Overview to typical technical activities in IWRM-concepts

I. INTRODUCTION

From 1983 until now the steel production was doubled leading to an enormous waste formation [6]. Along with growing world population the need of steel is still increasing and thus the environmental pollution caused by waste and waste-water from metallic working industries become a more and more emerging problem. In many countries, recently, novel stringent regulations and directives were introduced in order to both safe water quality and force the metal industry to a more sustainable handling of process water including new recycling strategies.

Against this background “Cleaner Production” (CP) management leading to savings in resources and costs turns out more attractive for the metal industry. The United Nations Environment Program (UNEP) define CP as follows: "Cleaner production means the continuous application of an integrated, preventative environmental strategy to processes, products and services to increase eco-efficiency and reduce risks to humans and the environment". Cleaner Production focuses on minimising resource use and directly avoiding the creation of pollutants, rather than implementing end-of-pipe technologies. It involves rethinking products, processes and services to move towards sustainable development.

The objectives of CP shall be reached by the means of conserving raw materials and energy, eliminating toxic raw materials, reducing quantity and toxicity of emissions and wastes, reduction of negative impacts along the product life cycle and incorporation of environmental concerns into designing and delivering services.

In addition to the environmental benefits also economic returns are to be gained, e.g. improved production efficiency, lower cost for waste disposal and waste-water treatment (WWT) as well as new and improved market opportunities via better image.

There is existing a wide variety of cleaner production methods for the diverse metal industry sectors including casting, finishing and machining processes [2, 4].

Within this work CP waste-water management for metal finishing applications, namely pickling processes was theoretically investigated. Additionally, more practical a pilot plant for integrated pickling bath recycling was developed and installed.

II. CP MANAGEMENT FOR PICKLING PROCESSES

1. Overview

Related to metal finishing a high amount of liquid and solid waste containing toxic metals, alkaline and acidic solutions are produced. Among others some of the most harmful components within metal finishing waste are chromium, cyanide and concentrated acids. They are accumulated in paint, inks, sludge, solvents, liquid waste and rinse water [3, 5, 8, 10, 13, 14].

Belonging to the category of liquid waste pickling bathes are one of the most significant sources of highly contaminated acidic process water.

Pickling is defined as the removing of inorganic impurities from a metallic surface by the means of a liquid, causing chemical solving processes and/or blasting of oxide coatings from the metallic surface [1]. The pre-treated metallic work piece is turned over to subsequent processing like galvanizing, painting or extrusion. Except for some additives the pickling liquor mainly consists of mineral acids particularly hydrochloric acids, sulphuric acid, nitric and hydrofluoric acids.

2. CP measures

Within the pickling process acid is consumed, water is evaporated and metal salts are enriched. As a result a certain amount of acids have to be continuously added to maintain free acid levels. However, if the metals content of the solution exceeds a specific value (e.g. 50-60 g/l in stainless steel pickling liquor) metal salts will start to crystallize leading to the dumping and replacement of the bath [1]. Thus, the pickling process, including subsequent process steps like rinsing, generates a considerable quantity of spent pickle liquor comprising large amounts of nitrate, fluoride, chloride, sulphate and metal salts. Spent pickling is considered as hazardous waste by EPA. In former days, it was disposed of in landfills following lime neutralization.

Nowadays advanced Cleaner Production strategies, which aim to in-situ regeneration, are applied to the pickling process. Besides this more elaborate and cost intensive techniques there are certain easy handling CP methods listed below for reducing the quantity of liquid waste within the pickling process [3, 10, 14].

• Careful and clean working

• Installation of drip trays and splash guards

• Drag out reduction through

- longer drainage times for batch treated work pieces

- squeezing of continuously treated work pieces

• Improving rinsing through

- cascade rinsing (reuse of rinsing water)

- spray rinse of the work pieces above the pickling bath

- air or mechanical agitation for better cleaning efficiency

These measures are very helpful for reducing waste-water from pickling processes. However, recovery/regeneration applications are obsolete, since the quantity of generated waste-water resulting form a bath change is much larger compared to drag out losses, etc.

The most important processes for the recycling of pickling acids are pyrohydrolysis and retardation.

Pyrohydrolysis bases on the principal of thermal decomposition. At high temperatures water and acid are evaporated, metal salts are decomposed into metal oxide and are continuously removed. This process route enables the total regeneration of both the free acid and the acid that was chemically bounded to metals. The metal oxides are transferred to diverse industries for reuse [1, 11].

In contrast to the total regeneration that is reached by thermal process retardation is limited to the recycling of free pickling acid. However, acid retardation is by far the most widely used system due to its low investment costs and simplicity, reliability and superior performance. Retardation of pickling bases on the selective adsorption and desorption of acid on specific ion exchange resins. Because of the discharge of acids bounded to the metals periodically a small quantitiy of fresh acid has to be added [1, 11].

Another CP strategy for a more environmental friendly and resource saving pickling process is the complete or part substitution of the chemical pickling process through mechanical pickling if suitable. Thus, surface impurities and oxide coatings are removed by using certain abrasive materials (e.g. glass or steel ball, carbide pats or rotating brushes) [14].

3. Case study

1. Situation: Due to more stringent legislations and the need for saving costs the plant engineering sector has a growing interest in more environmental and resource saving processes in the area of metal finishing. Among others they are searching for an alternative more efficient process for the reuse of pickling bathes.

Though nanofiltration is an established membrane process for separation of metals, so far, only a few research activities focus on the implementation of nanofiltration processes in the recycling of pickling acids [9].

Thus, within our work we investigated the behaviour of different nanofiltration (NF) membranes to corrosive acidic conditions in order to develop and implement a novel nanofiltration plant at pilot scale for recycling of pickling acids.

2. Materials and methods: First screening tests with different nanofiltration membranes were conducted (Tab. 1).

Table 1. Nanofiltration membranes
for screening tests [7]

Membrane	Material	pH	T_max	p_max
NF 1	PES	1-14	90°C	40 bar
NF 2	NN	0-14	70°C	35 bar
NF 3	PES	0-14	95°C	40 bar
NF 4	NN	NN	NN	NN
NF 5	NN	NN	NN	NN

The experiments were performed using a membrane test cell made of Hastelloy steel (Fig. 1).

The operation pressure is applied by a gas cushion and the mass transfer is realized by a magnetic stirrer. The filter cell features a maximum pressure of 100 bar and a filling volume of 400 ml. Filter areas up to 36 mm² can be tested within the filter cell.

The applied pickling acid was provided from a large steel company and is characterized as follows (Tab. 2):

Table 2. Composition of the nitric/fluoric pickling acid [7]

Parameter	Wert
pH	< 1
Fluoride	25.5 mg/l
Nitrate	7.4 g/l
Chrome	222 mg/l
Iron	1.2 g/l
Molybdenum	10 mg/l
Nickel	166 mg/l
Titan	0.75 mg/l

The main parameters that have to be determined during the screening tests are the filtrate flow and the retention of the different components.

Nitrate was detected by ion chromatography. Fluoride was measured by the means of an ion selective electrode. For the determination of metals an ICP-spectrometer was applied. The filtrate flow was gravimetrically measured.

3. Results: Basing on a variety of experiments the membrane screening of five different membrane types yields the following results (Tab. 3).

Though the NF2 features the best results the NF1 was preferred for subsequent experiments since the NF1 showed a higher resistance within the acidic milieu. The properties of the NF1 relating to the metal and nitrate retention are presented below (Fig. 2). In order to determine the acidic stability the rejection was measured before and after eight weeks storage of the membrane in the pickling acid.

The metal retention maintained relatively constant. The slight decrease of the rejection was probably caused by oxidation processes. In contrast the nitrate retention drastically changed from 75 % to -1.5 % which is very beneficial for the separation of nitric acid from metal salts, since the nitrate was even concentrated in the filtrate.

The behaviour of the permeate flux is shown below (Fig. 3).

Table 3. Evaluation of the screened membranes for nitric/fluoric acidic pickling:
+ positive, - negative, 0 indifferent [7]

Retention	NF1	NF2	NF3	NF4	NF5
NO₃^-	+	+	+	+	0
F⁺	0	0	0	-	0
Cr- and Fe-	+	+	+	+	+
Ni- and Ti-	0	0	0	+	+
Mo	-	+	-	+	+

The process chain consists of the pickling step followed by a rinsing process. Due to drag losses and evaporation water and fresh pickling liquor is continuously added. In order to prevent the nanofiltration plant from blocking by particles a microfiltration pre-treatment was established.

The nanofiltration plant is specified as follows (Tab. 4):

Table 4. Specifications of the nanofiltration membrane plant [12]

LWH	400019002100 mm
Materials	PE, Hastelloy C22, Teflon coated tubes
T (max)	30 °C
p (max)	40 bar
Feed (max)	1.7 m³/h
Membrane	6 Spiral wound module (22 m²/module)

A picture of the nanofiltration plant is shown below (Fig. 5)

Figure 5. Nanofiltration membrane plant for the filtration of nitric/fluoric acid pickling.

A quantity of 1.7 m³/h of pickling liquor with a mean concentration of 35 g/l metal, 50 g/l fluoric acid and 110 g/l was flowing into the membrane plant and concentrated up to 1.25 VCF.

As result, it succeeded in recovering 15.6 kg/h fluoric acid and 60.0 kg/h nitric acid. Depending on the prices of pickling liquor and the life time of the membranes a pay-pack period of maximum 5 years was calculated [12].

V. CONCLUSIONS

Particularly small and medium enterprises in the sector of metal finishing are sceptic towards CP methods since they fear high investment costs. However, already very simple measures like installations of drip trays and splash guards contribute to reasonable saving of waste and material.

In the long run the integration of advanced but more elaborate CP processes, e. g. membrane filtration, for closing process water loops sustainably improve the efficiency of existing processes and helps to reduce the amount of waste-water. One main advantage of membrane processes is the fact that there is no interference with the original process chain, since membrane applications are an additional modular operation step.

They are to be operated both continuously or batch wise, are place-saving and, with regard to pickling recycling, have significant lower energy consumption.

However, further research activities have to be conducted in order to increase the reliability of the membrane process and the lifetime of the membrane at strong acidic conditions.

REFERENCES

1. Brown C. J., 2002. Recovery of stainless steel pickle liquors purification vs. regeneration. CISA Intern. Steel Congr., Beijing.

2. Centre for Excellence in Cleaner Production, 2002. Waste minimization in metal surface Ffinishing. Australia.

3. Cleaner Production International LLC, 2009. The metal products industry, http://www.cleanerproduction.com, 08/2009.

4. Environmental Guidelines for small-scale activities in Africa, 2006. Micro- and small enterprises – Metal finishing.

5. EPA, 1993. Guides to pollution prevention - The metal finishing industry. 10/1992.

6. German Industrial Bank, 2009. Metal Industry 2020. Berlin.

7. Kleemann M., 2001. Aufbereitung einer Abfallphosphorsäure und einer Fluss-/Salpeter-Abfallmischsäure mittels Membrantechnik. Master thesis, Fraunhofer UMSICHT.

8. Koefoed M., C. Buckley, 2008. Clean technology transfer: A case study from the South African metal finishing industry, 2000-2005. J. of Cleaner Production, 16 : 78-84.

9. Lyko M., 2005. Zu sauer? – Nicht für Membranen. Chemie Ingenieur Technik, 77/5 : 598-592.

10. Reeve D.J., 2007. Environmental improvements in the metal finishing industry in Australasia, 2000-2005. J. of Cleaner Production, 15 : 756-763.

11. Rituper R., 1993. Beizen von Metallen. Leuze Verlag.

12. Siegert K., 2004. Untersuchungen zur Wiederverwendung von Abwasserströmen aus der Oberflächenbehandlung in der Stahlindustrie mittels Membrantechnik. Student research project, Fraunhofer UMSICHT.

13. Telukdarie A., C. Buckley, M. Koefoed, 2006. The importance of assessment tools in promoting cleaner production in the metal finishing industry. J. of Cleaner Production, 14 : 1612-1621.

14. UNEP Working Group for Cleaner Production, 1998. A cleaner production manual for the metal finishing industry. CRC for Waste minimization and Pollution Control, Ltd.

^{^[1]} VCF Volume concentration factor: relation between inlet- and retentate volume flow