivumäärä 28 sivua + 1 liite Aika 15.5.2020

Tutkinto insinööri (AMK)

Tutkinto-ohjelma bio- ja kemiantekniikka

Ammatillinen pääaine kemiantekniikka

Ohjaajat tuotepäällikkö Sakari Hiidenheimo lehtori Timo Seuranen

Insinöörityön toimeksiantaja oli Outotec (Finland) Oy. Työn tavoitteena oli tuottaa vakioituja virtauskaavioita raakavedenkäsittelyprosessista. Tuotettuja kaavioita voi käyttää referens- seinä ja malleina vedenkäsittelylaitosta suunniteltaessa. Lisäksi työn kirjallista osuutta voidaan käyttää suunnitteluoppaana virtauskaavioiden tueksi.

Virtauskaavioiden piirtämistä varten perehdyttiin raakavedenkäsittelyprosessiin, joka jaettiin osiksi tuotteina saatujen vesien käyttökohteiden perusteella. Jokaista vesilaatua kohden va- littiin yleisimmin teollisuudessa käytettyjä yksikköprosesseja ja laitteita, joiden toimintaperi- aatteet selvitettiin virtauskaavioiden piirtämistä varten.

Kirjallisen perehtymisen pohjalta laadittiin virtauskaavio jokaista tekstissä käsiteltyä yksik- köprosessia kohden eli yksittäinen kaavio kuvastaa vain tiettyä osaa koko käsittelyproses- sia. Virtauskaaviot laadittiin MicroStation ohjelmistolla, joka on laajassa käytössä prosessin- suunnittelutehtävissä. Kaavioiden piirrossa hyödynnettiin kirjallisista lähteistä opittua tietoa sekä yrityksen vanhoista projekteista saatuja referenssikaavioita. Kaaviot piirrettiin sellai- siksi, että ne voi tarpeen mukaan kopioida ja muokata suoraan työstettävään projektiin.

Insinöörityön tavoitteet saavutettiin hyvin. Työn seurauksena laadittiin kahdeksan virtaus- kaaviota, joita voidaan helposti hyödyntää prosessilaitosta suunniteltaessa. Tulevaisuu- dessa virtauskaavioita voidaan käyttää pohjana putki- ja instrumentointikaavioiden (PI-kaavioiden) tuottamiseen.

Avainsanat raakavesi, vedenkäsittely, virtauskaavio

Contents

List of Abbreviations

1 Introduction 1

2 Background 1

3 Pretreatment 3

3.1 Bar-screening 3 3.2 Straining & filtration 5

4 Cooling water treatment 8

5 Dust suppression, firefighting water and process water treatment 9

6 Chemical precipitation 10

6.1 Addition of chemicals and reagents 10 6.1.1 pH adjustment 11 6.1.2 Coagulation 11 6.1.3 Flocculation 11 6.2 Dissolved air flotation (DAF) 13 6.3 Sedimentation 14 6.4 Sand filtration 15

7 Softening 16

7.1 Lime-ash method 17 7.2 Ion exchange 17

8 Potable water 18

8.1 Activated carbon treatment 19 8.2 Disinfection 20

9 Desalination 21

9.1 Reverse osmosis (RO) 21 9.2 Ultrafiltration (UF) 22

9.3 Evaporation 23

10 Process flow diagrams 24

11 Summary 27

References 29

Appendices Appendix 1. Process flow diagrams

List of Abbreviations

PFD Process flow diagram. Diagram used to indicate the general flows of processes and equipment

DAF Dissolved air flotation. Process used to treat water utilizing air dissolved to water.

PAC Powdered activated carbon. Activated carbon that has been ground to fine powder. Used in activated carbon treatment processes.

GAC Granular Activated carbon. Activated carbon that has been ground to powder with grain size larger than powdered activated carbon. Used in activated carbon treatment processes.

RO Reverse osmosis. Membrane process used to separate salts and minerals from saline water.

UF Ultrafiltration. A form of membrane filtration which separates pollutants via a semi permeable membrane.

1 Introduction

This bachelor’s thesis was carried out for Outotec (Finland) Oy and is mostly literature based. The aim of this thesis was to study the process of raw water treatment and to produce standardized flowsheets to use as templates for designing raw water treatment plants in future projects. The water treatment process was examined from the perspec- tive of mineral processing industry.

The first step of the thesis was to study the actual process of raw water treatment. The process was split into several parts based on a typical treatment process chain, purified water usage points and a pre-existing flow sheet. All the major parts of the process were described in their own respective chapters. This written part of the thesis is to be used as a handbook for the design of raw water treatment processes. The goal was to give basic information and principles of operation of each stage of a water treatment process to aid in design work.

The second step was to produce individual flowsheets for each part of the process. The process flow sheets were made to be standardized representations of unit processes to treat raw water for different purposes within mineral processing plants. The flow sheets were drafted to aid process design engineers when designing raw water treatment processes for mineral processing plants. The drafted diagrams do not include all the available technologies which are described in this thesis. However, they illustrate some of the most common unit processes used in water treatment.

2 Background

Mineral processing plants require very large amounts of water to operate efficiently. To- day when sustainability and environmental friendliness are increasingly important values in industries the efficient and highly optimized use of resources is essential [1].

Nowadays mineral processing plants are being built farther and farther away from cen- ters of population, which often means that water for the process must be acquired from the site. Because the locations of these plant sites rarely have fresh water piping the

2 most common sources of water are raw water sources both surface water and ground water. Examples of surface water sources are lakes, rivers and streams. Due to the remote locations of many mineral processing plants a water treatment facility is often designed and situated near the actual plant. This will enable the new plant to make use of resources available on site as effectively as possible.

Different water types used in mineral processing plants are listed and described below in Table 1 [2].

Table 1. Water types used in mineral processing plants

Water type/use Description

Firefighting water Large volumes of solid free water Dust suppression water Process water Different qualities depending on mineral and unit process Chemically treated water Water for chemical make-up and gland water Demineralized water Process cooling circuits

Potable water Drinking water, lab water, safety shower and chemical water Desalinated water Process water desalination

Most of the types of water described in Table 1 require specific unit processes and equipment to accomplish the desired grade and purity. Whereas water for firefighting and dust suppression can be treated simply with just mechanical separation of solids, the treatment process for potable water is far more complex and requires many different unit processes.

Water treatment processes often progress step by step. Each step of the process tends to a specific goal and produces water for a particular unit operation or need. A single unit process is rarely enough for a whole water treatment process, because the equipment used in a later part of the process, for instance potable water treatment, require the water to be pre-treated to begin with. Table 2 is a simple representation of what treatment stages are needed for a specific type of water. If a cell in the table is marked with an x- symbol the treatment stage described on the top row is needed to achieve the water type described on the leftmost column.

Table 2. A representation of treatment stages needed for a specific water type

Screening Chemical Cooling water Softening Potable wa- Desalination + filtration precipitation treatment ter treatment Solids free water x

Cooling water x x x x Potable water x x x Desalinated water x x x Demineralized/sof- x x x tened water

3 Pretreatment

The first part in most if not all raw water treatment processes is the mechanical separation of suspended particles. The goal is to remove large objects and most of the larger contaminant particles. The removal of these objects and particles is essential to protect structures, equipment and piping downstream from blockages and mechanical damage. This will also prevent solids and debris transported by the raw water from interfering with the efficiency of the process downstream [3]. Mechanical separation can be done via the combined use of grids, screens and filters depending on the desired grade of water.

Mechanical separation of objects and particles is normally done in multiple stages, with each consequent stage removing smaller sizes of particles and objects from the water [3].

3.1 Bar-screening

The first unit of all water treatment plants is usually a series of bar-screens with differing gaps separating the screen bars [3]. The first bar-screens in a water treatment plant are coarse screens which are meant to remove larger solids from the water flow such as tree branches and other debris. After the coarse screening is fine screening which is used to separate smaller objects from the water flow [4]. The bar gap will decrease as the water

4 flow continues along the process. The bar gap can differ from larger than 40 mm with coarse screening down to around 6 mm with fine screening. Figure 1 shows a group of bar screens used in water treatment.

Figure 1. Bar screens used in water treatment [3]

The debris collected by the screens will remain against the screening elements. That’s why screens must be cleaned and unblocked periodically to avoid chocking the water flow to the rest of the process. Depending on the level of automation in the water treatment plant the bar screens may be automated or manual. Manual screens must be cleaned by hand whereas automated screens are automatically cleaned intermittently [4].

The advantages of manually cleaned screens are that they require very little maintenance. This makes them well suited for smaller plants with less individual screens. On the downside, manually cleaned screens require more labor in the form of frequent raking and removal of caught solids. This can also affect the other screens’ ability to capture solids downstream due to flow surges caused by manual raking [4].

Automatically cleaned screens have the advantage of reduced labor due to the removal of caught solids being completely automated. Automatic screens do not suffer from the same problems as manual screens described above. Therefore, they are better suited for larger treatment plants. The disadvantage of automatically cleaned screens comes from the higher procurement and maintenance costs [4].

3.2 Straining & filtration

Particles that are too small to be removed via the use of bar screens can be removed with the use of strainers. Strainers can be effective in the removal of large amounts of suspended solids from the treated water [3].

Strainers often consist of a perforated metal sheet with differing perforations depending on the fineness. Other common strainer element type is a stainless-steel cross-mesh. Drum and rotating strainers are a common type of strainer used in water treatment. They have a rotating drum which separates the solids from the waterflow by letting water pass and by carrying the solids away to be removed [3]. An illustration of a drum strainer can be seen in Figure 2.

Figure 2. Rotating drum strainer [3]

Suspended solids can also be removed from the water via filters and most commonly pressure filters. The two often used pressure filtration processes are bag filtration and cartridge filtration. In bag filtration the water flow is forced through a bag shaped filtration unit which includes the filter media. The solid particles are caught in the filter media and the treated water will pass through. The typical micron rating of bag filters is from 1 to 40 microns [5]. The working principle of a bag filter is illustrated in Figure 3 below.

Figure 3. Working principle of a bag filter [6]

Cartridge filters typically consist of a fabric or membrane which is placed around a filter element. The filter element is then housed inside a pressure vessel into which the water is fed. Figure 4 shows an assortment of filter cartridges and cartridge filter vessels. Car- tridge filters have a micron range of around 0.3 to 80 microns. The filters used often also require backwash to clean the filter media and to ensure the proper operation of the filters [5].

Figure 4. Different filter cartridges and cartridge filter vessels [7]

Another filter type utilized in water treatment processes are automatic filters. Automatic filters operate continuously and include systems which allow the filters to operate and self-clean automatically. Automatic filters require very little manual labor making them altogether low maintenance and therefore fairly cost effective. They can filtrate particle

8 sizes of around 800 µm – 2 µm. Automatic filters can be utilized in many parts of the water treatment process making them very versatile [8]. Figure 5 shows an automatic screen filter.

Figure 5. Automatic screen filter package [9]

4 Cooling water treatment

The three main problems that affect cooling water systems and the equipment in them such as cooling towers and heat exchangers that must be solved are listed below:

 biological growth  scaling  corrosion

The treatment of cooling water is largely chemical treatment. The chemicals used affect different factors and pollutants [3].

The water used as cooling water should also be free of suspended solids to prevent blockage of pipes and instruments. This can be ensured with the use of dispersing

9 agents which prevent the leftover particles from forming deposits in areas where flow speeds are lower. This will also prevent biological growth.

Oxidation and the use of biocides are common methods for dealing with fouling and biological growth. The most common method of oxidation is the addition of chlorines which also work as biocides. Other common oxidizers are brominated derivatives, ozone, and hydrogen peroxide. The purpose of oxidizers and biocides is to eliminate biological pollutants and algae from the water. This will prevent fouling and growth on the cooling system and will also prevent scaling.

Both corrosion and scaling can be prevented with the use of inhibitor chemicals. These inhibitors can affect different properties on the water such as pH and hardness. This will prevent the precipitation of, for example calcium carbonate and control the pH of the water so that it will not corrode the equipment and pipes [3].

Depending on the process the chemicals may be fed periodically or continually. This will require feeding tanks or replaceable barrels of the needed chemicals and chemical pumps to deliver the chemicals to the water stream.

5 Dust suppression, firefighting water and process water treatment

The amount of purifying required for process waters is highly dependent on the usage point of the water in the process. Different unit processes and equipment require differing grades of purified water. Therefore, the requirements for process water and the means to achieve the defined requirements should be reviewed case by case. However often the removal of suspended solids is enough for most process waters. For this reason, process water has been grouped with dust suppression and firefighting waters in this bachelor’s thesis.

Water used in dust suppression systems and firefighting have more straight forward requirements. With these water types the main concern is not the purity of the water. Ra- ther because dust suppression equipment typically sprays the water through nozzles, the water being used needs to be free of suspended solids. The same applies to

10 firefighting water, which is typically stored in tanks and when used is sprayed through nozzles. For this, the water may need to be filtrated further to remove any solids that might clog or block the hoses or nozzles [2].

This can be achieved with further filtration or straining with ever decreasing grades. The solids-free water is then often stored in tanks to be used when required.

6 Chemical precipitation

Chemical precipitation is a process used to remove particles which are too small to be removed via mechanical separation. Chemical precipitation includes multiple steps and makes use of different chemicals to optimize the separation of solids from the water. The steps involved in this phase of the water treatment process are listed below [10].

 pH adjustment  Addition of coagulation and flocculation chemicals  Sedimentation or dissolved air flotation (DAF)  Filtration

Most of the contaminants found in raw water are present as particles. These particles include contaminants such as clay, silt, sand, insect eggs, bacteria and viruses. Many of these have dissolved into the water medium are therefore cannot be removed via straining or other means of mechanical separation. Some of the contaminants listed can be dealt with by disinfecting the water, but excessive use of disinfectants can cause problems with the taste of the water. Also, some parasites cannot seem to be killed via disinfection. It is important to remove as much of the contaminants well before the actual disinfection stage to prevent scaling and to protect the process [11].

6.1 Addition of chemicals and reagents

Chemical precipitation process requires the addition of different chemicals to the water feed. The chemicals used are pH controlling chemicals, coagulants and flocculants. The latter mentioned make the suspended contaminants form larger particles which are

11 easier to separate. The coagulants and flocculants used in the process are very dependent on the pH of the water medium. Therefore, the pH must be set at the beginning of the precipitation process [12].

6.1.1 pH adjustment

The pH of most raw water is between 6.5–8.5 [13]. Some of the chemicals used for pH adjustment of water are sodium hydroxide and different oxides such as lime. These chemicals are used to raise the pH of the water. However, if the pH needs to be lowered some acids such as acetic acid, citric acid, sulfuric acid and hydrochloric acid can be used [14]. The chemical used depends of course on the pH of the water feed. The chemicals can be added to the water feed via inline mixers or in separate mixing tanks.

6.1.2 Coagulation

The next step is to add coagulants to the water feed. Coagulants can be also used to adjust the pH of the water in cooperation of the chemicals described in section 6.1.1. The main task of coagulants, however is to coagulate the solid particles in the water. This means that the particles start to form larger clump of solids in the water. The most common coagulants are iron or aluminum-based chemicals, such as ferric chloride and aluminum chloride [15]. These chemicals have very specific operating ranges in terms of pH of the water feed which makes the afore mentioned adjustment essential. The optimal pH range differs according to the coagulant being used. Therefore, pH control should be reviewed case by case [3].

The actual addition of the coagulants is often done in a tank equipped with a mixer. The clumping of the solids requires some time and the mixing will encourage the formation of larger clumps.

6.1.3 Flocculation

After coagulation, the water will be directed to another mixing tank which will then be used to add flocculants to the water. The flocculant is often added as a liquid solution. The point of flocculation is to collect the now formed coagulated particles into even larger

12 particles. These particles are often visible and in resemblance of snowflakes. While forming, the flocs need to be supplied with energy to induce the formation of even larger flocs. This can be achieved by agitating the water via the use of a mixer.

Figure 6. Simple illustration of coagulation and flocculation

The level of agitation should decrease as the flocs grow. In the first stages of floc formation the establishment of rapid floc formation requires high amounts of energy. There- fore, the level of mixing should be relatively higher during the beginning of flocculation. As the flocs grow and form ever larger particles the level of mixing should decrease. This is because too high mixing speed might cause the larger flocs to break apart [12]. For this reason, the efficient use of flocculants will require the use of two-part mixing units or two separate mixing tanks. Figure 6 shows a simple illustration of coagulation and flocculation.

6.2 Dissolved air flotation (DAF)

Dissolved air flotation (abbreviated as DAF) is a water purification process which is nowadays often used in as an alternative to sedimentation in water treatment plants. As a unit process DAF consists of multiple components as shown in Figure 7. In dissolved air flotation the suspended particles and flocs are separated from the water medium via the use of small air bubbles with diameters of around 30–50 μm. The particles are caught in these bubbles and are lifted to the top of the tank from where they can be collected by a scraper or just by overflowing. The purified water can then be drained and directed downstream from the mid-point of the flotation tank.

Figure 7. Diagram of a typical DAF system [113]

The bubbles are formed using so-called dispersion water. Dispersion water is made by dissolving high amounts of air into water in high pressures. The amount of air that can be dissolved in water can be increased by raising the pressure. The typical pressure used to prepare dispersion water is 4 – 6 bar. This requires the use of a pressure vessel such as a tank into which the water and pressured air can be directed. The dispersion water is then directed into the bottom of the flotation tank through a pressure regulator. When the dispersion water enters the flotation tank all the excess air is released from

14 the water as small bubbles. This is caused by the sudden drop of pressure since dissolved air flotation tanks are often open on the top [16].

The advantages of DAF when compared to more traditional methods like sedimentation is that it is much faster. Sedimentation is largely operated only via gravity which makes it much slower. DAF is also a much more reliable method for water purification than sedimentation [16].

6.3 Sedimentation

Sedimentation is a method which can be used instead of or in conjunction of dissolved air flotation. Sedimentation is an alternative method to separate the suspended and flocculated particles from the water medium. In sedimentation the water which has been treated with coagulants and flocculants is directed into large tanks called clarifiers. Sed- imentation as a process is very similar to thickening, the difference being that clarifiers are used to purify water and thickeners are used to concentrate solids [17]. Visually clarifiers appear very similar to thickeners as can be noted from Figure 8.

Figure 8. Cutaway picture of a clarifier. [17]

The water which has been treated with coagulants and flocculants is fed into the clarifier. Once in the clarifier the flocculated solids start to decent to the bottom of the tank via gravitation. The solids will form a bed of sludge on the bottom which will then be pumped out of the clarifier. The purified water can then be drained or pumped from the top portion of the clarifier. Clarifiers such as thickeners also typically include a rake which slowly stirs the sludge bed formed on the bottom of the tank. The purpose of this is to release any trapped liquid or gas from the sludge bed.

The disadvantage of sedimentation is its noticeably slower speed when compared to DAF. This often leads to much larger clarifier units than what the equivalent DAF unit would be. The settling of solids cannot be accelerated; thus, gravity is the only factor pulling the solids towards the bottom of the clarifier. On the other hand, sedimentation typically requires less energy than a DAF system [18].

6.4 Sand filtration

The final part of chemical precipitation chain is often a sand filter. Sand filters are a type of gravity or pressure filters where the water feed will flow through multiple layers of sand of varying grades. The coarsest sand is typically on the top of the filter (top feeding filters) and below are layers of ever finer sand. Any remaining suspended solids will be caught between the grains of sand while the water passes through the filter. The sand layers are typically supported by a perforated sheet at the bottom of the filter as can be seen in Figure 9. The filtrate can be removed from the bottom of the filter or from outlets situated along the side of the filter depending on the type filter used [19].

Figure 9. Cutaway illustration of a sand filter [19]

Despite their name, sand filters might also use different filling materials than sand such as plastic granules or anthracite. The actual operation of the filter is still the same re- gardless of the filter media [19]. Sand filters are a very efficient and low-cost method for removing suspended solids from water. They do, however require maintenance periodically in the form of backwash by flushing the filter media. This will prevent the filter from clogging up. The backwashing can be improved by blowing air into the filter via blowers [20].

7 Softening

Softening is done to remove dissolved minerals such as calcium and magnesium salts from the water. These salts make water hard; therefore, their removal will make the water softer. The aim of softening is to prevent the blockage of pipes caused by the

17 precipitation of minerals and corrosion. The mineral salts can be removed via the use of chemicals or with ion exchange process [12].

7.1 Lime-ash method

The use of chemicals for water softening is often referred as the lime-ash method. The name is derived from the most common chemicals used for softening which are sodium carbonate (also known as soda ash) and calcium hydroxide (also known as slaked lime). These chemicals are mainly used to precipitate the magnesium and calcium from the water making them easier to remove. However, the use of said chemicals requires an additional separation phase such as sedimentation or filtration to remove the now formed precipitates from the water [21]. Figure 10 depicts a water softening process via the lime- ash method.

Figure 10. Water softening process using the lime-ash method [22].

7.2 Ion exchange

The other method to soften water is an ion exchange process. The working principle of an ion exchange process is to switch the undesired and harmful contaminants with other less objectionable ionic substance. Hard water containing magnesium and calcium salts is softened by switching the calcium and magnesium ions with sodium ions. This is done

18 by letting the water flow through columns which include ion exchange resins. Ion exchange resins are porous materials which allow the water to flow through. Common materials for the resins are polystyrene and polyacrylate. The resin is saturated with sodium ions which will be exchanged with the magnesium and calcium ions.

Figure 11. Ion exchange process and backwash [23]

After numerous uses ion exchangers start to let magnesium and calcium ions through due to the resin becoming too saturated with said ions. Therefore, the resin must be regenerated/recharged periodically by backwashing it with brine solution which contains sodium. This backwashing is illustrated in Figure 11 above. If the ion exchanger is regenerated properly, it is a very reliable process for removing dissolved inorganics [23].

8 Potable water

The treatment of potable water is a multi-stage process consisting of removing any pollutants from the water and disinfecting the water to kill any micro-organisms such as bacteria, viruses and parasites. The full process which is required before the actual

19 potable water treatment stage is once again very dependent on the source of the treated water since water from different sources contain different amounts of different pollutants. For an example, ground water is generally cleaner than surface water and therefore requires less treatment to purify. The other required unit processes which are needed prior are described in chapter 2 and Table 2 above. Figure 12 shows an illustration for a possible treatment process for potable water.

Figure 12. Potable water treatment process [24].

Potable water treatment commonly done by purifying the water with activated carbon. Any bacteria or parasites can then be eliminated by disinfecting the water.

8.1 Activated carbon treatment

Water can be purified with activated carbon filtration which is often used to remove organic compounds and synthetic organic chemicals from the water. It is also used to ad- dress taste and odor problems which is why it’s often one of the final steps in a potable water treatment process. Activated carbon is available in two forms: powdered activated carbon (PAC) and granular activated carbon (GAC). Due to its highly porous composi- tion, activated carbon is very effective at absorbing contaminants from water [25].

In the water treatment processes PAC can be fed to the process as a dry powder or as a slurry via pumps. Dry feeding is often used if the need for PAC is infrequent or the doses are smaller. Slurry feeding systems are used if the need is more frequent and dosages larger. Different equipment such as hoppers and tanks are needed depending on the dosage method of PAC [25]

Activated carbon should be added to the water flow as early in the process as possible to allow time for the carbon to absorb any pollutants. For taste and odor control a mini- mum time of 15 minutes is often required. PAC should not be added to the water at the same time as disinfectants such as chlorine because these chemicals can be absorbed by the PAC [25].

Activated carbon can also be utilized in filters as a solid block. In this case, the activated carbon has been mixed with a binder and pressed into a solids block which is utilized as a cartridge inside the filter. Filters with solids blocks of activated carbon are often able to filter more pollutants from water than filters utilizing PAC or GAC. This is due to the fact that the carbon in the block is much denser and therefore able to filter more pollutants from the water flow [26].

8.2 Disinfection

Before its distribution potable water needs to be disinfected to remove any harmful micro- organisms. Disinfection has to important steps that affect the choice of disinfectant. The first one is the disinfectant’s bactericidal effect, which is the disinfectant’s capacity to kill micro-organisms in the water during the treatment. The other one is residual effect which is the disinfectant’s capacity to remain in the water after the actual treatment and therefore to maintain the water biological quality [3].

Before disinfection, the water should be as solids free as possible because bacteria and viruses tend to collect on suspended solids. This can protect the harmful micro-organisms from the disinfectant. Common disinfectants/methods used in the disinfection of raw water are listed below:

 Chlorine (Cl2)

 Chlorine dioxide (ClO2)

 Ozone (O3)  Chloramines  UV irradiation

All methods listed above have differing qualities when it comes to their ability to eliminate micro-organisms. These qualities are shown below in Table 3.

Table 3. Qualities of commonly used disinfectants [3].

Effect O3 Cl2 ClO2 Chloramines UV Bactericidal & viricidal +++ ++ ++ + ++ Protozoa cysts + 0 0 0 +++ Residual effect 0 + ++ +++ 0

If used in too high quantities, disinfectant chemicals can cause unwanted taste and odor problems which can make the water undrinkable. The only disinfectant that causes no taste or odor changes is UV irradiation. The drawback of UV is that it has no residual effect, which means that after irradiation the water is no longer protected from micro- organisms. Because of this, combinations of different disinfectants are often used. This will allow disinfection that will effectively eliminate micro-organisms and has good residual effect after treatment [3].

9 Desalination

On some occasions the water from the raw water source might be saline, such as seawater or brackish water. On these occasions the water must desalinated to make the water potable for instance.

9.1 Reverse osmosis (RO)

In reverse osmosis the water is forced through a semi-permeable membrane via pump- ing. The membrane will allow water molecules to pass, but salt molecules which are

22 larger will be blocked by the membrane. The pressure applied to the water must be higher than osmotic pressure for the process to work. The water being fed to the RO must be free of pollutants such as suspended solids, clay, bacteria and oils in order to avoid fouling of the membrane [27].

Figure 13. Comparison between the effective ranges of different desalination technologies [28].

Out of the technologies available for desalinating seawater or brackish water RO is the most economical in terms of capital investment and energy usage. It is also the most versatile being able to treat water with differing salt concentrations [28]. The comparison of different desalination technologies in terms of their ability to treat water with different salt concentrations can be seen in Figure 13.

9.2 Ultrafiltration (UF)

Ultrafiltration is a similar process to reverse osmosis, but instead of a semi permeable membrane UF uses hollow fiber membranes which filters water from inside out or outside in. The working principle of an ultrafiltration membrane can be seen illustrated in Figure

14. The hollow membranes used in ultrafiltration typically have a good chemical re- sistance to chemicals such as chlorine whereas membranes used in RO cannot tolerate chemicals like chlorine [29].

Figure 14. Ultrafiltration membrane [30].

RO has a much better ability to remove pollutants due to the much smaller pore size when comparing to UF. This level of filtration is not, however always necessary. One of the advantages of UF in addition to the ability to withstand chemicals is that UF normally operates at much lower pressures than RO eliminating the possible need for booster pumps [29].

9.3 Evaporation

Evaporation is a desalination process which can be used as a stand-alone process or in conjunction with other methods such as reverse osmosis. Evaporator units are used to separate salts from water by vaporizing as much of the water medium as possible and leaving behind solid salts or a liquid which has a high concentration of salt [31].

Figure 15. Illustrative diagram of an evaporator used in desalination [32].

In evaporation units the saline water or brine is fed into a vessel through heat exchangers. The heat exchangers usually utilize steam to heat the water flowing through them. Once inside the evaporator tank the water is flown and circulated through heat exchanger cartridges. The condensate, which is mostly salt free water, is collected from the tank at the end of the treatment. The now concentrated minerals and salts can also be collected and disposed [33]. Figure 15 illustrates an evaporation process.

10 Process flow diagrams

As a result of the thesis, seven individual process flow diagrams (PFD) were designed. Each diagram corresponds with a chapter in the text part of the thesis and is captioned according to the text. The PFD’s offer a standardized representation for a unit process in raw water treatment. The auxiliary flows such as pressured air, filter slurry and backwash have not been included to all of the PFD’s. The equipment drawn in the PFD’s represent different options and the equipment utilized in certain projects should always be implemented to the reference PFD’s.

The first diagram (PFD 1 in Table 4) shown in page 1 of Appendix 1 corresponds with chapter 3 of the text and illustrates a simple process for removing solids from the initial raw water flow. The water flow is first run through a bar-screen which separates most of

25 the larger solids from the water. The water is next run to a filter feed tank and from there pumped to filters. The filters are drawn as pairs in all the diagrams to represent a running filter and a stand-by filter which can be used, for instance during maintenance. The now filtrated water is then stored in a storage tank and pumped forward.

The second diagram (PFD 2 in Table 4) shown in page 2 of Appendix 1 corresponds with chapter 4 of the text. The PFD shows the treatment process for water used in cooling circuits. The water is first run through filters to remove remaining suspended solids. Next, chemicals such as biocide and oxidizers are added to the water flow via an inline mixer. the treated water is then run to a storage tank and forward to the process.

The third diagram (PFD 3 in Table 4) shown in page 3 of Appendix 1 corresponds with chapter 5 of the text and shows the treatment processes for firefighting, dust suppression and process waters. The water flow is first filtrated to remove suspended solids that might block the process or equipment downstream. The main flow is then split into three flows for firefighting water, dust suppression water and process water. The dust suppression and firefighting water are simply pumped into storage tanks and from there to the downstream process. The process water treatment should be reviewed and drafted case by case due to each process/plant having differing requirements for process water.

The fourth diagram (PFD 4 in Table 4) shown in page 4 of Appendix 1 corresponds with chapter 6 of the text. The diagram shows a chemical precipitation process which includes a DAF unit and sand filtration. The water flow is first pumped to the DAF unit from a feed tank. The DAF unit includes a coagulation/flocculation tank and the DAF cell itself. After the flotation, the water flow is run through sand filters. The PFD also includes the auxiliary flows required by the DAF and sand filters. The production and distribution of coagulant and flocculant are shown in the diagram as well as the distribution of alkali and acid for pH control before the DAF unit.

The fifth diagram (PFD 5 in Table 4) shown in page 5 of Appendix 1 corresponds with chapter 7 of the text and it shows a water softening process utilizing an ion exchanger. The water is first run through filters to remove solids. The diagram then shows a pair of anion and cation exchangers and resin traps. The ion exchangers are regenerated with

26 the use of an alkali solution and acid solution, both of which have been illustrated in the diagram.

The sixth diagram (PFD 6 in Table 4) shown in page 6 of Appendix 1 corresponds with chapter 8 of the text. The diagram illustrates the treatment process for potable water utilizing an activated carbon filter and disinfection. The water is first treated with an activated carbon filter to remove any pollutants and unwanted compounds. The water is then disinfected with either the use of a disinfectant mixed via an inline mixer or with UV- irradiation treatment. Once again, the method for disinfection should be chosen according to the need of the process.

The seventh (PFD 7 in Table 4) diagram shown in page 7 of Appendix 1 corresponds with chapter 9 of the text. It shows a process for desalinating saline or brackish water with the used of a reverse osmosis (RO) process. The water is pumped to the RO from a feed tank. First, the water passes through a filter to protect the equipment from any suspended solids. The water is the pumped through two membranes to separate the salts from the water medium. The PFD’s features two RO units with the other one being a stand-by unit to be used during maintenance. The brine from the RO can be treated with an evaporation unit if necessary. The PFD also incorporates a clean-in-place (CIP) unit to wash the RO membranes when needed.

The eight and final diagram (PFD 8 in Table 4) shown in page 8 of Appendix 1 is a continuation to the RO diagram. This diagram shows an evaporation unit which treats the brine from the RO unit. The brine which is fed to the evaporator tank is first heated via heat exchangers utilizing steam. Inside the tank the brine is flown through heat exchanger cartridges. Circulation pumps circulate the brine further down the tank. The treated condensate is then cooled with a heat exchanger and pumped to a storage tank. The saline concentrate is pumped from the evaporator tank to a tailings sump.

The reference diagrams can be combined to form a complete water treatment process according to all the water types needed. The diagrams needed for a specific water type are described in Table 4. The leftmost column shows the different water types and the top row shows the diagrams captioned according to the text. The diagrams needed for the treatment process of a specific type of water are marked with x symbols.

Table 4. Diagrams needed for specific water types in a treatment process

Water type Diagrams needed for complete treatment process

PFD 1 PFD 2 PFD 3 PFD 4 PFD 5 PFD 6 PFD 7 & PFD 8 Solids free wa- x ter Dust suppres- x x sion water Firefighting x x water Process water x x

Cooling water x x x x Potable water x x x Desalinated x x x water Demineral- ized/softened x x x water

11 Summary

The goal of this thesis was to study the process of raw water treatment and to produce process flow diagrams to be used as possible references for projects. When a plant or a process is being designed reference diagrams can be a very helpful tool in diagram drafting. Reference diagrams offer helpful information about the needed flows and equipment in specific unit processes and help in the selection of the equipment for the actual diagrams drafted for a project.

In this thesis, the raw water treatment process was split into parts according to the use of the product water. The treatment process was studied from a variety of sources in- cluding literature and scientific articles. Knowledge from experts within the company was also utilized as source material. The unit processes which were described in the text and drafted into the PFD’s were chosen according to their relevance in water treatment in general and their use in the company’s technologies.

The PFD’s were drafted using MicroStation as it is still widely used in process design tasks at Outotec. The use of MicroStation allows the diagrams to be easily utilized in process design task as copiable and modifiable references. In the future, these diagrams

28 could be converted to be used with other design programs. The diagrams should also be updated periodically with information acquired from sites and as technology concerning water treatment advances. The diagrams drafted during this thesis can also be used as a basis for piping and instrumentation diagrams (PID’s) in the future. Refence PID’s are also a very helpful tool during process design tasks

In general, there rarely is two water treatment plants which are identical to one other. This is because the treatment process is greatly dependent on the source of the raw water and the use of the treated water. Both of these can vary greatly on the sites because modern mineral processing plants and concentrator plants are often built in increasingly remote locations.

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Appendix 1 1 (8)

Process flow diagrams

Appendix 1 2 (8)

Appendix 1 3 (8)

Appendix 1 4 (8)

Appendix 1 5 (8)

Appendix 1 6 (8)

Appendix 1 7 (8)

Appendix 1 8 (8)