ABSTRACT: Balanced fluctuation of a solid contaminant at inlet of circuit for treatment of reused water causes accumulation of the contaminant inside of the circuit. Computer simulation, thermodynamic consideration and actual measurement have been used to determine impact of the input fluctuation. A relationship has been established between amplitude of the oscillation and the contaminant build-up. Methods of designing of a circuit has been proposed to alleviate negative effect of the fluctuation.

RC – row coal; GC – gravity concentration (2); FP – final products; 3 – thicken cone; 4,6 – hydrocyclons; 7 – radial thickener; FFF – flotation, filtration, flocculation

 Figure 1. Flowsheet (a) and graph (b) for actual water clarification system.

 

1 INTRODUCTION

 There are a great number of technologies which employ reused circulating water. Mineral processing, paper production, chemical technology are described by complex ramified flowsheets in which reused water circulates and carries a waste product. In reality, a raw treated product contains different quantity of waste material. Furthermore, its specific amount fluctuates during processing of the material. To investigate the effect of this oscillation, special numerical simulation has been conducted.

            A numerical model of circulation (Nazimko & Nazimko 1996) has been employed. A flowsheet of an actual circuit process of coal slurry treatment (Figure 1,a) was replaced by mathematical graph (Figure 1,b). Nodes of the graph represent actual equipment, whereas links connecting of the nodes simulate the pipes. The flowsheet is fed by discrete portions of a product during the every shortest cycle of the coal slurry circulation. In addition, duration of the every link has been input to simulate real delay of the product in local circuits, units of the equipment and links.

 2 RESULTS OF THE SIMULATION

 As an example, fluctuation of input feeding has been investigated for a coal preparation circuit. First, a fixed or predetermined portion of the slime with a raw coal had been input into inlet of the flowsheet at every cycle. Then stochastic fluctuation of the input product has been exited using pseudorandom generator. Chart of input slime content fluctuation in Figure 2.

  

Figure 2. Input slime content fluctuation.

 Distribution of input amount of the slime is shown in Figure 3. Process of the randomization has been checked to be balanced. It means amplitude of slime content deviation relatively the predetermined value has been distributed approximately uniformly

though the diapason ±15% (see Figure 3). In addition, total average amount of the input product was the same as the negative deflection. Material balance has been kept with an accuracy of -0,000839 or -0.08% per cycle. It means that equilibration between positive and negative fluctuations of the slime content during 160 cycles was unbalanced just to –13.3% of the elementary discrete portion of the material what may be neglected. In the other words, quantitative and qualitative balance has been satisfied and the input fluctuation may be considered as a random one.

 

  

  Figure 3. Distribution of input amount of the slime.

 

  

Figure 4. Accumulation of coal slime in radial thickener feed.

 

  

Figure 5. Accumulation of coal slime in reused water.

             Despite  of  this,  accumulation  of the coal slimedefinitely increased (Figures 4 and 5). The slime content    built   up   0.0309  or  3.09%  per  cycle  in thickener (node 7) and up to 0.0323 or 3.23% per cycle in circulating water (node 2). The distributions of the slime deviation became close to normal in the nodes and shifted to the right (Figures 6 and 7). It means positive magnitude of the slime content deviation was more than negative inside the circuit.

  

 

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 Figure 6. Distribution of slime deviation in radial thickener feed.

 

  

  Figure 7. Distributions of slime deviation in reused water.

 For example, maximum of the slime content was 0.3 or 30%, whereas minimum was –0.2 or -20% in the thickener. The same factors were 0.6 or 60% and –0.5 or -50% in the circulating water (node 2). As one can see, the node that accepts return flows is more sensitive to the input fluctuation of circulating product. The node 2 absorbs the flows of the slurry from the nodes 4, 5, 6, 7 and 8. Parameters of the fluctuation and reaction of the nodes are summarized in the table 1.

 Table 1. Parameters of the slime accumulation

Type of the node	Slime content accumulation, per cycle, %	Slime deviationmagnitude, %	Node accepts return flows
Node 1: input raw material	-0.08	±15	No
Node 7: thickener	+3.09	+30 -20	No
Node 2: takes reused water	+3.23	+60 -50	Yes

            Simulation has demonstrated that slime accumulation rate depends on topology of a flowsheet, upon factors of slime distribution among flows, on duration or delay in the flows and on amplitude of fluctuation input material at inlet of the circuit. It is hard to discover concrete relations between these parameters because of limitless diversity of the flowsheets. However it may be estimated approximately that fluctuation of a solid contaminant in input flow of ±10% relatively normally expected level may increase the material content at least by 2-4% in ramified circuits and amplify maximum amplitude of slime fluctuation up to 2-4 times inside the circuit. The more complex and ramified circuit and the more return flows occur, the more slime accumulation and its fluctuation inside the circuit are.

 3 DISCUSSION

 Circuit assemblage for treatment of reused water is a typical open thermodynamic system (Glensdorf 195?; Nazimko & Nazimko. 1996). The circuit absorbs an external flow of a product, mixes it with water, treats, separates from the water, discharges the product and return the water for reusing. Pumps move the water-product mixture. Therefore the circuit system puts through oneself the flows of material and energy. These flows are agents of thermodynamic flows I and forces X:

 I_s = ∑ L_ik X_k                                                                           (1)

 where X_k are Onsager’s factors.

Entropy S of the system is determined by (Glensdorf):

 ΔS = 1 / T (∑ L_ik X_k )                                                             (2)

 where T is the absolute temperature.

It has been shown that slime accumulation inside of the circuit system is the final manifestation of the entropy production (Nazimko & Nazimko. 1996). According (Glensdorf), the entropy production is caused by fluctuation of thermodynamic flows and forces.

In the case, the entropy production (or slime accumulation) occurs as a result of slime fluctuation at inlet of the circuit. Hence the results of computer simulation mentioned above have been predicted and explained theoretically.

            Measurements of slime content in reused water at “Proletarska” coal preparation plant were employed to corroborate the data of the simulation. It had been discovered that normal fluctuation of the readings of a slime content register was ±8% because of pen vibration and an adjacent pump influence. To investigate the effect of slime content fluctuation, raw input slurry was diluted with fresh water by random turning on and off a special pump. The fresh water was added in such a manner that input fluctuation of slime concentration changed by ±12%. Because the slurry had gone through different links and washery equipment, slime fluctuation at the point in question changed by range of +36% -22%(?) (Figure 8). Average slime concentration increased from 63kN/m³ up to 71kN/m³ or by 6%.

            The results of the investigation should be considered when the systems for treatment of circulating water are designed. Capacity of separate units of equipment should be increased relatively normal level according expected fluctuation of a solid contaminant in the water. It should be noted that this measure has nothing similar to the emergency measures like an inundation or an accident. Proposed above measure relates to everyday routine functioning of a circuit. Another way to alleviate negative effect of input fluctuation is selection of optimal topology, distribution factors and delays in the flows of a circuit in question.

 4 CONCLUSION

 Balanced fluctuation of a solid contaminant at inlet of circuit for treatment of reused water causes accumulation of the contaminant inside of the circuit.

Computer simulation of slime content fluctuation at inlet of coal preparation circuit has demonstrated that fluctuation of a solid contaminant in input flow of ±10% relatively normally expected level may increase the material content at least by 2-4% in ramified circuits and amplify maximum amplitude of slime fluctuation up to 2-4 times inside the circuits. The more complex and ramified circuit and the more return flows occur, the more slime accumulation and its fluctuation inside the circuit are.

     Capacity of separate units of equipment should be increased relatively normal level according expected fluctuation of a solid contaminant in the water. Another way to alleviate negative effect of input fluctuation is selection of optimal topology, distribution factors and delays in the flows of a circuit in question. Computer model has been developed for assessment and optimization of a circuit.