How to treat Zinc, Aluminium, Cyanide, chrome for plating unit (fancy Hardware fittings)

The plating industry is an important sector that contributes significantly to the economy. It is essential to ensure that the wastewater generated during the plating process is treated before discharge to the environment.
Effluent Treatment Plants (ETPs) are designed to treat industrial wastewater before it is released into the environment     
The wastewater from the plating unit contains harmful chemicals such as Zinc, Aluminum, Cyanide, and Chrome, which can be toxic to the environment.  
Here we aim to provide insights on the treatment of these chemicals.

Treatment of Zinc:   

Zinc is commonly used in plating units for its corrosion resistance properties. The wastewater from the plating unit contains high levels of Zinc, which can be toxic to aquatic life.

Steps involved:          
The treatment of Zinc involves the following steps:
a. pH Adjustment: The pH of the wastewater is first adjusted to a pH of around 9-10 using sodium hydroxide (NaOH) or lime (Ca(OH)2). This helps to precipitate the zinc as zinc hydroxide.

b. Precipitation: The wastewater is then treated with a coagulant such as ferric chloride (FeCl3) to promote the precipitation of the zinc hydroxide. This forms a sludge that can be separated from the water.

c. Solid-Liquid Separation: The sludge is then separated from the water using a clarifier or a filter press.

d. Discharge: The treated water can then be discharged into the environment or further treated, depending on the regulations.

The treated wastewater can then be discharged into the environment as shown in Figure 1.

Figure 1: Flow diagram showing steps which will help in removal of Zinc from effluent of plating industry in an ETP

Treatment of Aluminum:     

Aluminum is one of the common metals used in plating unit, and its treatment is necessary to avoid environmental hazards.       
The treatment process includes the following steps:

a. Neutralization:
In this step, the effluent is neutralized with the help of chemicals such as Sodium Hydroxide or Hydrochloric acid to adjust the pH of the effluent to around 7.0. This can be done after filtration also.    

b. Coagulation and Flocculation:       

This step involves the addition of coagulants such as PAC (Poly Aluminum Chloride) and flocculants such as PAM (Polyacrylamide) to the neutralized effluent to form large particles known as flocs, that can be easily settled. 

c. Sedimentation:         
The flocs settle down under the force of gravity in a sedimentation tank, and the clear water is separated from the settled flocs.           

d. Filtration:     
The clear water is then filtered through sand filters to remove any remaining impurities. We can also dewater remaining sludge using a filter press, which results in the formation of a solid cake.  

e.Disinfection:
The filtered water is disinfected using chemicals such as Chlorine or Ozone to kill any remaining bacteria.

The Process Flow Diagram for Aluminum Treatment can be understood below from Figure 2.

Figure 2: Flow diagram showing steps which will help in removal of Aluminum from effluent of plating industry in an ETP

Treatment of Cyanide:          

Cyanide is a highly toxic chemical used in the plating industry. The wastewater from the plating unit contains high levels of Cyanide, which can be harmful to the environment. It is essential to ensure complete removal of cyanide before discharging wastewater.       
Some basic steps of treating cyanide are shown in the flow diagram below in Figure 3.

Figure 3: Flow diagram showing  few steps which will help in removal of Cyanide from effluent of plating industry in an ETP

Steps Involved in Cyanide Treatment:         

The treatment of Cyanide involves the following steps:

a. Degassing:     
The wastewater is first degassed to remove any volatile cyanide compounds.

b. Alkaline Chlorination:        
The wastewater is treated with sodium hypochlorite and sodium hydroxide to convert the Cyanide into Cyanate, which is less toxic.           

c. Acidification:            
The wastewater is then acidified using sulfuric acid to reduce the pH of the wastewater.

d. Precipitation:           
The remaining Cyanate in the wastewater is precipitated using calcium hydroxide. This results in the formation of calcium cyanate, which can be easily removed.           

e. Filtration:     
The remaining solids are then filtered to remove any remaining impurities.

f.  Sludge Dewatering:
The remaining sludge is dewatered using a filter press, which results in the formation of a solid cake.        

The treated wastewater can then be discharged into the environment.

Treatment of Chrome:          

The treatment of chrome-containing wastewater from a plating unit involves several processes that aim to remove heavy metals and other pollutants before discharging the effluent into the environment.          

Steps:
The following is a brief overview of the treatment process and the chemicals involved in treating the effluent.

a. Primary Treatment:
The first step in treating chrome-containing wastewater is to remove any large solids and sediments using screens, grit chambers, or sedimentation tanks. This process is called primary treatment.       

b. Chemical Precipitation:      
The next step is chemical precipitation, which is used to remove dissolved metals such as chromium, copper, and nickel. Chemical precipitation involves adding chemicals such as lime, sodium hydroxide, or sodium carbonate to the wastewater, which causes the dissolved metals to form insoluble precipitates that can be removed by settling or filtration. The chemical reaction is as follows:          
                                                                         Cr3+ + OH- → Cr(OH)3 ↓    

c. Coagulation-Flocculation:  
The next step is coagulation-flocculation, which is used to remove small suspended particles that cannot be removed by sedimentation alone. Coagulation involves adding chemicals such as aluminum sulfate or ferric chloride to the wastewater to destabilize the suspended particles, while flocculation involves stirring the wastewater to form larger particles called flocs that can be removed by sedimentation or filtration.           

d. Biological Treatment:          
Biological treatment is used to remove organic compounds such as surfactants and complexing agents that may be present in the wastewater. This process involves using microorganisms to degrade the organic compounds into simpler, less harmful substances such as carbon dioxide and water. The most commonly used biological treatment systems are activated sludge and trickling filters.

e. Disinfection:
The final step in treating chrome-containing wastewater is disinfection, which is used to kill any remaining bacteria or viruses in the effluent. Disinfection can be achieved through chemical disinfection using chlorine, ozone, or ultraviolet radiation.

The following figure (Figure 4) is a simplified process flow diagram for treating chrome-containing wastewater:

Figure 4: Flow diagram showing  few steps which will help in removal of Chrome from effluent of plating industry in an ETP

In short, the treatment of chrome-containing wastewater from a plating unit involves several processes, including primary treatment, chemical precipitation, coagulation-flocculation, biological treatment, and disinfection. The specific chemicals and processes used will depend on the characteristics of the wastewater and the discharge regulations in the local area. A well-designed and operated ETP can effectively treat chrome-containing wastewater and ensure compliance with discharge regulations.

Summary:

Plating industry is an important industry for making bathroom fittings and hardwares. But during the manufacturing process harmful chemicals such as Zinc, Aluminum, Cyanide, and Chrome are discharged into the waste effluent. We need proper ETP plant with ability to provide needed processes for an effective treatment before they are discharged into the environment.